Chapter 12: Properties of Stars

A Hertzsprung-Russell diagram showing color and size of stars.

As astronomers studied the properties of large samples of stars, they found stars with many different combinations of luminosity and temperature. Most of these stars had properties that placed them at different positions on the main sequence of the H-R diagram, slanting from high luminosity and high temperature to low luminosity and low temperature. However, some stars had properties dissimilar from any main sequence star.

Comparison between the red supergiant Antares and the Sun, shown as the tiny dot toward the upper right. The black circle is the size of the orbit of Mars. Arcturus is also included in the picture for size comparison

Other stars are brighter than the giants or the main-sequence O stars. Application of the Stefan-Boltzmann law showed that they were larger than giant; they came to be called supergiant stars. For example, the enormous red giant Antares, in the constellation Scorpius, would dwarf the Sun. Antares is only ten times the mass of the Sun but it is so bloated it is almost 900 times the size; the Sun would fit into Antares 700 million times over!

Main sequence stars in scale.

Hertzsprung, Russell, and their followers showed the systematic properties of different types of stars: one type was the main sequence; the giants, supergiants, and white dwarfs were distinctly different types of stars. These types refer to the luminosity class of a star. The main sequence — seen as stars whose properties place them on a diagonal of the H-R diagram — is the set of stars converting hydrogen into helium, a set that includes the Sun. Half of the most prominent stars in the sky are giants or supergiants. Although the whales among the fishes are rare, they account for many of the familiar stars. Their combinations of size and luminosity mean they must have a different mechanism for energy production than the Sun and other main sequence stars. Further work showed that these groups occur all over the sky and at many distances from the Sun. This meant that universal physical processes could be sought to explain the different non-main-sequence stars.

The size of Betelgeuse compared to our solar system.

There is an enormous range in the physical properties of stars in the H-R diagram. Astronomers have the practical problem that stars with the same color might be quite different in their other properties. To see this, consider stars of different luminosity that have the same surface temperature (stars aligned vertically in the H-R diagram). Arcturus is over ten times larger than the Sun but has the same color. Betelgeuse is more than 10,000 times larger than Proxima Centauri but they both have a reddish color. Without knowing distances, is there any way to distinguish main sequence stars, giants, and supergiants of the same color? Yes. The more diffuse atmosphere in the larger stars gives fewer atomic collisions and a narrower absorption line. Spectroscopy can be sufficient to establish the luminosity class (main sequence, giant, supergiant) of a star. This allows a distance to be estimated, even if no parallax measurement is possible. However, spectral diagnostics apply to the outer layers of a star so they are indirect; the main differences between these star types stem from their masses and their energy production mechanisms.