Quick! What's the brightest Star in the sky?

If you said Sirius, then give yourself a blue gold star. But I'm sorry to say: Bzzzzt. You're wrong. Your problem is you know too much astronomy; Sirius is the brightest star in the night sky. The correct answer is, duh, the Sun.

The Sun is a star just like all the others in the sky, more or less, but it just so happens to be substantially closer. The funny thing is, it's so close that it's incredibly bright and we can't even look at it. Well, we can, but it hurts. And the fact that it's there every day (Seattlites: ignore that part) and we can't even glance at it means we take it for granted. It's a vast, mighty, seething cauldron of energy, and even though solar astronomers have studied it for centuries, there's a lot about the Sun that's still not understand. And if they don't get it, then I'm pretty sure that you're unaware of one or two things about it too. I'm fuzzy on one or two (or a thousand) things about it myself.

So here's my list of stuff you may not grok about our nearest star. It's the fourth in my Ten Things You Don't Know series (the others are black holes, the Milky Way, and the Earth). How many didn't you know? I'll be honest: before I wrote my book -- which has a whole chapter about our dangerous star -- I didn't know some of these things about the Sun. But I ain't sayin' which ones.

1) You won't go blind looking at it. Probably.

You've heard this from your mother, your neighbor, and pretty much everyone else: don't look at the Sun or you'll go blind! Well, that's not strictly true. To be clear: no one has ever been permanently and totally blinded by looking at the Sun (despite a recent 30 Rock episode). You can hurt your eyes, but the damage is usually not total, and a lot of it heals (though not always completely).

Usually, damage to the eyes from looking at the Sun happens during a total solar eclipse. The eclipse itself doesn't hurt you -- after all, the point of the eclipse is that the Sun is covered by the Moon! -- but the damage happens in the moments right after the eclipse. While the Sun is blocked, your pupil dilates to let in more light, so when the first sliver of the brilliant Sun reappears your eye is flooded with light. This can cause damage to your retina called solar retinopathy. It's actually not heat damage, but photochemical; the flood of UV light actually alters the chemistry of your cells, damaging them.

In general, the damage is minor and can heal well, though there can be some permanent though relatively minor effects (in other words, you still shouldn't stare at the Sun). Usually the damage is worse in children because their lenses let in more blue light (the lens yellows with age, acting as a natural filter for UV light).

So you won't go permanently and totally blind from looking at the Sun... unless you do it looking through binoculars or a telescope. But then in those cases there are Darwin Awards to consider.

Incidentally, using sunglasses to look at the Sun can actually make things worse, since they block visible light and your pupil dilates to compensate. If you want to observe the Sun -- and I recommend it, because it's fascinating and utterly beautiful -- then read Mr Eclipse's guide to safe solar viewing. It's a site for sore eyes.

2) The Sun is not an average star.

If I had a nickel for every time I have heard someone say or write that the Sun is an average star... well, I'd have a lot of nickels. But just because someone says something a lot doesn't mean it's right.

With a few exceptions, stars come in pretty much one shape (round), but lots of sizes. Also different temperatures, color, energy output, and so on. All of these characteristics are due to one overriding quality: Mass. The mass of a star determines a whole lot about its life: how big it is, what color it is, how much energy it emits, and even how long it will live. Low mass stars (say, less than half the Sun's mass) are cool, red, dim, and live a long time. High mass stars (more than about 10-20 times the Sun's mass) are blue, incredibly luminous, and die after a few million years.

Where does the Sun fit on that scale?

As with most things in nature, the number of objects depends on the size. There are very few high mass stars, more intermediate mass stars, and gazillions of low mass stars. Roughly 10% of all stars by number in the Milky Way Galaxy are like the Sun, which means that very few are more massive. Even being conservative, I'd say that the Sun is more massive than 80% of the stars in the Galaxy. That's hardly average!

Now, if you take the lowest mass star and the highest mass star and average them, then yeah, the Sun looks a bit feeble (it's way down toward the low mass end). But that's unfair, because the sheer number of low mass stars drops that average considerably. The picture you usually see is like the one above on the left: the Sun compared to some ginormous star like Aldebaran, a red giant. But those stars are rare! Red dwarfs outnumber more massive stars 5:1 or better, and brown dwarfs may be even more common. So the picture on the right is more fair.

Pretty much no matter how you slice it, the Sun is a powerhouse, way way bigger, brighter, hotter, and more impressive than most of the stars in the sky.

Which brings up the question, how does it keep this up?

3) The Sun converts matter into energy.

OK, so the Sun is pretty bright. Ever wonder why?

It's because of that whole E = mc² thing. Seriously. The Sun is pretty big; at 1.4 million kilometers across it has a million times the volume of the Earth! That means that at its core, at the very center of the Sun, the pressure and temperature are ridiculously high (340 billion atmospheres and 16 million degrees Celsius (27 million degrees Fahrenheit)); so high that hydrogen can undergo nuclear fusion and turn into helium.

That's how a hydrogen bomb works! When H is turned into He, some of the matter (the m in the equation) is converted into energy (the E). Thing is, that pesky c in the equation is the speed of light, which all on its own is fast enough, but here we're squaring it too, making it a seriously ridiculous number (about 10¹¹ km²/sec² if you're keeping track at home). So a little tiny piece of matter converts into a vastly powerful and scary amount of energy.

We know how much energy the Sun emits (4 x 10²⁶ Joules per second; again, I hope you're keeping notes), and so we can use Einstein's equation to calculate that, to power the Sun, it must convert 5 million tons of matter into energy every frakking second. That's roughly the mass of seven fully loaded oil supertankers, to give you an idea of the kind of scale we're talking here.

Is your mind boggled yet? Then think on this: knowing the efficiency of hydrogen fusion, that means that 700 million tons of hydrogen are converted into 695 million tons of helium every second of every day of the billions-of-years lifespan of the Sun.

Egads.

That sounds like the Sun will run out of hydrogen quickly, but remember, the Sun is big. 5 million tons is just 0.00000000000000000025% of the Sun's mass, so we're safe for a few billion years.

If you want to hear more about this, I talked about it in detail in the second episode of my doomed video series Q & BA:

As a comparison, the total nuclear weapon arsenal on Earth is about 20,000 bombs. Assume each has a yield of one megaton (which is certainly an overestimate). How does the Sun compare to this mighty cache of weapons?

It emits the energy equivalent of 5,000,000 times the total yield of the entire Earth's nuclear weaponry stash. Every second. And that's a conservative estimate. All that, just 150 million kilometers (93 million miles) away.

Sleep tight.

4) It would be invisible to the naked eye 60 light years away.

For all the Sun's sound and fury -- and despite the point I make that it's way above average for a star -- it's incredible how quickly it would fade to invisibility if we were to move away from it. For a normal person the faintest star you can see has a magnitude of about 6, where magnitudes are a logarithmic scale with higher numbers for fainter objects; the full Moon has a mag of about -13, Venus is around -4, the bright star Vega about 0, and Polaris is about 2.

Of course, the farther away an object, the fainter it gets. It turns out that the Sun fades to 6th mag at a distance of roughly 60 light years. Now, that's a long way by human standards -- 600 trillion kilometers, or 360 trillion miles -- but on the galactic scale that's still breathing down our necks; the Milky Way is 100,000 light years or one quintillion kilometers across. That's 1,000,000,000,000,000,000 kilometers. A lot. In the illustration above of the Milky Way, 60 lights years is less than half a pixel. The galaxy is huge.

When you go outside at night and look at the stars, almost all the stars you see are within 100 light years from us, and only a handful of the extreme brightest ones can be seen from farther away. If you were to pluck the Sun from the solar system and plop it down in some random location in our Galaxy, there's a better than 99.99999% chance it would be invisible to the naked eye.

5) The Sun is not yellow.

This one causes a lot of confusion. Let me be clear: the Sun emits light at all different colors; red through violet. The amount it puts out at different colors is different, though. In fact, it emits most strongly in the blue-green part of the spectrum (around 480 nanometers)... but it doesn't look green because our eye (with help from the brain) combines all those different colors.

Technically, the Sun is white. This is easy to show: a piece of paper held up to sunlight appears white, as do snow and clouds. If the Sun were yellow, those would look yellow.

However, a lot of people perceive the Sun as yellow. There has been quite a bit of research into this (and never-ending discussions), but I don't think it has yet been fully understood why that is. Do people compare it to the blue sky? Are they used to seeing it lower in the sky (it hurts to look at it when it's up high) when atmospheric effects make it look yellow/red/orange? Hard to say.

But it's white. So there.

6) It solved a major problem in physics... and then created another one.

The idea that stars use nuclear fusion to generate energy is credited to Hans Bethe, who thought of it in the 1930s. In the 1950s, an incredible paper came out describing the detailed nuclear reactions going on in the cores of stars (I read it as an undergrad astronomy major a long time ago, and it's actually pretty cool).

The physics involved made a prediction: the Sun should be emitting neutrinos, energetic subatomic particles that are a severe pain in the neck to detect. The first solar neutrinos were detected in the 1960s, which was very cool. But there was a problem: only 1/3 of the predicted neutrinos were found.

This was no small thing; the very nature of what we understood about quantum mechanics and particle physics was threatened by this finding! The missing neutrino problem, as it was called, was considered one of the most outstanding mysteries in astronomy at the time. Of course, every crank and crackpot on the planet came along and said they had the solution to the missing neutrino problem, usually told to them by aliens or the ghost of Einstein.

Finally, though, the problem was solved. Through real science. Duh. The detectors astronomers had used to look for neutrinos were only sensitive to one kind of neutrino, the kind the Sun emitted. There are actually three kinds, so if, somehow, neutrinos could change their type as they flew from the Sun to here we'd only expect to see 1/3 the predicted number (2/3 would change to the kind we couldn't detect). If that were true, then statistically seeing 1/3 the total would make sense. But then the Canadian Sudbury Neutrino Observatory came online (pictured above), one that could measure the effects of all three neutrino flavors. When the results were combined and extrapolated, it was seen that the number of neutrinos coming from the Sun was just as predicted. Yay!

However, there remained a problem: the only way neutrinos can change their flavor is if they have mass. Only a tiny amount, but it's still some, and The Standard Model (yes, capitalize it when you use it!) of particle physics said they shouldn't have any mass at all. The Standard Model is, therefore, wrong. Or more properly, incomplete. The SM does pretty well for lots of other particles, but neutrinos are still something of a mystery; they're still difficult to detect and difficult to study. Someday we'll have this more under control, but for now we can be happy that the Sun, at least, is behaving as expected.

7) The Sun can blow out satellites and even cause blackouts on Earth.

The Sun may look steady and peaceful, but every now and then it gets a bee in its bonnet, and gets seriously ticked.

The Sun's magnetic field changes over time, with a 22 year cycle. At the start of the cycle the magnetic field is very weak, and grows in strength. About 5.5 years later (it varies) it reaches its peak, then fades again. It dips to zero, then strengthens again, but this time the polarity is reversed, with north becoming south and vice-versa (note the Sun doesn't physically flip over, just the polarity of the field). You can think of it as a double 11 year cycle if you'd like.

At any time, the magnetic field of the Sun is fiendishly complex, but near the peak of the magnetic cycle it gets very strong, too. It can get tangled up near the surface, like a sack full of bed springs, and store a huge amount of energy. If something happens -- a line gets crossed, for example -- that energy can be released. It can cause a solar flare, a massive explosion of matter and energy from the Sun's surface, or a coronal mass ejection (or CME), a vastly larger eruption of material. A CME can slam us with high-energy subatomic particles, and a good flare can send high-energy gamma rays our way as well, a double-whammy of nastiness.

Either of these can fry a satellite. They generate massive amounts of currents in the materials making up satellites, which can arc and blow out circuits. Most satellites can withstand a minor onslaught, but quite a few have been lost in bigger events.

We can be affected down here on Earth's surface, too. The Earth's magnetic field gets rung like a bell when slammed by a CME, which can generate vast currents under the Earth's surface. These geomagnetically induced currents, or GICs, can dump a lot of extra current into the power grid, causing widespread blackouts.

And this is not just a guess: in March 1989, just such a GIC blacked out Quebec, causing millions of dollars in damage. The picture above shows a transformer that was totally destroyed in the 1989 solar eruption. Other events like that are not hard to dig up.

The problem now is that the North American grid is carrying so much current that it's almost at full capacity, like a pipe full of water. Any extra current and down it goes... and the Sun is gearing up for the next magnetic maximum. The last one, in 2003, was a record breaker, with huge flares and CMEs. Will it happen again?

And when? Probably not when you think...

8) The peak of solar activity is not at the same time as the magnetic field peak.

You would think that all that violent activity would peak at the same time as the magnetic field peak. But it's a little more complicated than that.

The Sun's field actually peaks twice; there is a first peak which then declines over about a year, then a slight resurgence for about a year, then a decline which leads down to the minimum -- it looks like a double-humped camel's back (technically, a Bactrian camel, if a zoologist happens to be reading this). The peak of flare and CME activity is actually associated with that second peak, with more and more violent solar explosions occurring then. This last occurred in November 2003, when the Sun went nuts, blasting out flare after flare in a series that stunned astronomers (I attended a special meeting for solar astronomers about this event, and they were all aghast at what the Sun had done). The image above (in false color) shows one flare that happened on November 4, 2003, and you can see it was quite a bruiser.

The next peak in the solar magnetic field should occur sometime in 2012 or so. As you can imagine, all the Mayan prophecy goofballs are making a lot of hay with that. But the actual peak in activity won't be until 2013 or 2014! So their end-of-the-world scenarios will have to be delayed a bit... not that this will stop them.

The last peak was a real wowser, but no one knows what will happen this time. The minimum lasted much longer than usual -- we're still in it -- and solar physicists aren't sure if that means we'll have a weaker or stronger than normal cycle. This is an incredibly complex subject, and the only way we'll know what happens for sure is to wait.

Sometimes, the best teacher in science is patience.

9) It's getting hotter.

The Sun is fusing hydrogen into helium in its core. It doesn't have enough mass to squeeze the helium enough to fuse it (it takes far higher pressure and temperature to fuse helium into carbon), so that helium builds up in the core, like ash in a fireplace. As more of it piles up, its own gravity squeezes it. When you compress a gas it heats up, and that helium is basically just a gas, though a very hot and high-density one. So for billions of years, as the very center of the Sun has had more and more helium pile up, it's been getting hotter.

That extra heat works its way out from the center and eventually out through the surface. So, over time, the Sun itself, even its surface, is getting hotter. A hotter star is brighter... which means the Sun is getting brighter, too. In fact, it's about 40% brighter now than it was when nuclear fusion first switched on 4.5 billion years ago.

And it's still getting hotter now. That's bad. It's been calculated that if the Earth's average temperature were to increase by about 10 degrees Fahrenheit, a runaway greenhouse effect would be triggered... and the Earth is being heated by the ever-hotter Sun. It turns out this magic moment will happen in about 1.1 billion years. At that time, the Earth will become a little too warm for comfort; the ice caps will melt, and Antarctica will be a pretty good vacation spot.

But only for a little while. 2.4 billion years later (3.5 billion years from now) the Sun will be so hot (about 40% brighter than it is today) that the Earth's temperature will rise enough to evaporate all the oceans!

That will, to use a scientific term, suck.

But (he says, chuckling in an evil fashion), it gets worse. Oh yes, far worse.

10) The Sun has 6.3 billion more years to live.

Right now, the Sun is 4.55 or so billion years old. It's been getting hotter all that time, but in general, from day to day, it's been a stable star shining steadily.

In 6.3 billions years, that'll all change.

At some point, there won't be any more hydrogen in the Sun's core to fuse. All that will be left is helium, contracting (about 30 meters per year, which is not a whole lot, but we're talking millions of years here) and getting hotter. Eventually, the core will heat up enough that hydrogen will fuse in a thin shell surrounding the core. This will add even more heat to the Sun, and what happens then is the outer part of the Sun will balloon outward (because hot gas expands). Over about 700 million years, the Sun will expand from about 1.5 times its current size to about 2.3 times.

After that time, the helium core gets so compressed under its own weight it becomes degenerate, a bizarre quantum mechanical state that is complicated and weird and you probably don't need details here. But when this happens it gets even hotter, the outer layers of the Sun swell even more, and it'll bloat out to an awesome 160 million kilometers in diameter, about 100 to 150 times its current size! Weirdly, that will add so much surface area to the Sun that it will actually get cooler; the average amount of heat per square centimeter of its surface will drop... but it will have a lot more square centimeters so it'll be brighter. Cooler stars are red, and the Sun will be huge, so it will then be a red giant.

After that, things get complicated. It will go through several periods of contraction and re-expansion. Eventually it will have some serious paroxysms which will eject over half the mass of the Sun into space. This exposes the ultra-dense blindingly bright core of the Sun. There won't be any more fusion; all the helium will be used up, and all that will be left is an Earth-sized ball of matter with about half the Sun's mass, first white hot, but then cooling over billions and hundreds of billions of years, ending up a cold black dwarf, invisible and quite thoroughly dead.

And what of the Earth? It's possible the Earth will be swallowed by the Sun when it becomes a red giant in 7 billion years, but it's also possible we'll be too far away to get eaten by our star. Either way, though, the Earth will be fried, blasted, bombarded by searing solar matter... and its surface heated well beyond the melting point of rock.

In other words, ouch. Unless we move the Earth far enough out from the Sun (like, billions of kilometers), then we'll have to find some better real estate somewhere else in the Milky Way. This one will have a little bit too much lava for my taste.

The actual series of events leading up to all this is significantly more detailed and interesting; if you want more, than read Chapter 7 of my book, Death from the Skies! and you can follow along as the Sun dies over the next 7.765 billion years. It's actually an incredibly fascinating story, and was one of my favorite chapters to write.

Conclusion:

Familiarity breeds contempt.

Oh, you want more? OK. It's easy to take the Sun for granted, but even though it's 280,000 times closer than the next nearest star we know of, it's loaded with mysteries. I've only listed a few, and I had to cull easily ten more from this list. Even to professionals in the field, the Sun is laden with questions, and the answers are still being sought.

Further reading:

If you want to learn more about the Sun, here are a few places to check out:

Big Bear Solar Observatory

SOHO, the Solar Heliospheric Observatory, a multi-nation mission which has been studying the Sun for more than thirteen years! It has great imagery and fantastic videos.

The Nine Planets has all kinds of info on the Sun and planets and is one of my favorite sites.

Image Credits:

Solar eclipse: luc_viatour's Flickr stream

Sun comparison: NASA/CXC/M.Weiss and Wikipedia.

Neutrino detector: Sudbury Neutrino Observatory

Quebec transformer: NASA.

Solar flare: SOHO, NASA, and the ESA.

Desert photo: bachmont's Flickr photostream.

Red giant sun illustration: James Gitlin/STScI

Spectacular Green Flash image: Mike Baird's Flickr photostream.

Source : Discover Magazine.com