Introductory textbooks such as ours work hard to present the material in a straightforward and simplified way. Nearby stars thus serve as benchmarks for more distant stars because we can assume that two stars with identical spectra have the same intrinsic luminosity. Once astronomers take a spectrum of a nearby star for which we also know the parallax, we know the luminosity that corresponds to that spectral type. The distances measured using parallax are the gold standard for distances: they rely on no assumptions, only geometry. The H–R diagram method allows astronomers to estimate distances to nearby stars, as well as some of the most distant stars in our Galaxy, but it is anchored by measurements of parallax. (For historical reasons, astronomers sometimes call this method of distance determination spectroscopic parallax, even though the method has nothing to do with parallax.) As before, if we know how luminous the star really is and see how dim it looks, the difference allows us to calculate its distance. Since the diagram plots luminosity versus temperature, this means we can now read off the star’s luminosity (once its spectrum has helped us place it on the diagram). With both its spectral and luminosity classes known, a star’s position on the H–R diagram is uniquely determined. By studying details of the spectrum for each star, astronomers can determine which luminosity class they fall in (whether they are main-sequence stars, giant stars, or supergiant stars). Stars of the same temperature (or spectral class) can fall into different luminosity classes on the Hertzsprung-Russell diagram. The dashed portions of the lines represent regions with very few or no stars.įigure 19.15 Luminosity Classes. Figure 19.15 illustrates the approximate position of stars of various luminosity classes on the H–R diagram. The specification for an M2 giant is M2 III. For example, a main-sequence star with spectral class F3 is written as F3 V. The full spectral specification of a star includes its luminosity class. IV: Subgiants (intermediate between giants and main-sequence stars).These luminosity classes are denoted by Roman numbers as follows: The most widely used system of star classification divides stars of a given spectral class into six categories called luminosity classes. But if there are subtle differences between the solar spectrum and the spectrum of the distant star, then the distant star may be a giant or even a supergiant. It is then reasonable to conclude that this distant star is likely to be a main-sequence star just like the Sun and to have the same luminosity as the Sun. Suppose, to start with the simplest example, that the spectrum, color, and other properties of a distant G2 star match those of the Sun exactly. If we look in detail at the spectrum of a star, we can determine whether it is a main-sequence star, a giant, or a supergiant. This knowledge is very useful because giant stars are larger (and have lower pressures) than main-sequence stars, and supergiants are still larger than giants. Remember, for example, that we can detect pressure differences in stars from the details of the spectrum. We can learn more from a star’s spectrum, however, than just its temperature. A G2 star could be a main-sequence star with a luminosity of 1 L Sun, or it could be a giant with a luminosity of 100 L Sun, or even a supergiant with a still higher luminosity. (The types are O, B, A, F, G, K, M, L, T, and Y each of these can be divided into numbered subgroups.) In general, however, the spectral type alone is not enough to allow us to estimate luminosity. As discussed in Analyzing Starlight, a detailed examination of a stellar spectrum allows astronomers to classify the star into one of the spectral types indicating surface temperature. If we can observe the spectrum of a star, we can estimate its distance from our understanding of the H–R diagram. In this case, it turns out the H–R diagram can come to our rescue. Suppose, for example, we need the distance to a star that is not varying, or to a group of stars, none of which is a variable. Distances from Spectral TypesĪs satisfying and productive as variable stars have been for distance measurement, these stars are rare and are not found near all the objects to which we wish to measure distances. Another way involves the H–R diagram, which shows that the intrinsic brightness of a star can be estimated if we know its spectral type. Variable stars are not the only way that we can estimate the luminosity of stars. Examine how these techniques are used by astronomers today.Understand how spectral types are used to estimate stellar luminosities.By the end of this section, you will be able to:
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