1.2 The main classes of stars
The main classes of stars are shown in Figure 5.
The main sequence is 'main' in the sense that about 90% of stars fall into this class, and 'sequence' in the sense that it is a long, thin region that trails across the H-R diagram, covering a very wide range of temperatures and luminosities. The Sun is a main sequence star, of very modest temperature and luminosity, and correspondingly modest radius. It is yellowish-white. Sirius A is a main sequence star rather hotter than the Sun, and appears bluish-white. It has the greatest apparent visual brightness (most negative apparent visual magnitude!) of any star in the night sky. This is, as we have seen, not because it is very luminous, but because it is both fairly luminous and rather close - at 2.63 pc it's the seventh closest star after the Sun.
Above the lower part of the main sequence we come first to the red giants. These stars are cool, hence their orange tinge, and are of order 10 to 100 times larger in radius than main sequence stars of comparable temperatures. Thus if our Sun were a large red giant, its surface would extend a considerable distance towards the Earth (as we saw in Question 2)!
If you knew that a red giant was larger than a main sequence star of comparable temperature, what could you say about its luminosity?
From Equation A we could say that its luminosity is greater than that of the main sequence star. (This conclusion is borne out by Figure 5.)
The bright star Aldebaran A ( Tau) is a red giant. (It's actually a visual binary, but the red giant is dominant.)
Above and to the left of the red giants we come to the supergiants. These are larger, and thus more luminous than red giants of comparable temperature, but they also extend to higher temperatures, where they are larger and more luminous than main sequence stars of comparable temperature. Rigel A is a hot supergiant, which appears bluish-white whereas Betelgeuse is a cooler supergiant, and it appears distinctly orange-white.
Though we have not marked it on Figure 5, there is a class of stars that comprises the red giants plus the stars to their left that lie between the main sequence and the supergiants. This class is the giants and is broader than that of red giants alone.
You can see from Figure 5 that white dwarfs are, as their name implies, hot and small, only about the size of the Earth (Question 2). Consequently their luminosities are low. Indeed, there are no white dwarfs sufficiently close to us to be visible to the unaided eye. The closest is Sirius B, the faint companion to Sirius A, but its visual magnitude is only 8.4, well outside the limit of about 6 for very good, unaided human eyes, in the very best observing conditions. Even if it were a bit brighter, its light would be swamped by Sirius A, and we would still be unable to see it.
The width of spectral lines gives an indication of the luminosity of a star. We can now see how this is reflected in the H-R diagram and the description of stellar spectral types. Giant stars have narrower spectral lines than dwarf stars and stronger lines due to certain ionized atoms. These characteristics are used to define a luminosity class, designated by roman numerals I to V, with I being brightest. Class I is often sub-divided into Ia and Ib.
Figure 6 illustrates the positions of these luminosity classes on the H-R diagram.
From your knowledge of the Sun's position on the H-R diagram, what is its luminosity class?
The Sun is a main sequence star so its luminosity class is V.
The full designation of a star's spectral type also includes its luminosity class. The Sun is spectral type G2 V and Betelgeuse is spectral type M2 Ia.
White dwarfs are usually designated by a prefix 'D' or 'w' as in the case of Sirius B which is spectral type D A5 or w A5. Other suffixes are used for special characteristics such as 'e' for emission lines or 'p' for peculiar spectrum.
The tendency for stars to concentrate into certain regions of the H-R diagram is clearly meaningful. But what does it mean?