Friday, July 31, 2009

Answers to the informal quiz, Part 3: The Sun

I know it's been a while since my last post, but research and conferences have been taking up all of my time least I've got a new first-author paper submitted to the journals! Nonetheless, I extend my deepest apologies to my loyal readers.

So with my mea culpa out of of the way, let's get to the next set of answers!
  • Compare the Sun to the stars.
So, this is a bit of a trick question. The standard answer is that the our Sun is an "average-to-small" star. In some sense, this is true - stars can range from 100 times more massive to 10 times less massive than our Sun. They can also be 8 times hotter, or 2 times cooler. Finally, they can be 1,000,000 times brighter or 10,000 times dimmer than our Sun. So, in this sense, our Sun is in the middle of these ranges (though a little on the smaller side).

However, there are far, far more small, cool, dim stars than big, hot, bright stars...the vast majority of stars in our universe are small little red dwarf stars. In fact, the distribution is so lopsided that the mass of all the small stars put together is many times larger than the mass of all the big stars put together (though exactly how many times is still debated). So, in that sense, our Sun is actually bigger, hotter, and brighter than most other stars.
  • Why does the Sun shine?
Deep in the core of the Sun, hydrogen atoms are packed under such great pressure and temperature - being gravitationally compressed by the outer layers of the Sun - that nuclear fusion ensues. This is similar to how a hydrogen bomb works. In our Sun, four hydrogen atoms will collide together to eventually make one helium atom.

However, the resulting helium atom weighs just slightly less than the four hydrogen atoms that went into it. This tiny bit of missing mass is actually converted into pure energy via Einstein's famous equation E=mc^2. This energy, which leaves the nucleus of the atom as an extremely energetic gamma ray photon (a particle of light), bounces around the interior of the Sun countless times, getting absorbed and re-emitted by surrounding atoms and losing just a little energy to them each time. By the time it reaches the surface of the Sun - a process which takes, on average, about 1 million years - the photon has lost enough energy to emerge, on average, as yellow light...and that's why the Sun looks yellow.

This is also what makes movies like "The Core" utter nonsense. If the core of the Sun suddenly stopped undergoing fusion, we wouldn't really notice the effects until a million years later, once all the photons had managed to escape.
  • What happens to the Sun at night?
Okay, this was a softball question: the Sun is still there, it's just shining on the other half of the Earth. At any given time, it's always day for one half of the Earth, and night for the other half.

Still, I've heard some pretty wacky wrong answers for this, like the idea that space itself is lit up during the day, and dark at night. Meanwhile, Socrates thought that every night the Sun passed through a giant hole in the middle of the Earth. Those wacky ancient Greeks...
  • What is the sun made of?
Mostly hydrogen (about 75% by mass), some helium (about 24%), and about 1% everything else (carbon, oxygen, iron, etc.). The Sun's hydrogen is primordial - it comes from the big bang. The helium is partially from the big bang, and partially manufactured from the Sun turning hydrogen into helium. That 1% of everything else comes from exploded stars which seeded the interstellar gas cloud from which the Sun formed.

Note that this also goes for the planets and everything on them...So, all that hydrogen locked up in the water in your body came from the big bang, while all the carbon, oxygen, iron, etc., in your body originally came from ancient exploded stars.

If you're a romantic, you can say, "we are all stardust." Meanwhile, if you're a pessimist, you can say, "we are all nuclear waste products."
  • What causes a solar eclipse?
This occurs whenever our Moon passes between the Earth and the Sun and blocks out the sunlight. When the Moon only blocks out part of the Sun, it's a partial solar eclipse. If the Moon manages to block out the entire disc of the Sun - though this is only visible in a narrow range of locations - it's a total solar eclipse (which just happened in Shanghai a couple weeks ago).

Now, it doesn't happen every New Moon because the Moon's orbit around the Earth is tilted 5 degrees to the orbit of the Earth around the Sun. Only when the orbits line up during New Moon (about once every 6 months), does the Moon block out the Sun's light...otherwise it passes a couple degrees below or above the Sun from our perspective.

Just by chance, both the Moon and the Sun span about half a degree on the sky, so they have to be lined up just right for a total solar eclipse to occur, and it's only visible at just the right location on Earth. This wasn't always the case - in Earth's past, the Moon used to be quite a bit closer to us, meaning it appeared quite a bit larger in our sky.

Currently, the Moon is moving away from Earth at a rate of about 1.5 inches per year. This is all because of the tides...the difference in the Moon's gravity felt on the Earth raises two bulges on the Earth which rotate roughly once per day (anyone who's lived near the ocean knows the cycle of low-tide and high-tide). These act as a very subtle brake on Earth's rotation, just like slightly depressing the brakes in your car.

This causes the Earth's rotation to slow down ever so slightly - this is why every now and then you'll hear about the powers-that-be inserting a leap second to keep the clocks accurate. So, the days on Earth are getting slightly longer...but all this rotational energy the Earth is losing has to go somewhere. The moon ends up absorbing it, causing it's orbit to slightly spin-up, which makes it's orbit slightly wider every year.

So, in the distant past, total eclipses were much more common. Likewise, in the distant future total eclipses will no longer occur - the Moon's apparent size will just be too small to block out the entire Sun. It's a rather happy circumstance we were all born at a time in Earth's history when the two brightest celestial objects are the same size on the sky.


  1. Q: Why does the sun shine?
    this was a sequence of trick questions on the qualifying exam.
    A: it shines because it's hot. Blackbody.
    Q: How did it get hot?
    A: gravitational collapse
    Fusion can only happen after it gets hot, and that's the final exchange
    Q: How does it stay hot so long?

    As for the earth, for much of our lifetimes the rotation of the crust and mantle has been getting faster

  2. My main reason for asking the "why does the sun shine?" question was just to combat the painfully popular misconception that the Sun is made of fire.

    It's true that any of your above answers work, too...but ultimately fusion is the "sine qua non" without which none of the other answers could exist on geological timescales.

    The concept of gravitational compression (i.e. potential energy converting to heat) as the sole driver of sunshine was the first physical explanation given for the Sun's energy production. However, it was determined early on that at it's current luminosity, this mechanism would only produce about 20,000 years of sunshine...even before nuclear physics was understood, scientists knew there must be some other method of energy production.

    As for the Earth's rotation, you're right, fluid motions of the Earth's mantle redistributing angular momentum are more important on short, human time scales. In the long run, however, these cancel out to be zero sum and tidal energy losses are much more important.

  3. If you're a romantic, you can say, "we are all stardust." Meanwhile, if you're a pessimist, you can say, "we are all nuclear waste products."

    Thought that was incredibly hilarious! Also, and I know we'll be long gone by then, what happens when the Moon moves away from the Earth? Will the Earth's orbit be altered? Why are tidal energy losses important?

  4. Lastly, I read a previous post on interstellar space travel. At speeds up to 12 percent the speed of light, you said a spacecraft would be subject to collision with hydrogen atoms, which would eventually tear the spaceship apart. Is there a way around this? Also, what's your take on science-fiction 'shields' and 'warp speed'. I read an article that said warp speed could potentially cause a black hole? Have you seen that one? I should have chosen astronomy, because this stuff is what I find exciting!

  5. I always wanted to ask how are heavy metals like u-235 are formed because the heaviest and stablest mental formed in core of star is iron. then how are heavy metals such as u-235 or pu-239 found on earth? i also recommend you to watch this nice video -


  6. Hi Mike,
    could you perhaps add the following question to the list?
    "why do stars appear to twinkle?"

    We were taught that its because of continuous variations in density (and hence refractive index) of the air in the atmosphere...

    I was wondering, as the light from the stars reached us, it might be bent a little left and right , up and down by gravity of the passing bodies (planets, stars, black homes) which might cause the flickering effect?

    I would would be glad if you could shed some light on it

  7. Mike, does the discovery of large beds of water on the moon pretty much prove the 'big impact' theory?