Moon and Antares 
To truly understand a star, you have to know how far away it is. Yet measuring astronomical distances is tough — even for stars that are fairly close.
As an example, consider Antares, the brightest star of Scorpius, which is to the lower left of the Moon this evening.
Antares shines reddish orange. Stars of that color are either really big and bright, or small and faint — too faint to see with the eye alone. So the fact that it’s visible at all means it’s a giant.
But the type of giant depends on its distance. If it’s within a hundred light-years or so, then it would be a normal giant — the final stage of life for a star like the Sun. But if it’s much farther than that, then it’s a supergiant.
Astronomers measure a star’s distance by plotting its position relative to other stars when Earth is on opposite sides of the Sun. Viewed from these different angles, the star moves back and forth a tiny bit — like holding out your finger and looking at it with first one eye, then the other. The size of the shift reveals the star’s distance.
For Antares, the best measurement was made by an orbiting satellite. But it took years for astronomers to analyze and then re-analyze the data. Based on the latest analysis, the distance to Antares is about 550 light-years — one of the most-distant stars that’s visible to the eye alone. So Antares is not just a giant, but a supergiant — one of the biggest and brightest stars in the entire galaxy.
Script by Damond Benningfield, Copyright 2012