Slacker Astronomy has a new chit-chat show. In it they play an audio comment I sent them with a little physics pop quiz in it. It actually turned out kind of humorous. Check it out!

Some of my astronomy friends started up a really cool podcast called Slacker Astronomy. It’s a weekly 10-minute MP3 file with news and stories from astronomy. MSN just ran a news story about Slacker Astronomy, which gives a good flavor of what it is about. Aaron is a friend of mine from the AAVSO and Pamela Gay is a cool person I met recently at an astronomy conference. I am glad to see their little podcast taking off! Go check it out.

A given star, at a given time, has a certain temperature and a certain luminosity. It turns out if you plot temperature vs. luminosity of a whole bunch of stars, they aren’t distributed randomly — there is a certain pattern that is apparent. The pattern is dominated by a swath that goes through the center of the plot. This swath is called the Main Sequence.

Stars spend most of their life on the main sequence.

The problem is, there are two things that can make a star appear bright or dim — the intrinsic brightness of the star and how far away the star is. If you know a bunch of star are all roughly the same distance away, like in clusters of stars, you can assume all the differences in brightness you observe are due to intrinsic brightness differences in the star. Then rather than plotting luminosity and temperature, you get the same result plotting the color of the star vs. the brightness of the star.

I tried this little experiment with my telescope on a cluster known as M38. The resulting plot showing the main sequence looks like this:

So in a very minor sense, using my own equipment, I have proven a basic observational fact in stellar evolution: the existence of the main sequence.

There is an equation which you can derive which relates the temperature of a dust grain (or any object, really) to the distance it is from a star:

In this equation T* is the temperature of the star, R* is the radius of the star and Tg is the temperature of the dust grain.

The distance that the earth is away from the sun is called an astronomical unit or A.U. so earth is 1 A.U. from the sun.

If you put the temperature of water freezing and the temperature of water boiling in the above equation you find that from our Sun, liquid water is possible from 0.51 A.U. to 0.95 A.U.

If you are surprised that liquid water is not possible here on Earth there is, of course, an explanation and it is the greenhouse effect. The earth also has some stored heat in its core. So the earth’s average temperature just happens to fall exactly in the realm of liquid water.

This range — 0.51 A.U. – 0.95 A.U. — is extremely small in astronomical terms. At roughly 0.5 A.U. water boils. At right around the orbit of the earth, water freezes. Aren’t we lucky to have ended up right here in just the right place?

Of course, those that didn’t aren’t around to blog about it.