Moon and Antares 
It's sometimes hard to tell where a star ends and space begins. A case in point is Antares, the brightest star of Scorpius, the scorpion. It's just to the left of the Moon at the first blush of twilight tomorrow, and shines with a distinctly orange color.
Antares is a red supergiant -- a star that's far larger than the Sun. In fact, if it took the Sun's place, it would extend well beyond the orbit of Mars.
Astronomers aren't sure just how far beyond Mars, though. In part, that's because there's still some uncertainty in how far away Antares is. The best measurement to date says it's about 550 light-years, but that's still subject to change.
The other problem is that Antares just kind of tapers off into space. The star's core is quite dense, but its outer layers get progressively thinner as you move away from the core. Its outermost layers are basically a vacuum, so you can see right through them.
And to make it even tougher, Antares has surrounded itself with a giant cloud of gas and dust -- material blown into space from the star's surface -- enough material to make another star. So when astronomers look at Antares through a telescope, there's no sharply defined surface for them to measure. They know the star is a big 'un -- but just how big remains a bit cloudy.
Regardless of its exact size, though, Antares is one of the brightest stars in the night sky -- and it shines quite close to the Moon before dawn tomorrow.
Script by Damond Benningfield, Copyright 2009