Color magnitude diagrams of nearby stars

For nearby stars, say within 20 pc, we can determine the distances from trigonometric parallaxes. From the apparent magnitudes and the distances we can calculate the absolute magnitudes, i.e., the magnitudes which the stars would have if they were at a distance of 10 pc. This means that for absolute magnitudes we compare the brightness the star would have if it were at a distance of 10 pc with the actual brightness of Vega at its actual distance, i.e. with its apparent brightness. It turns out to be quite instructive to plot the absolute magnitudes of the stars as a function of their B – V colors. In Fig. 5.1 we do this for the nearby stars. While we might have expected that stars with a given color could have quite different absolute magnitudes, it turns out that this is generally not the case. Most of the stars with a given B – V color have the same absolute magnitude. Most of the stars fall along one line in the color magnitude diagram. This line is called the main sequence. The intrinsic brightnesses and the colors of these stars are obviously determined by just one parameter, since they follow a one-dimensional sequence. It turns out, as we shall see in Volume 3, that this one parameter is the mass of the star.