In the decade 1905-1915 Hertzprung and Russel observed a group of near-by stars whose distances they knew (using parallax). For each star they recorded its color and calculated its brightness as it would be measured at a distance of 1 l.y. (using the 1/(distance)2 law, see Sect. 8.2.1). Then they plotted this brightness versus the color; what they found is that most stars (90% of them) lie on a narrow band in this type of plot which they called the main sequence (see Fig. 8.6).
Suppose we now obtain the HR plot for stars which are far away, say on the other side of the galaxy, about 105 light years (1018 km). If we choose these stars such that they are not too far apart (there are good astronomical indicators for this) the distance from Earth to any such star will be more or less the same. It is found that, as for the near-by stars, 90% of these far stars will again fall on a main-sequence strip in the color vs. brightness plot.
On the other hand all these stars are dimmer than the near-by stars originally used by H-R; the decrease in brightness is due to the fact that brightness drops as the square of the distance (Sect 8.2.1). Comparing the two main sequences (for near and far stars) as in Fig. 8.7, we can extract the distance to these far-away stars. This method can be used to determine distances up to 300,000 l.y.; for larger distances the main sequence stars are too dim to obtain a reliable estimate of their brightness.