Size and Mass

Once astronomers know a star's temperature and absolute brightness, they can estimate its diameter and then calculate its volume. However, there is no direct way of finding the mass of an isolated star. Instead, astronomers are able to calculate the masses of many stars by observing the gravitational interaction of stars that occur in pairs. From such observations, astronomers have determined that, for most stars, there is a relationship between mass and absolute brightness. Astronomers have found that many stars are similar to the sun in size and mass.

Figure 11 These streetlights all have about the same absolute brightness. Inferring Why do the nearby streetlights appear brighter than the distant ones?

Composition

A spectrograph is an instrument that spreads light from a hot glowing object, such as a light bulb or a star, into a spectrum. Astronomers can use spectrographs to identify the various elements in a star's atmosphere.

Each star has its own spectrum. The elements within a star's atmosphere absorb light from the star's photosphere. Each element absorbs light of different wavelengths, removing these wavelengths from the star's continuous spectrum. The result is a bright spectrum, such as the one shown in Figure 12. It contains a set of dark lines called absorption lines that show where light has been absorbed. Just as fingerprints can be used to identify a person, a star's absorption lines can be used to identify different elements in the star.

Absorption lines of most elements have been identified in the spectra of stars. Observations of such lines in many stars have shown that the composition of most stars is fairly similar. Most stars have a chemical makeup that is similar to the sun, with hydrogen and helium together making up 96 to 99.9 percent of the star's mass.

Figure 12 This is the spectrum of a star. The dark absorption lines indicate the presence of various elements in the star.