Counting Stars

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This is a good evening for counting stars. The glowing band of the Milky Way arcs high overhead, from W-shaped Cassiopeia in the north, through the bright Summer Triangle overhead, to teapot-shaped Sagittarius in the south.

In all, about 5,000 individual stars are visible to the unaided eye. Millions more blur together to form the Milky Way's hazy band. They're all in the broad disk of our Milky Way galaxy.

The total number of stars in the Milky Way is uncertain. Estimates range from a hundred billion up to about four hundred billion.

There's no way to actually count the stars in the galaxy. For one thing, there are just too many of them. For another, many stars are hidden behind dense clouds of gas and dust, so we can't see them.

So astronomers have to estimate the number. They use the galaxy's total mass, then estimate how the stars break down by mass.

The total mass is about a trillion times the mass of the Sun. But most of that is in the form of invisible dark matter. Normal matter, like stars and planets, makes up around a tenth of the total.

The question then is how many stars are big and heavy, and how many are small and lightweight. From counting the stars in our neighborhood, astronomers know that only a small percentage are much heavier than the Sun, and most are much lighter than the Sun.

So astronomers apply different estimates -- and come up with wildly different counts of the number of stars in the Milky Way.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.