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Little Questions Revisited

After a few more negative, cynical, and controversial blog entries it’s time to get back to the “little questions” again, the ones I started discussion about a month ago.

I’ve already dealt with “Why is the Moon round?”, “Why is the sky blue during the day?”, and “Why is the sky dark at night?” so what’s next? How about “what is a star?”

I did a “pop quiz” and found that most people know roughly what a star is, something like “a big burning ball of gas”.

That’s not quite true: stars are neither gas, nor burning. Stars are made from plasma, not gas, but gas is a fair approximation. Also they aren’t burning, they are undergoing nuclear fusion which is a far more efficient process. If a star was burning a conventional fuel (like coal for example) it could burn for about 6000 years, but nuclear fusion allows a star to shine for billions of years.

Interestingly there was some confusion abut the difference between the Sun and a star. People seemed to realise the two things were similar but some thought the Sun was brighter and hotter because it is bigger.

Of course the reality is that the Sun is a star. There is absolutely no difference between a star and the Sun. The Sun seems brighter simply because it’s much closer. The Sun is just 150 million kilometers away but the stars visible without a telescope are between 40 trillion and 100 million trillion kilometers away. There are stars much further away than that but a large telescope is needed to see them.

How do we know? Stars are so far away that they can usually only be seen as single points of light so couldn’t they be something totally different from our Sun? Almost 200 years ago the great philosopher, Auguste Comte, said that humans will never be able to visit the stars, that we will never know what stars are made out of, that that’s the one thing that science will never understand. He was right that visting the stars isn’t likely in the foreseeable future, but it was only a few years later that science discovered what stars are made of thanks to spectroscopy. And it turns out that stars are made of similar things to the Sun.

And how do we know the distance to the stars? That turns out to be an extremely difficult problem. The most direct measure, which works for close stars, is to use parallax. When a star is observed from one side of the Earth’s orbit, then from the other side 6 months later the observer on the Earth has moved 300 million kilometers compared to the Sun. That causes the star to seem to move against distant objects like dimmer stars and galaxies. But how much does it move? Not much. Imagine a small coin at a distance of 5 kilometers. That is how much the a close star seems to move, and the more distant the star the less the apparent movement. But the measurement has been done and the distances are known. Looking at the apparent brightness and size of a star and knowing its distance allows an estimate of its real size and brightness.

The Sun is not even a particularly big or bright star. It’s quite average and almost all the stars visible in the sky at night are much brighter. For example, the star Rigel in Orion is over 100,000 times brighter than the Sun, and another star in that constellation, Betelgeuse, is a billion times the Sun’s volume.

I think the facts above are quite significant. They mean that the Sun – the most significant thing to us (apart from the Earth itself maybe) – is just one of many other objects and isn’t really special in any way. How many stars (or Suns) are there in the universe? About the same number as there are grains of sand on our planet!

That’s an extraordinary thing really. Imagine going to a beach and picking up a handful of sand and counting the grains of sand. Now think about how many grains are on the beach and how many there are on all the beaches on the whole planet. There are a similar number of stars in the universe (the exact number is debated a bit but it’s about 300 trillion trillion) and each of those is like our Sun (which is well over a million times the volume of the Earth) and probably has orbiting planets like the Earth.

Yes, we think that planets might be quite common. Because of the distance to stars which might have planets orbiting them (apart from the Sun) is so great it’s difficult to observe anything except the star, but recently telescope technology has progressed sufficiently (the Kepler space mission and various big ground-based telescopes) to allow planets to be detected. And plenty have been found, including some much bigger than any of the planets in our Solar System, and others very similar to the panets we already know.

And just to complete the whole story about the insignificance of anything in relation to the universe as a whole, we don’t even know the size of the universe and according to some ideas it might be infinitely big. The universe we see, including its apparent beginning in the Big Bang, could be just a localised part of a much bigger (possibly infinitely big) multiverse. Then there really would be an infinite number of stars not just a few trillion trillion like we can see in the visible part of the universe.

Start with the question “what is a star” and finish with almost philosophical musings regarding the universe on the grandest of scales. Another little question – another big answer!

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