How to see the planets for yourself

You have almost certainly heard much about the planets of our Solar System, and have likely seen the beautiful pictures space probes have made of each of them from nearby. But what some people don't know is that you don't need to go to other planets to see them: they're all visible from Earth. In fact, a couple of them are just about the easiest things to find in the sky, and each of them is fun to see. So for those who are curious, but haven't seen the planets yet (or not all of them) I have written a little guide, starting with the easiest planet to see in 2013 and working my way up to the hardest. Give it a try some night, seeing another planet is not much harder than seeing the Moon once you know where (and when) to look! All you need is at least one eye and a clear sky.

Jupiter

(Image credit: NASA)

The Romans named Jupiter after the king of their gods, and if you've ever seen it on a dark night, it's not hard to see why. Jupiter is far brighter than any star and easily dominates the night sky if the Moon isn't out. This makes it the easiest planet to see in many years, including 2013. Jupiter is very bright due to its enormous size, which means it reflects a lot of light.

Right now, Earth is passing between Jupiter and the Sun, and this means Jupiter is at its closest to us(and therefore brightest), and that Jupiter rises right as the Sun sets and sets as the Sun rises. That makes any cloudless night these months an awesome time to look at Jupiter. In the early evening, you can see it shine brightly in the east, even from the most light-polluted cities. Around midnight, it passes very high through the south, easily the brightest “star” in the sky. Early in the morning, it sinks down into the west as the Sun rises in the east. Due to Jupiter's brightness, you probably won't need a star chart or any knowledge of the sky to find it. It's brightness makes it stand out easily: for most of the night it's the brightest object in the sky other than the Moon (which, by the way, will approach Jupiter very closely in the sky on 26 December, and even cover it seen from South-America and Africa). But in case you need more help, it's in the constellation of Taurus. It's easy to find due to a large hexagon of very bright stars in the sky, often called the Winter Hexagon. It's near the star marking its right corner, Aldebaran, but way brighter than this star:

(Image of the brightest stars of the winter sky made using SpaceEngine. Jupiter is the bright object on the right of the large hexagon. Brighter stars are larger in the picture than smaller ones. It helps to look at this picture in full view)

While Earth is passing between Jupiter and the Sun now, it will slowly move away during 2013's early months. Jupiter will stay in the same area of sky, but will set earlier every day. By March, it'll be in the south around sunset, and sink into the west during evening, setting around midnight. In May and June, Jupiter will pass on the other side of the Sun and therefore be invisible from Earth. But in July it will become visible again very early in the morning, rising about three hours before the Sun in the east. In the final months of the year, Earth will approach Jupiter again and it will rise earlier all the time, so by December 2013 it will once again be visible the whole night long.

(Image credit: NASA)

Jupiter is a beautiful sight with the naked eye, but if you have binoculars or a telescope you can see something very cool: its four largest moons! They're called Io, Europa, Ganymede, and Callisto, and each is the size of a small planet (Ganymede is in fact larger than Mercury). The only reason we can't see them with the naked eye in the first place is that Jupiter's bright light is in the way, but with binoculars, even poor ones, you can see them separate from Jupiter. They're visible as four “stars” close to Jupiter (or three, or two, or occassionally just one; they often pass behind or before Jupiter so they're not visible), roughly on a straight line. If you look at them from day to day, you can see they're in different places each time due to their fast orbits. With good binoculars it also becomes possible to see Jupiter as more than a point of light, but actually see it as a sphere. You won't be able to see its beautiful clouds, though: you need a telescope for that.

Venus

(Image credit: NASA)

Maybe you have seen something very strange in the sky once. A mysterious light hovering above the western horizon in the evening, or maybe it was above the eastern horizon early in the morning before the Sun was up. It was obviously far too bright to be a star, but unlike a plane it didn't move at all. It just hung there silently. Was is a spaceship from another world?

Well, no. In fact, it was another world: Venus. Venus is ludicroudly bright, as much as six times brighter than Jupiter. Other than the Moon and the Sun, nothing in our skies is brighter. She gets so bright because A: she's the closest planet to Earth, B: she's covered in bright white clouds, reflecting a lot of light, and C: she's close to the Sun, so brightly lit. Venus is bright enough even to cast shadows in places that are very dark. (in any place in or near a city these very faint shadows are washed out by the light pollution though)

So if Venus is so bright, why do I consider Jupiter the easiest planet to see? Well, there's a catch: Venus is closer to the Sun than Earth, so in the sky she's always near the Sun. Jupiter can be nearly overhead in the sky in the middle of the night, but Venus is always near the Sun so it's usually day when she's in the sky. However, at dawn or dusk she is often visible and makes even Jupiter look faint then. Right now Venus rises shortly before the Sun, but she's about to pass behind it, becoming invisible to us Earthlings from mid-January on. She won't emerge on the Sun's other side until June, and then she'll be visible low in the evening sky for the rest of the year. She'll follow the setting Sun, setting one to three hours after it and gracing the western evening sky with her brightness. You really don't need a star chart to see Venus; it's a matter of looking to the west in the second half of the year about an hour after the Sun sets: Venus is unmistakable.

Much like Jupiter, Venus also has a bonus if you look at her with binoculars. They need to be binoculars on a tripod, though: holding them in your hand won't make them steady enough. But if you have binoculars on a tripod and aim them at Venus, you can probably just make out that the planet doesn't look round, but has the shape of a crescent Moon. This is because Venus is closer to the Sun than Earth, so we're looking at it more or less from behind.

I've said it before: Venus is bright. But in fact, she's so bright she's visible at day! You've probably seen the Moon at day some time, but not Venus. This is because Venus is very easy to miss. Unlike the Moon, she's just a tiny little dot of white among a sea of blue sky. But she's visible if you know where to look. October will likely be the best time to try: Venus will be at her farthest away from the Sun. To try it, find a spot where the Sun is just out of your sight: this will prevent your eyes hurting from looking roughly in its direction. So you need to be in the shadow, but left of the Sun (in the east) you need to be able to see the sky. Sitting in a place where a tree or building covers the Sun is probably the best idea.

Venus will be a bit over forty degrees to the Sun's east. To measure out these forty degrees, spread your hand out at armlength. Provided you are not very weirdly proportioned, the distance from the tip of your thumb to the tip of your pinky is roughly twenty degrees at arm's length. So you look two of these distances left of the Sun. The best times to do this would likely be 14:30 (at that time, Venus will approximately at the same height as the Sun). Most likely, however, you will see nothing there at first, just blue sky. Well, keep looking at the area, scanning it carefully with your eyes: the detail of a tiny white spot against the blue sky is too slight to be picked up by your peripheral vision, so you need to see it with the centre of your sight. Don't give up: it takes a while to find.

There's another situation when Venus is visible at day: when it passes exactly between the Sun and Earth. It's visible as a little black dot during these Venus transits. This happened in June, but unfortunately won't happen again until 2117.

Saturn

(Image credit: NASA)

Just like Jupiter, Saturn is huge. But it's almost twice as far away from the Sun, so it gets less light, and is also further away from Earth. Because of this Saturn isn't as bright as Jupiter, but it's still a bright planet. It's about as bright as the brightest stars, so you'll need a little knowledge of the constellations to recognise it. Not much, though, as it's bright enough to easily stand out among all but the brightest stars. Saturn is in between the constellations Virgo and Libra at the moment, and fortunately there's only one bright star nearby:

(Image of the spring sky near Saturn made using SpaceEngine. The circled object is Saturn. Looking at the picture in full view helps)

Saturn doesn't rise until the morning right now, not long before sunrise. But in the first few months of 2013 it'll rise earlier all the time, until Earth passes between Saturn and the Sun in April, when it'll be visible all night long as it rises when the Sun sets and sets when the Sun rises. From June to September it'll set during the night, and therefore will be visible in the evening only. During October and November, Saturn will pass behind the Sun, and therefore will be invisible. In December it'll start rising before the Sun in the early morning again.

If you look at the southern sky in April or May, you will see two bright “stars” near each other, with a third one higher up in the sky. Saturn is the brightest of these three, at the lower left of the triangle they form. It's also the only one that's yellow-whitish; Spica (the star near Saturn) is a little bluish, while Arcturus (the star above Saturn and Spica) is a bit reddish. There are likely other stars visible as well, but none of them are as bright as Saturn.

With a binocular on a tripod, Saturn's beautiful rings can just be seen. To see them clearly separate from the planet you need a telescope, but in binoculars the planet looks oddly elongated, a bit egg-like due to the rings. Saturn also has one big moon, Titan, which is just in the range of binoculars. It's very big, bigger than Mercury, but Saturn's distance to the Sun makes it a lot harder to see than Jupiter's big moons. A faint star nearby Saturn is likely to be Titan, particularly one that moves from day to day.

Mars

(Image credit: NASA)

After Venus, Mars is the closest planet to Earth. This is both a blessing and a curse for seeing it: when Earth passes between Mars and the Sun, the planets get very close and Mars becomes brighter than any star, sometimes even brighter than Jupiter. It's a fiercely bright red thing in the sky at these times. But Mars' proximity also means it moves very fast compared to Earth, and that at its furthest, it's as much as five times farther away than its closest. It's very faint by comparison during these times, and this is one problem.

The other problem is this: Mars orbits the Sun once every 687 days*, while Jupiter takes twelve years over an orbit. This means that if Mars and Jupiter were both behind the Sun seen from Earth (and thus invisible), we need to wait only half a year for the Earth to pass between Jupiter and the Sun: Jupiter moves little in that time. Mars, on the other hand, will be long gone after half a year, and has completed a good part of its orbit by then, so while it's not precisely on the opposite side of the Sun any more it's still quite far away and it'll be another half year before Earth gets close to it.

This means there are great Mars years, when the planets are close to each other all year long and Mars is really bright. It also means there are awful Mars years, when Mars is in hiding behind the Sun most of the year. 2013 is one of these awful years, and this is why I've listed it behind Saturn (in a good year I might have listed it as the easiest to see planet). In January 2013, Mars sets less than two hours after the Sun. After this, it'll pass behind the Sun for most of the year, and won't be visible again until August, when it'll start rising before the Sun in the early mornings (along with Jupiter, but Jupiter will be as fiercely bright as always while Mars will look like an average star). It'll slowly rise earlier and become brighter, but even in December it still doesn't rise until 1:00 and is no brighter than Saturn.

Mars' rapid movement also makes it hard to give a star chart for its location, as it moves constellations about once every two months, and is constantly on the move within those constellations. So instead of making a dozen star charts, I will tell you in which constellations it is in each month. Then you can look that constellation up on Wikipedia for its star chart; the bright star that's not on it will be Mars. In December 2012 and January 2013, it's in Capricorn, low in the southwest in the evening. In July, it reappears in the northeastern morning sky in Taurus. In August it'll be in Gemini, and in September in Cancer. It will spend October and November in Leo, and end the year in Virgo.

Mercury

(Image credit: NASA)

Mercury is the smallest planet, but it's also closest to the Sun and therefore very brightly lit. This makes it quite bright despite its size: under favourable conditions it's brighter than any star, though not as bright as Jupiter. There's one problem, however: it's closer to the Sun than even Venus. While Venus gets a decent distance away from the Sun, Mercury is always sticking very close to it and never visible out of twilight. Even when it's visible in twilight, it'll be low above the still-lit horizon the Sun just set under/is about to rise above. This makes it far more difficult to see than the previous planets. In fact, I have never seen it myself.

When Mercury gets far enough from the Sun to be visible in twilight, it's only for a few weeks at most: the planet orbits in only 88 days, so it moves fast. It's visible in the early mornings right now, but only until 20 December. It's pretty close to the far brighter Venus, actually. After 20 December, it'll pass behind the Sun, but it'll be visible in the evening, just after sunset, from 8 to 26 February. Then it'll pass between Earth and the Sun, but when it reaches the other side of the Sun it'll never come high enough to be visible. But half an orbit later it'll reemerge in the evening sky from 19 May to 22 June, getting quite close to Venus once again in June. It passes before the Sun, and shows up in the mornings again from 24 July to 16 August. When it reaches the other side of its orbit it'll once again never get high enough to see, and its final visibility of the year will be from 7 November to 7 December.

I've established how hard Mercury is to see, but there are things that make it easier: Venus or the Moon being close by is the main one, since they are bright enough to be seen easily in twilight. The Moon (a crescent) will be close to Mercury on 11 February, 10 and 11 July, 5 August, and 1 December. From 24 to 27 May it'll be close to Venus and Jupiter. It'll also pass near Saturn on 26 November, but since Saturn is fainter than Mercury this is unlikely to help much. A star chart wouldn't be of much help either, as the Sun's glow near the horizon washes out the stars.

Uranus

(Image credit: NASA)

Uranus is only half as big as Saturn, and twice as far away from both Earth and the Sun's light. Due to this, it's far dimmer, so dim in fact it wasn't discovered until 1781. Uranus is just bright enough to be seen with the naked eye outside of cities, but this is hard to do. With binoculars or a telescope it's visible quite easily, with just one problem: how do you distinguish it from thousands of dim stars?

Well, with a map of course. I've found it myself using a map like the one linked below: it's a tiny blue light in a telescope, which becomes a minuscule little blue sphere at high magnification. Right now Uranus is in Pisces, where it'll be visible in the evenings during the first months of the year, behind the Sun in April and May, visible in the morning from June to September, and visible all night long as the Earth passes between it and the Sun in October and November. It's a difficult planet to find, but very cool to have seen.

The chart is located here: http://dcford.org.uk/findercharts.php?obj=uranus&year=2013. It's no mistake that Uranus doubles back on its path in August: this is a perspective effect caused by Earth overtaking it.

Neptune

(Image credit: NASA)

Neptune is even farther away than Uranus, and so even dimmer. It's theoretically visible to the naked eye under absolutely ideal conditions (very, very far away from any lights (We're talking Antarctica here), an incredibly clear sky, preferably on a mountain top or in space, when the Sun is very far beneath the horizon and Neptune high in the sky, there's no Moon, no Venus, and you've got very good eyes. Then you might just be able to glimpse a hint of it), but I don't think anyone has ever actually seen it with the naked eye. It's still visible through binoculars and telescopes though (though I have never seen it myself). You'll need a map of the area of Aquarius where it can be found (best in August, when Earth passes between Neptune and the Sun). This map is fortunately supplied by the same site that gave us the one for Uranus: http://dcford.org.uk/findercharts.php?obj=neptune&year=2013

*=Coincidentally** exactly as long as an elephant pregnancy.

**=Or not. Maybe elephants come from Mars.

Seven interesting things about the Moon

(Image credit: Gregory H. Revera)

In the long, cold winter nights that the northern hemisphere get right now, the Moon is often very high in the sky. The full Moon especially is impressively bright and beautiful as it shines almost directly above your head and illuminates the dark winter night. If you looked at it last night, like I did, you probably even saw a very bright “star” close to the Moon (It wasn't actually a star, but Jupiter). It was a wonderful sight, and inspired me to write about the Moon for a bit. Unlike with the Sun, I don't really have a singular story, and will instead just tell several quite interesting things about the Moon.

(Image credit: NASA)

1: The Moon was created when Earth collided with another planet. The planets formed from a disc of gas and debris around the proto-Sun. Little particles would collide with each other and attach to each other, forming pebbles, which would keep colliding to form rocks, and as these series of collisions kept going, the rocks became bigger and their collisions more violent; especially once they became big enough that their gravity began to play a role. In the outer Solar System, this led to four heavy planets being formed, which had gravity strong enough to keep hydrogen and helium, the most common atoms, from escaping, and as a result collected gigantic atmospheres of those two gases. But closer to the Sun, the planets formed were smaller and not heavy enough to get atmospheres of hydrogen and helium, but only of heavier, far rarer gases. These five planets were Mercury, Venus, Earth, Theia, and Mars.

Hold on, Theia? Well, Theia was a planet about the size of Mars; two times smaller and ten times lighter than Earth. It happened to have formed in almost exactly the same orbit as Earth. Normally, Earth's gravity would've caused the two planets to collide and form a single one with them being so close together, but Theia was in exactly the same orbit, orbiting the Sun in one year, just like Earth. As a result, the two planets never came close to each other since they orbited at the same speed.

So why doesn't Theia exist any more then? Well, the Sun's gravity isn't the only thing that affects a planet's orbit. It's the most important one, but not the only one. The planets also affect each other's orbits a little. It's a very small effect, but the little pulls Venus, Mars, and Jupiter gave Theia and Earth were enough to slightly speed one planet up and slow down the other, causing them to approach each other over the course of 50 million years. Finally, they got close enough to be affected by each other's gravity, so they sped towards each other and collided.

The catastrophic energy of that impact was enough to instantly destroy the Earth's entire surface and melt and deform the planet, while Theia was completely pulverised. Within hours, the collision was clouded in a huge sphere of debris as the two molten planets became a single one that glowed almost as hot as the Sun, vaporising the top layer of rock so it added to the debris. Much of the debris rained back to this molten new Earth, and much of it escaped the gravity and began following its own orbit around the Sun, but the rest of the debris stayed in orbit of Earth. And as the Earth slowly cooled down from the collision with Theia, the debris coalesced into bigger and bigger clumps in a small-scale repeat of the origin of the planets. Eventually, these clumps became two large moons, which finally collided to become the single one that still orbits us. (The traces of this second moon can still be seen in the form of the highlands on the far side of the Moon)

(Image made using SpaceEngine)

2: The Moon used to be far closer. It's not quite sure how much closer, but it is thought it might be as much as eight times farther away now than when it was newly formed! Imagine how gigantic the Moon would've been in the sky at such a distance; it would create planet-wide solar eclipses every month! But if it was this close, how come it's now much farther away? The answer is the tides. The tides are caused by the Moon's gravity, and when the Moon was eight times closer, its gravitational influence on Earth would've been 64 times greater. That means the sea didn't rise up six or seven metres at best with flood, as it does now, but about 400 metres!

Imagine a gigantic tsunami like that rising up every single day and washing deep inland. And there's something else: days used to be shorter back then. The Earth rotated in only six hours, so there were days and nights of three hours each. Every day, you'd get this giant tsunami caused by the Moon. All that water in a 400 metre high tidal wave had a lot of mass together. Every time the flood would hit a continent, the Earth would get a pretty big hit in the direction opposite to its rotation by all that water. The Earth is incredibly heavy, of course, far heavier than the massive flood, but all that water crashing into the continents every day eventually slowed its rotation down over millions of years. By the time the dinosaurs walked the Earth, days lasted 22 hours. They're currently 24 hours and still increasing with a few microseconds each year due to the tides.

But there's a natural law saying energy can't simply disappear, and rotation is a form of energy. So all that energy the Earth lost as it rotated slower needed to go somewhere. In fact, it went into the Moon's orbit. Every day, the tides made the Earth rotate a bit slower and made the Moon orbit a little faster, so it moved into a higher orbit, and that's why it's now much farther away than it used to be.

(Image credit: NASA)

3: The Moon also rotates. Earth obviously rotates once a day, causing day and night. Yet the Moon always has the same side turned to the Earth. The reason one side of the Moon is always facing us is because it rotates once every 27 days; precisely the same time it takes to orbit the Earth. What an incredible coincidence, both its orbit and its rotation take 27 days! What are the odds?

Of course, it's not really a coincidence. This is another thing that's caused by the tides. Remember the tides being 64 times stronger when the Moon was newly formed? Well, just imagine how strong they were on the Moon itself. Earth is 80 times heavier than the Moon, so the tides on the Moon were about five thousand times stronger than they currently are on Earth! Of course, the Moon has no oceans for these tides to cause ebb and flood, but water isn't the only thing affected by the tides. Even the solid rock a planet consists of it affected by it. It's a far smaller effect, and as good as immeasurable on Earth, but when you make that effect 5000 times stronger, it becomes quite powerful. The Moon's surface would've risen up and down every day due to Earth's gravity, probably causing big earthquakes (well, moonquakes) and heavy volcanism (in fact, this might be what caused the dark “seas” of solidified lava on the Moon to form). While it's not quite as intuitive as the idea of a tidal wave smashing into the continents slowing the Earth down, this pulling on the Moon's surface also caused its rotation to slow down due to friction. And since the tidal effects on the Moon were 5000 times stronger, the Moon's days rapidly became longer and longer. All the way until it rotated exactly once for every orbit it made, always facing the Earth with one side.

(Clause Joseph Vernet – Seaport by Moonlight. Work in the public domain)

4: Moonlight is amazingly bright. If you've only ever been outside on a night with full Moon in a place with street lights, you may not have noticed this, but the Moon reflects a lot of light from the Sun. The full Moon gives off enough light to read by and easily find your way around with. It also means you see far fewer stars on night when the Moon is out: the dimmer stars just can't compete with the far brighter Moon. Moonlight casts clear shadows on the ground, and when it's partially clouded it often illuminates one side of the clouds but not the other, creating a very beautiful effect.

(Image credit: Claude Schneider)

5: You can see Earthlight on the Moon. Given how bright moonlight is, it stands to reason the Earth shines even brighter on the Moon. The Earth is four times the size of the Moon, so it has a sixteen times bigger surface to reflect light with. Added to that is that the Earth's surface simply reflects more light than the Moon: the Moon's surface consists of dark grey rock, while the Earth has lots of bright white clouds that reflect far more light. Oddly enough, the oceans are actually quite dark and don't reflect much light; the difference lies mostly in the clouds. All this means Earthlight on the Moon can get up to forty times brighter than Moonlight on Earth. This is actually bright enough to see the Moon illuminated by it from Earth with the naked eye (though binoculars or a telescope make it easier).

Full Earth gives off forty times more light than full Moon, so you'd think full Earth is the best time to observe Earthlight. Unfortunately, when it's full Earth, it's new Moon which means the Moon is very close to the Sun in the sky and invisible. The best time to look for Earthlight is when the Moon is a crescent: the Earth is still close to full seen from the Moon then, but the Moon is visible at night near sunrise or sunset. By the time it's half Moon, Earthlight is getting weaker (since it's also half Earth then) and the Sun's daylight on the Moon makes it harder to see the earthlight on the Moon's night side anyway. It's still visible with binoculars or a telescope at half Moon, but usually not with the naked eye.

(Image credit: NASA)

6: The Moon is the largest moon in the Solar System compared to its planet. It's not the actual largest (It's a big one, but three moons of Jupiter and one of Saturn are bigger), but all other moons are absolutely tiny in comparison to their planet. Mars has two moons the size of mountains. Jupiter, Saturn, Uranus, and Neptune also have big moons, just like Earth, but since these planets are all far larger than Earth, their moons are still tiny compared to them. Our Moon, on the other hand, is only four times smaller than Earth, and eighty times lighter. This is so close some people don't even consider it a moon, but instead consider Earth and Moon two planets that orbit each other (a double-planet). The Moon is certainly the size of a small planet: Mercury, the smallest planet, is only 1.5 times larger (and actually looks a bit like the Moon). The reason the Moon is so big compared to the Earth is probably the unique way it was formed: the other moons in our Solar System probably weren't formed by a planetary collision.

(Image credit: NASA and Buzz Aldrin)

7: Twelve humans have walked on the Moon. Their names are Neil Armstrong, Buzz Aldrin, Pete Conrad, Alan Bean, Alan Shepard, Edgar Mitchell, David Scott, James Irwin, John Young, Charles Duke, Eugene Cernan, and Harrison Schmitt. All of them did so between 1969 and 1972. However, India, China, and Russia all have plans for a manned Moon landing in the early 2020s.

During the six Apollo missions where a manned Moon landing happened, a bunch of stuff was left on the Moon. There's six American flags (which are by now probably just pieces of white cloth due to the Sun's radiation bleaching them), a lot of footprints (there's very little erosion on a world without air or water), six landing modules, six crashed ascent modules, and the three Moon buggies the Apollo 15, 16, and 17 crews used to ride around on the Moon and make bigger trips than they could walking. These are the locations where the Apollo missions landed; they're all visible with the naked eye on the Moon:

(Image credit: NASA)

By the way, if you're in the southern hemisphere you're usually seeing the Moon upside-down compared to this picture.

Well, I guess that's enough about the Moon for now. I hope you found it interesting.

What happens when the Sun runs out of fuel?

Last week I talked about how the Sun works, and how it formed in the first place. If you haven't read that post, you might want to read it now after all. Because this one more or less follows it, and I'll assume you've read it so I don't annoy people who did by explaining things twice.

I talked about how the Sun gets its energy from fusing hydrogen into helium, preventing gravity from collapsing it further with the energy produced in the core. Yet I didn't touch an important question: what happens when the hydrogen runs out?

Well, the good news is that that won't happen for a long time. The Sun has been fusing hydrogen for five billion years, and is only halfway through its supply. The Sun still has five billion more years left before its hydrogen runs out. An amount of time like that can't be imagined at all, really. In that time, the entirety of life on Earth happened; all of human civilisation could be repeated 500000 times in it. Think of the oldest person you know; perhaps a grandparent, or a parent, or just someone else. Imagine how many seconds their life has lasted so far. Well, take double that amount, but take years instead of seconds, and you've got five billion years. And that's the amount of time we'll have to go into the future to see what happens when the Sun runs out of fuel.

I can't even begin to speculate what Earth looks like this far into the future, but let's assume it's still there at least. If humans still have any descendants now, they probably look nothing like us any more. The first thing you should realise is that the Sun running out of fuel doesn't mean the Sun now consists of 100% helium. In fact, most of the Sun still consists of 75% hydrogen and 24% helium. However, all this hydrogen does the Sun no good: the pressure and temperature simply aren't large enough to overcome the powerful electric repulsion the atoms have for each other in most of the Sun. In the only place where it's so hot and so compressed hydrogen atoms can collide - the core - there is only helium left by now. You'll recall a hydrogen nucleus consists of one positive particle named a proton, while a helium nucleus consists of two protons and two non-charged particles named neutrons, all held together by a powerful force named the strong nuclear force. Helium's two protons repel other helium more strongly than hydrogen's single proton repels other hydrogen, and this means the helium nuclei don't collide yet.

So what happens? Gravity, for the first time in ten billion years, can compress the Sun's core further, so it begins to shrink. The pressure increases, the helium is pressed even more closely together, and as a result the core's temperature rises. The core gets smaller and smaller and hotter and hotter, and this increased heat warms up the layer around the core. While this layer was of course always millions of degrees hot, it never was quite hot enough for nuclear fusion, so it still consists mostly of hydrogen. But as the temperature here increases, hydrogen starts to fuse in this layer around the core. Crisis averted, you may think. Unfortunately, it's not averted.

You see, this shell of fusing hydrogen now holds up the outer layers of the Sun just like the core did, but with an important difference: the non-fusing core still lies under it, and it's still contracting. It's not just getting smaller from the pressure of its own gravity, but also from the energy produced all around it. So the core keeps shrinking and heating up, and in doing so it also continues to heat up the shell where fusion now takes place. When the Sun was fusing hydrogen in its core, it did so at about fifteen million degrees, but now in the shell around the core, it could easily be fifty million degrees. At this higher temperature, the hydrogen atoms bounce around even more rapidly and powerfully, and therefore they collide more often. Much more often. The shell fusion burns through its supply of hydrogen way faster than the core did, and creates far more energy. A thousand times more energy, possibly even ten thousand times more! The Sun used to be a 385 Yottawatt lightbulb, but now it's becoming a 3850000 Yottawatt one.

This enormous increase in energy causes the outer layers of the Sun to be pushed away. The fusion energy pushing from inside always used to balance out with gravity, but now it's getting much stronger, making gravity lose temporarily. The Sun's outer layers expand, and so the Sun grows larger. And as it grows, the energy produced inside, while much greater than it used to be, is spread out over a far bigger surface than it used to, and this means every square kilometre of surface gets less energy than it used to and cools down. As the Sun grows several times as large as it used to be, its nearly white light becomes more pronouncedly yellow. As it keeps expanding further and further, the Sun cools down further, slowly going orange.

At about this point, Mercury meets its fiery maker as the Sun's surface expands beyond its orbit. The rocky little world melts as the Sun's surface gets closer, and finally gets engulfed by it. Yet it survives as a single piece of molten rock, still orbiting inside the Sun, for a surprisingly long time: as the Sun expanded, its mass didn't increase, so its outer layers have become incredibly sparse: it's the same amount of Sun, just spread out over a far larger space. The Sun's sparse outer layers still erode Mercury and slow down its orbit so it falls deeper into the Sun, making it reach warmer and denser layers where it will truly be destroyed, but this will take a long time.

And the Sun keeps expanding: Venus soon shares Mercury's fate as it too gets engulfed by the ever-huger star, which is now a fiery red. So what about Earth? Well, eventually the Sun's expansion stops, and this will be a bit outside Earth's current orbit. So that seems to be the end of the world, but there's a 'but': as the Sun's outer layers grow, they become very tenuous and get very far away from the centre of its gravity. Combined with their heat, the Sun is leaking lots of gas now, and getting lighter in the process. The Sun getting lighter causes its planets to move into higher orbits, and this might just be enough to save Earth. As knowledge stands now, there seems to be about a fifty-fifty chance of Earth following Mercury and Venus in or it orbiting just above the Sun's surface. I'll assume it survives for the rest of this post.

Not that that will actually save it: the Sun increased in brightness by at least a thousand times, so while the Sun's surface is only half as hot as it is now, its proximity and size still roast the Earth. The oceans boiled away, the atmosphere heated up so much the planet's gravity couldn't hold it down any more, and eventually the surface itself, as well as the Moon's, melts. Seen from this molten world, the Sun would fill almost the entire sky; a gigantic red ball of fire. Its surface wouldn't be nearly as bright as it is now, though; you could probably look straight into it while squinting, and even see darker spots on it with the naked eye. However, its enormously increased size in the sky still makes it far brighter than it ever was, even if you can now look into it.

Even aside from its colour and dimmer surface, the Sun wouldn't look much like it used to. The outer layers now contain gigantic convection cells which take material from as deep as the hydrogen-fusing shell and take it all the way to the surface, while gas at the surface gets submerged and taken into the deep. These gigantic convection cells and the Sun's gravity's tenuous hold on its distant surface make the Sun bubble and bulge like a boiling pot of water, even distorting its shape: the Sun no longer is a sphere, but an odd, more-or-less-round bulgy thing. The Sun is now a red giant.

(Image made using SpaceEngine)

The core, meanwhile, has reached a pressure so incredibly high it simply can't get any smaller. The helium is squeezed so incredibly tightly together it really can't get any closer without “breaking” the particles. In stars several times heavier than the Sun, this will actually happen and cause extremely strange things to happen. But the Sun's mass isn't big enough to pack the helium any tighter. You might think the helium will begin fusing too at some point if it's this close together. And you're right: it does. Two helium nuclei collide on occasion, and form a nucleus with four protons and four neutrons named Beryllium-8. There's just one problem: Beryllium-8 is a very unstable nucleus. The strong nuclear force just doesn't seem to have a good grip on it, and within a fraction of a second, it falls apart into two helium nuclei again. So the helium-fusion isn't going anywhere.

In the shell around the core, hydrogen fusion happens at an incredible rate. The Sun's days of slow and stable fusion are over; the hydrogen in the shell around the core gets squandered a thousand times faster than the core hydrogen was. Within only a few million years, the hydrogen here is gone too, and gravity once again has free play. The entire Sun's mass once again rests on the core, which is already as small as it could be, and now surrounded by a layer of new helium. This causes the core to heat up even further.

When the core reaches a hundred million degrees, something happens. The helium, which has occasionally been fusing with other helium to form beryllium-8, reaches a point where fusion is so common that it becomes possible for the beryllium-8 formed in the fusion of two helium nuclei to be hit by another helium nucleus before it decays. The third helium nucleus adds to the nucleus so that it has six protons and six neutrons; it's become carbon. And carbon is quite stable: you should know, as you mainly consist of the stuff. Sometimes, the carbon gets hit by a fourth helium nucleus, fusing to form oxygen. The fusion of three helium nuclei to carbon – or four to oxygen - creates a great deal of energy, and in the strange conditions that now rule in the Sun's core, this energy immediately sets off more helium fusion, which causes more helium fusion in a chain reaction that makes the Sun burn through a fifth of its helium in a single moment called the helium-flash.

The helium-flash is incredibly energetic, and makes the core expand, yet the Sun is so huge and distended by now it's barely noticeable by the time the flash reaches the surface. But after the helium-flash, the Sun's core continues fusing helium in the core at a slower pace. With fusion once again taking place in the core, the Sun's energy production lowers, and gravity contracts the red giant it has become. It looks like the Sun might be returning to its old days: it shrinks, becomes hotter, and since gravity has a stronger hold on the smaller surface, the convection cells stop making the Sun look like a bulgy bubbling mess. It once again becomes smooth and round and yellowish. The Sun doesn't shrink down all the way to its old size, but for a while, it has entered a second youth.

But this second youth doesn't last as long as the first. Not only does the Sun still burn much brighter than it used to, squandering its resources rapidly, but helium fusion also produces far less energy than hydrogen fusion, and therefore happens quicker to produce the same energy. The Sun's second youth lasts about fifty million years before trouble arises once again as all the helium in the core has fused to carbon and oxygen. The core contracts to its absolute limit once again, this time causing helium to fuse in the shell around it. But the helium-fusion in the shell around the core produces so much heat that in another shell around the first shell, hydrogen is also fusing to helium now. The Sun's inside is a bit like an onion now, with all these layers, and its heat quickly makes it grow to a red giant again. But this time, its growth doesn't make it reach a stable endsize: the Sun keeps growing and shrinking alternately. That's because the helium fusion is very sensitive to temperature, and in the shell where helium fuses to carbon and oxygen, the temperature varies. This causes the Sun's energy output to fluctuate wildly, and with it, the Sun's outer layers contract and expand rapidly.

Every time the Sun expands, it loses a lot of gas. The outer layers are just too far away from the core; there is very little gravity working on them at this distance. So the hot gas escapes from the Sun's gravity, expanding and cooling down like a smoke ring. Every few weeks or months, the Sun expands and contracts again, and every time it blows a bit of its own outer layers away. A heavier star will eventually begin to fuse carbon and oxygen to neon, magnesium, sulphur, and silicon; and then fuse silicon and sulphur to iron and create all the other light elements in the periodic table before exploding in an explosion brighter than an entire galaxy, in which heavier elements like gold and uranium are also formed. But our Sun isn't heavy enough to reach the 600 million degrees needed to fuse carbon or oxygen, and slowly blows its outer layers away instead, creating a beautiful nebula around our Solar System.

(Image credit: NASA and ESA)

As the Sun loses its mass, the fusion in the core slows down, with gravity pulling less hard. The core cools down as the pressure decreases. Over the course of millions of years, fusion eventually stops entirely, as all that's left of the Sun is the core: an incredibly dense thing the size of the Earth consisting mainly of carbon and oxygen. It glows a fierce white-bluish from its heat, but it's so tiny that the Earth now cools down deep below freezing, its Sun becoming a single bright point of white light in the sky. The Sun has become a white dwarf, and there's nothing left for it to do but to slowly cool down over billions of years. The white dwarf slowly becomes cooler and fainter, its white light eventually fading to yellow. Then orange, and then it only shines a very faint red light. Eventually that last light dims too, and all that's left is a cold, dark dense object called a black dwarf. The Sun is dead.

But the universe is still young. Stars continue to be formed, and the nebula that was once the Sun's outer layers - a very sparse cloud of mainly hydrogen and helium, with a bit of the carbon and oxygen fused by the Sun - mixes with other similar clouds, and becomes part of new suns, and their solar systems. The silicon and iron Earth consists of, the carbon in our bodies, the nitrogen and oxygen in our air, were all once created inside a star. Only hydrogen and helium were formed in the Big Bang; all the heavier elements come from stars. Carl Sagan used to say: “We're made of star-stuff.” And the Sun's atoms too will become part of worlds of star-stuff in the far future.

How does the Sun work?

Where I live it was was a cloudy, overcast day today, with a little rain. Since it's November and I live at pretty high northern latitude, this meant it was quite a dark, gloomy, grey day. Yet something you almost never realise, is that even on a day like this, there is a huge amount of light about. If it's evening or night when you read this, you may think you're sitting in a pretty well-lit room.

Well, you're wrong: unless you've got some kind of light fetish and stocked your room with big industrial lights, it's likely your room is about a thousand times darker than the outside is on a cloudy day. Your eyes and brain do an amazing job of compensating for it; if they didn't, the lit room would look almost completely dark; or being outside at day would be like constantly staring into the Sun.

Of course, that's where all that light is coming from in the first place: the Sun. Compared to the 60 Watt lightbulb that might very well provide all the light in your room (as one does in my room), it's a huge light, to illuminate things a thousand times brighter. But what's more than the light level is this: it beats that light bulb by a thousand times while being a hundred and fifty million kilometres away. An amount of distance like that is completely unimaginable; the largest thing a human mind can actually picture is probably our own Earth, yet the distance to the Sun is about twelve thousand times its size. If you imagine everything a thousand times smaller, a human is under two millimetres tall and the Earth is the size of a city; twelve kilometres. A big globe, but quite imaginable. Yet the Sun is still 150000 km away like that; almost half as far as the Moon really is.

So the Sun is a ridiculous distance away, and even at that distance it easily outperforms a 60 Watt lightbulb. So just how bright a lightbulb is the Sun? As it turns out, the Sun shines with a ribonkulous 385 Yottawatts! Yotta- is one of the metric prefixes, like kilo- and milli-. In fact, it's the very largest of the set. Like a kilometre is a thousand metres, a Yottawatt is a septillion (a one with 24 zeroes) watts. Now that's one gigantic light bulb.

Obviously, one major difference between the Sun and a light bulb is that the Sun isn't connected to the electricity net, and it's a good thing too, as its bills would run very high. So it makes its own energy, but how does it do that? How does the Sun work?

To answer that, we need to go back in time five billion years, to the Sun's birth. At this point, there was no Sun yet, just a very large cloud of very sparse gas. The cloud, like all really large things in the universe, consisted of 75% hydrogen, 24% helium, and about one percent heavier atoms: mainly carbon, oxygen, sillicon, and iron. As I said, the cloud was amazingly sparse; it was far less dense than any vacuum we can make on Earth, and in fact less dense than Earth's atmosphere at 400 kilometres height, where the International Space Station can orbit without any trouble.

Yet this cloud was also very heavy, as it was big. It was heavier than the Sun is, in fact, and the Sun's weight makes even its brightness look small. At some point, the cloud began to get smaller. This probably happened at first because of a bright star passing by. You see, when light shines on something, it pushes that thing with a tiny bit of pressure, like the wind. It's a very weak force, and completely unnoticable to a human. But the cloud wasn't going anywhere, and the star probably took about a million years to pass, so the tiny bit of force built up over aeons of time and compressed the cloud a bit. It was still way sparser than any vacuum humans can make, but it was now getting compact enough that its gravity began to matter.

Again, however, the forces we're talking about were miniscule, and it probably took an entire human life's length for the cloud's atoms to “fall” a single metre closer to the centre of the cloud. But only the first metre. Because the next metre would've taken significantly shorter. Gravity doesn't drop things fixed distances: it continually increases their falling speed. Otherwise, a fall from a kilometre would be no more deadly than a fall from a metre. So the first metre takes ages, the second shorter, the third even shorter, etcetera. If you can afford to wait a few million years, you'd see the entire cloud get smaller and smaller and smaller.

And the speed with which the entire cloud contracted increased further: as it got smaller, everything got closer to the centre, and therefore gravity's pull got stronger and sped the atoms of the cloud up even more. As the cloud got smaller, it began to rotate. This may seem odd, but you can test it out yourself if you have a chair capable of turning on its axis: just give it a spin while you sit on it with your arms wide, then retract your arms: you'll instantly start spinning faster. The same happens to the cloud: as it got thousands of times smaller, it rotated faster and faster. As this happened, the outer parts of the cloud started going so fast they stopped falling: they were now in orbit of the centre of the cloud, and will stay that way. Eventually eight planets, a couple hundred moons, and millions of other objects will form from this stuff, but lets look to the centre of the cloud.

The gas that managed to get in orbit is only a tiny fraction of the cloud: more than 99% of the total mass is still contracting, getting ever smaller. In fact, in the centre of this huge bulge of hydrogen and helium, pressure is getting higher, and would easily crush a human being at this point. The atoms in the core are getting pressed closer together continually, and often get to near collisions with each other, only their mutual electric repulsion keeping them apart. These near collisions give them energy and make them move rapidly and erratically, bouncing around like vigintillions of tiny bouncing balls. And it just so happens a high temperature is nothing more than atoms bouncing around: when it's extremely cold, atoms are sluggish and do little, and when it's hot they bounce around. So, the temperature rises. It rises a lot, in fact, and soon it gets so warm in the core of the proto-Sun it glows red.

Ever since it was just a cloud, the proto-Sun has done nothing but get smaller all the time, but now that is reaching its end: the heat's energy pushes back the atoms further outside and slows down the proto-Sun's collapse. It gets smaller more slowly, but it doesn't end yet: because gravity never wears off and always keeps pulling, but the heat leaks away out of the surface of the proto-Sun in the form of lots of infrared radiation and some red light and so pushes less. The proto-Sun loses heat from this, and so gravity wins bit by little bit. The proto-Sun gets smaller and denser, although more slowly than before, and its temperature keeps increasing. The red light it sheds gets brighter and brighter, and slowly becomes very bright and orange, then yellow.

At this point, the core is a raging inferno. The temperature is about a million degrees, and the pressure is so high no simile will suffice to describe it. Since temperature is the speed atoms bounce around with, you can imagine the atoms in the core are now going completely bazonkers. They're pressed tightly together, yet bouncing around like they're each attached to rockets. But despite all this bouncing in close quarters, they never actually touch. They get close, sure, but the electric force pushing them away from each other is still large enough to keep them apart.

Until this point, when the core is about a million degrees hot. As you recall, the proto-Sun consists almost entirely from hydrogen and helium. The nuclei of these atoms are the two smallest and lightest nuclei in the universe. Every nucleus consists of a certain number of protons, positive particles, and neutrons, non-charged particles. These are bound together by an odd force called the strong nuclear force. It works only over very small distances, but is strong enough to overpower the electric force that pushes the protons apart. But only over small distances: over distances larger than the ones inside the nucleus, the electric force is the only one that matters. The nucleus of hydrogen is very simple: it consists of a single proton, while a helium atom's nucleus consists of four particles: two protons and two neutrons.

At some point, hydrogen atoms begin to collide. As soon as they do, they get within range of the strong nuclear force. It instantly binds the two protons tightly together. Meanwhile, another force called the weak nuclear force changes one of the protons into a neutron and a negative particle called an electron that is shot away (don't ask me how that works). So now, we have a proton and a neutron sticking together in a nucleus called deuterium.

What happens next is that the deuterium hits another hydrogen nucleus. The strong nuclear force, which is amazingly strong as its name indicates, again says “Gotcha!” as soon as the proton that makes up the hydrogen nucleus gets in its range, but this time the weak nuclear force doesn't do anything. So we have a nucleus with two protons and one neutron, which is known as Helium-3.

While my narration may make it sound like this is an isolated event, it's happening all over the place. Lots of helium-3 is getting formed. So two helium-3 nuclei can collide with each other. You'd think this would result in a nucleus with six particles, but in the collision two protons actually get blasted away. The remaining two protons and two neutrons form a single nucleus, however. If this sounds familiar, that's because it's a helium nucleus!

So since those two Helium-3 nuclei were each originally formed from three hydrogen nuclei, what has essentially happened is that six hydrogen nuclei became one helium nucleus and two hydrogen nuclei. But we forget something: during each of these nuclear reactions, energy was released too. A humongous amount of energy, in fact. Well, a humongous amount compared to the size of the atoms. But there are a lot of atoms in the proto-Sun's core, and therefore a lot of this nuclear fusion happens. It starts slowly, but once it gets going, the core heats up to fifteen million degrees, and emits so much energy gravity can't make it any smaller any more. And with this, the Sun is born. The nuclear fusion's energy can go on for as long as the Sun has hydrogen to fuse, which is about ten billion years. During this time, gravity is powerless to make the Sun collapse any further.

However, it's not this same energy that the Sun eventually emits as light and heat. The energy from the Sun's core gets about halfway to the surface before it stops at the underside of a layer called the convective zone. The convective zone constantly has currents flowing through it that very slowly take extremely hot gas from its lower parts and take that to the cooler upper part, and they also take cooler gas from its upper parts and submerge it until it finally reaches the blazing underside of the layer. It takes about ten thousand years for the superhot gas from the bottom of the convective zone to reach the upper part.

When it finally gets there, it warms up the outer layer of the Sun, called the photosphere, to about 6000 degrees. This is warm enough for it to glow a bright, nearly white, yellow. And this glow due to the heat is that 385 Yottawatts of energy we saw earlier. It's enough to heat up a planet a hundred and fifty million kilometres further away, and even enough to cause blindness if you stare into it long enough from that distance.

Of course, there's far more to tell about the Sun, but I really think I've gone on long enough for now. I hope you enjoyed reading about our 385 Yottawatt lightbulb.

Alpha Centauri has a planet!

Exoplanetologists have found a planet orbiting Alpha Centauri B! This is a pretty huge discovery, since Alpha Centauri is the closest star system to our own, and even if this planet is far too warm to harbour life, this discovery shows there might well be other, more friendly planets in the same system.

Alpha Centauri consists of three stars orbiting each other: there's Alpha Centauri A, a yellow dwarf like our Sun, Alpha Centauri B, an orange dwarf about half as bright as our Sun, and Proxima Centauri, a tiny red dwarf that emits very little light. Alpha Centauri A and B orbit each other once every 71 years at about 3 billion kilometres, approximately 20 times the radius of Earth's orbit, or about the radius of Uranus' orbit. Proxima Cenaturi orbits very distantly from the other two, at 2 trillion kilometres, orbiting once in many thousands of years. The newly discovered planet, Alpha Centauri Bb, orbits very closely around Alpha Centauri B, only 6 million kilometres from it. It's nearly ten times closer to its sun than Mercury, so its temperatures are so high there's a good chance the planet's surface is molten.

Alpha Centauri Bb is only a little bit heavier than Earth, and probably about the same size too. If you were there, the skies would look much like they do from Earth, as most stars are far more distant. However, the constellation Cassiopeia (the W shaped one) would have a sixth bright star added to it, continuing the constellation's zigzag pattern. This star would of course be our Sun. Alpha Centauri B would look like a gigantic ball of intense light, 300 times brighter than the Sun from Earth, and its light would burn you to a crisp within a second. Alpha Centauri A would be a single point of very bright light that would complete a circle around the sky every 71 years, like a very bright planet. It would be 100 times dimmer than our Sun, yet still 4000 times brighter than our full Moon. Its light would be similarly bright to a street lantern. Proxima Centauri, the final sun of the system, would barely be visible at all; it's so dim and so distant you would only be able to see it on dark nights, and then it would look like just another dim star.

Curiosity and the exploration of Mars

As you've probably heard by now, the Curiosity rover landed on Mars yesterday morning (or the night of the day before yesterday, if you live in the Americas). This is the next big step in the exploration of Mars that has been going on for millennia. “Millennia?” You may ask. Yes, millennia, because the exploration of Mars didn't simply start with the first spacecraft sent there, but much before that. With its discovery, to be precise.

It's unknown who first discovered Mars, but what is known is that he or she lived thousands of years ago. The ancient Egyptians, Babylonians, and Chinese all knew of the existence of Mars and the other four planets that can be seen with the naked eye a thousand years before Rome was founded. You see, as they observed the stars, they realised all stars moved a little each night, rising and setting four minutes earlier than the night before. But always by that amount; it was as if the sky was a giant sphere with light on it that rotated around the Earth very slowly. But seven things in the sky didn't obey the rules: there were the Sun and the Moon, but aside from them there were five bright stars that moved a little among the stars each night. The Romans called them Mercury, Venus, Mars, Jupiter, and Saturn, after several important gods.

It was thought that these five, the Sun, and the Moon, all moved in circles around the Earth below the stars. The Earth was in the centre of the universe, then you had the Moon, then Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and beyond Saturn were the stars. There were some oddities that were difficult to explain like this, however. Mercury and Venus always stayed close to the Sun, for example, never reaching the opposite side of the sky. Mars, Jupiter, and Saturn did reach the opposite side of the sky, where they would be said to be in opposition, but once there they did a very strange loop in the sky over the course of a few weeks.

Mars was the most troublesome of the planets for this geocentric model: not only was the loop it made at opposition biggest of all three planets that did one, but it did many other things that perplexed ancient astronomers: it would stray off the path the other planets followed across the sky a little, and it would vary in brightness. One year, it would be brighter than anything in the sky except for the Sun, the Moon, and Venus. The next, it wouldn't be all that impressive, and all the other planets would get brighter, as would a couple of stars. It would also vary in brightness a lot during a year: all planets were brighter at opposition than when they were close to the Sun, but Mars showed much bigger variation than any of the others.

The big loop at opposition and the difference in brightness over the year were explained in 1543 when Nicolaus Copernicus published a book in which he claimed the Earth wasn't at the centre of the universe, but the Sun. The Earth was the third planet moving in a circle around it, and Mars the fourth. This would bring them very close together at opposition, much closer than Earth and Jupiter, for example, so by comparison they would be much further away from each other when Mars was on the other side of the Sun. The loops Mars, Jupiter, and Saturn made in the sky were a perspective effect caused by Earth passing those planets, and Mars made the biggest because it was closest.

However, Mars' deviation from the path of the other planets and its differing brightness across the years wasn't explained until 1615, when Johannes Kepler realised the planets didn't move in circles around the Sun, but in ellipses. Sometimes they got closer to the Sun, and sometimes further away. Most planets have an orbit very similar to a circle, but Mars' orbit is more elliptic than all others (except Mercury), getting about ten percent closer to the Sun at its nearest (which is called perihelion) than at its farthest (which is called aphelion). The difference in brightness was because in some years, Mars reached opposition while at its closest the Sun (and therefore the Earth), while in others it was at its furthest from the Sun.

During the 17th century, as telescopes were being pointed at the sky for the first time, Mars was found to look bigger at times when it was brighter and smaller when it was dimmer. This made perfect sense, of course, since the planet was sometimes closer to the Earth than at other times. The first details on Mars were being seen in this century and the next as well: white polar caps and dark spots on its reddish surface. It was found to rotate in 24 hours and 39 minutes, just a bit slower than Earth, and it had an atmosphere. Mars seemed a world that was a lot like Earth: the dark spots could be oceans, or perhaps forests.

In the 19th century, this view was confirmed even more as Giovanni Schiaparelli saw long, straight lines on Mars. Some thought these were canals built by the Martians to irrigate the dry areas of the planet. At the time, canals were a huge new product of technology, the Suez canal having just been completed while work had begun on the Panama canal, so canals were a big part of public consciousness. There were even ideas to help the Martians see us as we saw them, by bigging trenches thousands of kilometres long in the Sahara, filling them with oil, and lighting them on fire! A Mars-craze erupted, as it was considered very likely there was life on Mars by now. H. G. Wells wrote a great book, The War of the Worlds, about a Martian invasion of Earth, which was among the first science-fiction books.

However, around 1900 telescopes became better, and it was getting clear the dark spots on Mars were no seas, as they showed mountains and canyons in them. The canals turned out to be an optical illusion caused by smaller spots and craters lying roughly in lines. The concept of Martians remained a big part of culture and speculation for much of the 20th century, but for a few decades Venus was considered a more likely planet to harbour life by science. When the first interplanetary probes were launched in the 1960s, they were therefore going to Venus, but when they discovered that planet to be far more hellish than expected, Mars once again gained the spotlight. It was highly unlikely by now that little green men lived there, but there could be plant life or microbes on the red planet.

The Russians were the first to send probes to Mars. The first six, from 1960 to 1964, all failed; four of them before even reaching Earth orbit, and the other two while on their way to Mars. The Americans built two probes to Mars; Mariner 3 and 4, but Mariner 3 failed as well. It was almost getting suspicious, seven probes all failing before they got to Mars. Maybe the Martians existed after all and were protecting their secrets? Fortunately Mariner 4 launched perfectly, three weeks after Mariner 3's failure. It became the first successful probe to fly by Mars, and for the first time showed photographs taken of another planet from close by, showing a rough landscape full of craters and mountains. It also showed Mars to be harsher than expected: the atmosphere was very thin, providing only 1/150th of Earth's pressure. This also means the planet's heat easily escapes into space at night, bringing temperatures down below a hundred degrees below zero, though it also warms back up quickly when in daylight, sometimes reaching temperatures similar to a summer's day on Earth: 27 degrees. For the most part, its daytime temperatures stay quite a bit under freezing, though.

It made multicellular Martian life unlikely, as very few Earth organisms could survive long in conditions like that. But a subsequent set of probes launched in 1969, and managed to get into orbit of Mars, observing it for a while. They found structures that looked like dried-up riverbeds, and discovered the tallest mountain in the entire Solar System on Mars. It's called Olympus Mons, and is nearly 22 km high. With the discovery that water may have once flowed on Mars, it was clear the only way to find out more about the planet was to land on it.

In 1975, this was finally accomplished by the Viking 1 and Viking 2 landers, which each landed on the surface after making orbital photographs for a while. They each carried three experiments intended to detect microbial life if it existed on Mars. Two of them turned out negative, but a third was positive. The most common explanation of this is that life wasn't detected, but the third experiment set off by other factors, however, this is far from certain. For the most part, the Viking experiments proved inconclusive. Yet attention turned away from Mars, and this coupled with big budget cuts to both the American and Russian space programs meant Mars was mostly ignored for twenty years.

By the 1990s, it became possible not just to land on Mars, but to ride around there, so experiments could be done on many more places than just the single location the Vikings had been able to. The first such rover, Sojourner, was a small, cheap thing with few instruments on board, mainly intended to prove the rover concept was doable. In the next few years, three probes were sent to Mars: the American Mars Global Surveyor and Mars Odyssey, and the first European mission to Mars, called Mars Express. These three probes researched and photographed Mars in greater detail than ever before, and found many traces of water on Mars: dried-up riverbeds and lakes, gullies, and even signs of ancient rainfall. Global Surveyor failed in 2006, but the other two are still in orbit and returning data every day.

Along with Mars Express, the European Space Agency had tried to put a rover named Beagle 2 on Mars, intended to find signs of life, but contact was lost with the rover during landing, and it has presumably crashed or otherwise failed. The Beagle 2 was intended to land in a rather strange way: by surrounding it with inflated balloons and simply letting it bounce on Mars' surface until it would finally stop, at which point the ballons would be deflated and the rover released. The next year, however, NASA landed two rovers, Spirit and Opportunity, on Mars using the same method. These two mainly researched the geological history of Mars, including the period, long ago, in which there was liquid water there. Opportunity has done so for eight years by now, but Spirit became stuck in soft soil in 2009 and was eventually terminated the next year after many unsuccessful attempts to get it loose.

And now, finally, we get to Curiosity. Like always happens in science, the answers Spirit and Opportunity provided led to even more questions. And so the largest and most advanced rover yet was launched. Curiosity was lowered from a floating platform by a crane yesterday (no, I'm not making this up, that's really how it was landed. The balloon method was simply too dangerous to use with the car-sized Curiosity), and returned its first pictures not much later. It's intended to work for at least two Earth years (one Martian year), but will most likely continue to work for several years afterwards. One of the things it's going to start looking for is the building blocks of life. Its data will also serve to prepare a future manned Mars landing. Curiosity landed in the Gale crater, where there is a mountain called Aeolis Mons, which it'll be slowly climbing over the course of its mission, while examining many of the things it will come across.

What happens next? Well, it partially depends on Curiosity's findings. It is likely Curiosity will be followed up with an even more advanced rover around 2020, which might finally settle the question of life on Mars. And by the 2030s, a manned mission will land on Mars, most likely after earlier manned missions to an asteroid and to Mars' moon Phobos. Several countries have plans for Mars missions: America and Russia, of course, but Europe, Japan, China and India as well. The most likely way for a manned mission to Mars to happen, however, is as an international mission. The International Space Station has proved big, expensive space missions are best performed through international cooperation, sharing both the expenses and the knowledge and expertise required. A manned mission to Mars will likely require such international cooperation to succeed. Once on Mars, there is the possibility they would stay for several months and conduct research rovers can't do; but there is also a different idea floating around: it could be a one-way trip. The astronauts could be sent with everything required to set up the first Mars base and stay on Mars, greatly reducing both the cost and difficulty of the mission, while immediately beginning the colonisation of Mars.

Well, that turned out a little longer than expected. I hope you enjoyed reading it, though, or otherwise stopped reading it when you realised you didn't enjoy it.

How much matter does the universe contain?

A single look at the night sky will reveal there's far more nothing than something out there. The tiny specks of light are stars, made of something, and the vast oceans of blackness surrounding them are nothing. The universe contains gigantic amounts of void, but how much, I wonder. To visualise this, I decided to picture all the matter in the universe clumped together. Not super-tightly, like in a neutron star, but still put together closely so that the density is 1 kilogram per litre: the density of water. The main reason I chose this density is, of course, that it makes it very easy to calculate. So if we pack all matter in the universe together like this, is the result larger or smaller than a single galaxy?


Before I answer this question, I should point out I will give large numbers first in the scientific notation, then give their name according to the long scale. I'm using the long scale because it's more sensible and because it's the scale I grew up with using. But for people who are more keen on the short scale (the one generally used in English-speaking countries) I will put the short scale name of large numbers in parentheses. A second thing I should point out is that I'll be using all the matter in the observable universe, not the entire universe.

First of all, we have to find out the total mass of matter in the universe. Wikipedia gives several estimates of the mass of the universe. I will be using the calculated mass based on the critical density, 1.53×10⁵³ kg, as it seems to be the best corresponding to our observations of the universe. Basing the mass of the universe on stars alone is silly due to the presence of dark matter, and assuming a steady state-universe requires the pretty big assumption that our universe is a steady-state universe.

So, since we assumed the density of water for our clump of all matter, that means it's 1.53×10⁵³ litres in volume. 153 octilliard (153 septendecillion) litres is unimaginably huge, so we'll have to use bigger units to make this more imaginable. Let's start with converting to cubic metres. Thanks to space being three-dimensional, a cubic metre contains not ten, but a thousand litres. So that means this clump of matter is  1.53×10⁵⁰, 153 octillion (153 sexdecillion) cubic metres in volume.

We can get closer to imaginable numbers by converting to cubic kilometres. Similarly to how a cubic metre contains a thousand litres, a cubic kilometre contains a milliard (billion) cubic metres. That means our clump of all matter is 1.53×10⁴¹, 153 sextilliard (153 duodecillion) cubic kilometres.

To get closer to home, we'll convert to cubic megametres. The megametre is a pretty rarely used unit, but it's equal to a thousand kilometres. The clump of matter has a volume of 1.53×10³², 153 quintillion (153 nonillion) cubic megametres.

To take another step, I'll now convert to cubic gigametres. A gigametre is a million kilometres, a bit more than a journey to the moon and back. We divide by a milliard (billion) and get 1.53×10²³,153 trilliard (153 sextillion) cubic gigametres.

We're still not using imaginable numbers, so we convert to cubic terametres. A terametre is a milliard (billion) kilometres, 1.5 times the distance from here to Jupiter. The clump of matter is 1.53×10¹⁴,153 billion (153 trillion) terametres. Hey, that's approaching numbers we use (on occasion) in our daily lives.

But let's take another step and convert to petametres, one billion (trillion) kilometres. I would like to give an example of how big this is, but there are few meaningful distances close to a petametre. It's 220 times the distance from here to Neptune, or a fiftieth of the distance from here to the nearest star (aside from the Sun). All the matter in the universe taken together is, if my calculations are right, 1.53×10⁵, 153000 cubic petametres.

While a hundred-and-fifty-thousand is quite a low number, we can use cubic lightyears instead. A lightyear is approximately ten petametres. Actually, it's 9,5 petametres, but considering how rough our estimate of the universe's mass was I think we can get away with rounding it to ten, thus putting a thousand cubic petametres in a cubic lightyear. One final calculation, and we get 153 cubic lightyears. A cuboid of five by five by a little over six lightyears. That's all the matter in the universe. Not only would it fit inside a single galaxy, if its centre was at the Sun's position it wouldn't even reach the nearest star. That's all the matter in the universe, everything else is void.

But where is all this void exactly? You might think the gigantic gaps between galaxies are where it is, but these are actually relatively small. The Milky Way is a hundred thousand lightyears across, though only ten thousand lightyeard thick, and the nearest big galaxy is at two million lightyears, only twenty times the Milky Way's own diameter. If we count dwarf galaxies the nearest other galaxy is much closer even. If you imagined the big galaxies as coins, they would be less than a metre apart.

The real distances that are responsible for all this void are the distances between stars. When I tried to give an example of the size of a petametre and failed we already saw there is a huge gap between distances within a solar system and distances between stars. Our Sun is 1.4 million kilometres across, but the nearest other star is more than four lightyears away. If we imagine the Sun as a grape in the centre of Amsterdam, the nearest starsystem consists of two grapes and a grain of pepper located near Brussels. And that's a good comparison for interstellar distances: a piece of fruit in every European capital. Those distances are mainly responsible for the huge amounts of void in the universe.

Time travel guidelines

If you have recently acquired a time machine, then please, read through these guidelines carefully before using it.

The Timeline

If you're going to travel through time, the first thing you should try to find out is the shape of the timeline. This is very important to know as a time traveler, as it will affect how causality works and what happens if you try to change the past.There are three possibilities, the Linear timeline,the Changeable timeline, and the Branching timeline, which I will each explain and what the results of attempting to make a grandfather paradox (killing your grandfather before your father was born, thus preventing you from ever being born to kill him) would be.

In a Linear timeline, past, present, and future are all one line, and when you go back in the past, you will have already gone back there, and since whatever you will do in the past will have already happened, you can never change it. If you attempted to kill your own grandfather in a linear time, you would fail, as you have evidently been born, so your grandfather apparently survived your assassination attempt. Or the man you thought was your grandfather was not actually your real grandfather. Either way, whatever happens was part of the timeline all along, and therefore you can't create a paradox.

In a Changeable timeline, you can actually change the past by going back in time. As a result, whatever you do in the past will have an effect on your present self. This is the only timeline where you can actually cause the grandfather paradox, as killing your grandfather would actually have an effect on you. While this timeline may seem the most intuitive, the fact that paradoxes could be created here seems to indicate it is also the least likely.

In a Branching timeline, the universe splits into two universes whenever someting happens, and one of the possible outcomes becomes a reality in each of the universes. Thus, when you go back in time you will arrive in a new branch of the timeline. If you tried the grandfather paradox here, your grandfather would die in the new timeline, but you would be fine since the grandfather from your own timeline was alive and well. Paradoxes would be impossible in this one too.

If you built a time machine, presumably you discovered the shape of the timeline during this. If you didn't, try this: leave your house for about an hour. When you come back, look into an empty drawer and make sure it is actually empty. Then, go 30 minutes in the past and put a sock in the drawer. Go back to the future (about an hour would suffice so you don't run into yourself), and check the drawer. If the sock isn't there, you're probably in a Linear timeline. You just have to worry about the fact that apperantly someone broke into your house and took the sock while you were away. If the sock is there, you are probably in a Changeable or Branching timeline. Repeat this experiment a few times to be sure. If you keep finding you actually made small changes to the past, you're now certain you are not in a Linear timeline. Finding out whether you are in a Changeable or Branching timeline will be a bit trickier, but there is a way that you can use for it.

Go back in time 3000 years. Depart to the present immediately, to make sure you don't actually change anything. If you're in a Changeable timeline, your effect on the timeline should be negligible, and history will have played out just as you remembered it. However, if you're in a Branching timeline, your arrival in 989 BCE will have created a new alternate timeline where you have arrived in. At every small little happening in history, even the smallest, this universe split again, and you end up in the year 2011 CE in a random one of these universes. Thus, when you get out of your time machine in your new timeline's present, history will probably have happened vastly differently. By the way, this is probably a good moment to mention the fact that it will be impossible to return to our original timeline, though with many careful time trips you may manage to end up in a very similar one.

So now that you know the shape of the timeline, this tells you how careful you'll want to be in the past. If you're in a Linear timeline, you don't have to worry about changing the future, and only have to watch out for your own neck. If you're in a Changeable timeline, be very careful, as a single mistake can vastly change the whole world history, and you may find your own present gone if you are not very careful. Thanks to the butterfly effect, even a really small change could culminate into massive differences over a long period of time. If you are in a Branching timeline, changing the past means going back to a present similar to your own present will be difficult, but by traveling back even farther you can return to a present similar to the one you knew. 

Some things to watch out for

When you arrive, if your time machine does not have some way to tell you the date, try to find out the year. Don't go asking random strangers what year it is, though, as that question is considered weird. If newspapers are printed in this time, just look at the date on them. Or go into a store and ask to buy a calendar. If you're in the past, some careful references to historical events in casual conversation can help you pinpoint the date (And if an event turns out not to have happened yet and you're asked about it, just say it was just something minor in your home country).

Try to blend in as well as possible. When you're time traveling, try to wear something that will look not completely bizarre in most eras. When you arrive, try to switch it for era appropriate clothing as soon as possible. Looking at how most people around you dress can provide you with a good guideline.

Don't tell people you're a time traveler. If you're lucky, they'll think your nuts and ignore you. If you're only slightly lucky, they'll think you're nuts and lock you up. If you're unlucky, they'll believe you, kill you, and steal your time machine.

Don't act smug about your era when you're in another era. First of all, talking about your era strongly borders on telling them you're a time traveler. Also remember that you're a guest in their era. Acting condescendingly about their era is considered rude. If you're in the past, remember that change is gradual, and that to reach the situation of the world in your home era, it had to pass through other eras, and that your own home era may be viewed just as condescendingly in the future. If you're in the future, remember that they went through the present and went on from that. Presumably they have good reasons for changes made since your own home era.

Whether you're in the past, alternate present, or future, always wash your hands with ethanol or another bacteria killing substance. A squirtbottle of ethanol can be found in most labs and classrooms, and they presumably buy them from somewhere, so find out where and buy some to take on your journey. The past had many potent diseases that your lazy 21st century immune system will not be prepared for. In the future new diseases will have evolved that you will not have had yet, so you can easily be infected by them. There's also the danger of spreading your own germs around and causing diseases.

Interactions with Yourself

When you travel through time, meeting yourself should generally be avoided, especially in a Changeable timeline (Who knows how the timeline will change when your past self meets your present self). If you're in a Linear timeline and you can't remember ever having met yourself, then don't try it as you will not succeed and may in fact be run over or something before you reach your past self. Even in a Branching timeline it should be avoided, as meeting yourself is a great way to get into trouble. Meeting your future self can be considered somewhat safer, as both parties will be aware of the situation (you're in it and your future self remembers it unless your in a Branching timeline), and your future self will probably remember the encounter and thus be able to prepare for it, but I still discourage it.

To avoid meeting yourself, try keeping a log of where you were at any given time. This can also come in handy for many other reasons. Just make sure it doesn't fall into the wrong hands (Wouldn't want to make it too easy for Terminators targetting you).

If you HAVE to interact with yourself and can't hide your identity, there are some basic rules:

1: NEVER kill yourself. This one should go without saying, but aparrently it doesn't. Killing yourself is never a good idea, especially not in a Changeable timeline, where you will in fact cause a paradox. 

2: Avoid confusion between you. If you have a middle name, now is finally the time to use it. One of you (If it's the home era of either of you, that should be the guest) uses his/her middle name as a first name. Alternatively, you can try nicknames, numbers, or letters. If your in the home era of either of you, that person always gets first pick. If you both traveled to a year that's not your home era, just work it out like adults.

3: Talking about avoiding confusion: have some sort of password to your own mind, which you can use to prove you are really the same person. Never tell this password to anyone, and never write it down, and if someone knows your password, assume they are either yourself or someone deeply trusted by yourself. This one will be more effective if you think of that password even before you even consider time traveling. Seriously, choose a mind password right now. Your future self may thank you for it.

4: If you're in a Changeable timeline, watch out for paradoxes. During interaction with yourself, these are very easy to cause. In a Branching timeline, try not to get your other self killed and/or in big trouble and/or with a completely ruined life/world. While paradoxes may not occur, that person is still you and you don't want that kind of stuff to happen to you, do you? In a Linear timeline, be careful not to do anything you can't remember while interacting with your past self. If you tried it, evidently you were stopped by something, and that something may actually be very dangerous to you.

Three is a crowd

Don't visit the same moment more than twice. While avoiding one other you is quite doable, avoiding two or more other yous will quickly cause a mess. There's also the problem that the chance increases that people who know you see you at the same time in different locations when there's more you's running around. Watch Primer. They went back in time across the same 4 days multiple times, and made a real mess. That's the kind of thing you want to avoid.

Danger

Speaking of Primer: time travel can be hideously complicated. Try watching Primer and understanding it the first time through. That's the kind of complicated you'll be dealing with. Time travel is an inherently complex activity, and should not be attempted unless your mind can keep track of it all.

Time travel is also an inherently dangerous activity. Even discounting the possibility of flaws in your time machine, danger lurks everywhere. The past was quite often a dangerous time, full of violence and diseases. In the present, you have to always be watchful of running into yourself (or, if that is part of your plan, to prevent trouble if you do, see the section 'Interactions with yourself' for that). If you go into the future, you never know what you might find, and if you're really unlucky you could end up appearing straight in the middle of a nuclear detonation. In a Changeable timeline, even a slight change in the past could forever change the present, and quite possibly not for the better. Any change in the past in a Changeable or Branching timeline could make it very difficult to return to the version of the present you call home. Your time machine could get destroyed or stolen in a different era and you;d be stuck there forever.

P.S.: I just thought of another handy tip: If you don't need to get to a specific date, but just a year, try going at Carnival or a similar holiday featuring people dressing up and lots of alcohol. Even if you completely dick up your attempts to blendin with the time, you won't stand out.