Thursday, January 31, 2008

Star light, star bright

A sign at the beginning of our driveway here in Exuma says "Starlight House." Two reasons for the name. It was the movie Frankie Starlight that paid for this tropical retreat. And we came here for the stars.

That is to say, we came here for the dark, a precious commodity that was in short supply at home, and even here is now being seriously eroded. Our starry universe is shrinking around us.

But not as fast as we might think. The stars we see in a dark night sky are not representative of the stellar population.

Within 20 light-years of Earth there are about 100 known stars. Of these, nearly 70 are tiny red dwarf stars, much less bright than the Sun, barely hot enough to ignite the fires of nuclear fusion that blaze at a star's core. These stars are so faint that they are not visible to the naked eye, even though they are among our closest neighbors.

The same neighborhood includes about 15 orange stars, hotter and bigger than the red dwarfs but not as hot or bright as the Sun. There are six yellow stars, including the Sun, with surface temperatures of about 6,000 degrees Celsius.

Only four stars in our 20-light-year neck of the Milky Way are more luminous than the Sun: Alpha Centauri, Procyon in Canis Minor, Altair in Aquila, and Sirius in Canis Major. Sirius, a white-hot star, is the big boy on the block.

A larger neighborhood, say 2,000 light-years in radius -- still just a corner of the Milky Way Galaxy -- would show the same pyramidal distribution, but with a few blue-white supergiants sitting near the top, 2,000 times more luminous than Sirius, including Deneb, Rigel and the stars of Orion's belt. The capstone stars pay a price for their dominance. They come and go quickly.

The red dwarfs at the bottom of the pyramid burn their hydrogen fuel so slowly they live for hundreds of billions of years. Since the universe is only about 14 billion years old, every red dwarf star that was ever born is still with us -- which at least partly accounts for their dominant numbers.