Continuous as the stars that shine
You learned that stellar spectral types correspond to the stellar surface temperatures, which are plotted on the x-axis of the H-R Diagram. O stars are the hottest and bluest, M stars the coolest and reddest, with stars like the Sun, a G star, at intermediate temperature. Astronomers also have a system for specifying luminosity classes, which correspond to stellar luminosities. These are a rougher classification scheme than the spectral types, and are more approximately represented on the y-axis of the H-R Diagram (Figure 1).
A star's luminosity correlates with its radius. Given two stars with the same temperature, the bigger one will be more luminous. Supergiants like Betelgeuse are bigger than the Earth's orbit! The luminosity classes are specified with Roman numerals:
• Supergiants are denoted classes I and II.
• Giants are denoted class III.
• Subgiants are denoted class IV.
• Dwarfs are denoted class V.
The Sun is a dwarf star, so it is a G V star. All Main Sequence stars are dwarfs (class V), with the exception of some extreme, most luminous blue stars, which are blue supergiants (class I).
The luminosity classes are associated with different age stellar populations, because most stars leave the Main Sequence and increase in luminosity near the ends of their lives. Stars just leaving the Main Sequence are subgiants (class IV), and red giants (class III) have clearly left the Main Sequence. Thus, both luminosity class III and IV stars are associated with older stellar populations. On the other hand, massive stars maintain the same, extremely high luminosity, for most of their short lives. Therefore, supergiants (classes I and II) are all massive stars and are associated with young populations. Betelgeuse, in the constellation Orion, is clearly red even to the naked eye, and is an example of a short-lived, massive star that has become a supergiant. Note that almost all of the red stars visible to the naked eye are red giants (class III) or red supergiants (class I), because red dwarfs (class V) are too faint to be seen.
In this activity, you will work in groups to to see how different stellar luminosity classes trace different stellar populations in our Milky Way Galaxy, and how the disk of our Galaxy is related to them.
Last modified: 10/31/12 by MSO
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