We now have the necessary tools in place to answer the question of why the stars in Orion look so different. By plotting stars on an HR Diagram (Fig. 7), we can easily compare their properties. Two stars that look distinctly different in Orion are Betelgeuse and Rigel, one being red and the other blue. In Figure 7 below we can identify the location of these two stars on the HR Diagram with Betelgeuse, labeled 1, on the right hand side and Rigel, labeled 2, on the left. Recall that stars to the left of the diagram are hotter and towards the top they are more luminous. Thus we can see that Rigel and Betelgeuse are both luminous stars, but have widely different temperatures. Since Betelgeuse in not a main sequence star, it does not adhere to the mass-luminosity relationship like Rigel does. Instead, we find the high luminosity of giant and supergiant stars to be a function of their radius rather than their mass. Simply stated, although Betelgeuse is an extremely cool star, it is highly luminous because of its huge surface area.

Another interesting comparison can be made between Chi 1 Orionis (10), a main sequence star and Phi 2 Orionis (9), a giant star. While they are similar in color, belonging to the same spectral class, detailed study through the HR Diagram reveals their marked differences. The remainder of the numbered stars are located in the upper left corner of the diagram, identifying them as main sequence stars. The HR Diagram tells us that this collection of blue stars are not only high in luminosity and temperature but massive as well, details which are an indication of their young age.

Figure 7
Seventy-two of the stars in the Orion are plotted on this HR Diagram (data from Table 1). The absolute magnitude, a measure of a star’s intrinsic luminosity (as opposed to its apparent magnitude which is dependent upon its distance from us), is plotted on the vertical axis. The horizontal axis labeled B-V, reflects the color index of the star, a measure of the blue light the star emits minus the measure of the visual light the star emits, the resulting value of which corresponds to the temperature of the star. Also included on the horizontal axis is the number of stars of each spectral type and the range over which they lie, as indicated by the solid line. The labeled stars in the diagram, which also appear in Figure 1, correspond to the following list:
1-Betelgeuse; 2-Rigel; 3-Bellatrix; 4-Alnitak; 5-Alnilam; 6-Mintaka; 7-Saiph; 8-Meissa; 9-Phi 2 Orionis; 10-Chi 1 Orionis.