2 - Magnitude, color, and distance

Summary

What is the first thing you notice about the stars? Quite likely, it is their differing brightnesses. Although this appears obvious, it is the single most important concept with which you should become familiar before you can be a variable star observer.

Why do stars differ in brightness? Is it because they are at different distances from us, so that the farther stars appear fainter, like the glow from lamps at the far end of the street? Or are the stars themselves of different brightnesses? As you have likely guessed, both are correct. Sirius, a blue star off the southeast corner of Orion the Hunter, is normally the brightest star in the night sky, not so much because it is actually large and bright, but because it is close. At a mere 8 light years away, Sirius is one of our nearest neighbors. However, there exists a star only a little farther away, Wolf 359, whose intrinsic brightness is so low that it cannot be seen with the unaided eye. Meanwhile, the brightest star in Cygnus the Swan, Deneb, is well over 1000 light years away from us. S Doradus, intrinsically one of the brightest stars of all, appears to us as a faint star shining dimly because it is so far away, actually in a neighboring galaxy.

To reconcile these two factors, astronomers have created two independent systems of describing brightness or “magnitude”.