Moon and Antares

StarDate: May 10, 2009

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Many of the stars that are visible to the naked eye are giants or supergiants -- stars that are tens or hundreds of millions of miles across. Yet they're all so far away that they look like mere pinpoints of light. In fact, they look like pinpoints even through most telescopes.

But with the help of modern technology, astronomers can actually measure the sizes of some of these stars. An example is Antares, the brightest star of Scorpius, the scorpion. It rises a little above the waning gibbous Moon late this evening, and leads the Moon across the sky.

Antares is a supergiant -- one of the biggest and heaviest stars in the galaxy. Astronomers have measured its diameter at around 800 times the diameter of the Sun.

Getting a precise measurement is a bit tricky, though -- and not just because the star is so far away.

Antares blows a strong "wind" of hot gas into space -- enough gas every million years to make a star as heavy as the Sun. The gas forms a fairly thick cloud around the star. In fact, the gas is so thick that it's hard to tell where the surface of the star ends and the surrounding cloud begins.

The gas gets thinner and thinner as it moves away from Antares. Even so, the cloud is huge. It even surrounds a companion star, which leaves a broad wake in the cloud as the two stars orbit each other.

Look for giant Antares -- a star with a hard-to-define surface -- leading the Moon across the sky late tonight.

Script by Damond Benningfield, Copyright 2009

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

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