Comparing stars
Comparing stars

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Comparing stars

1.4 Stellar masses and stellar evolution

Measured masses range from about 0.08M to about 50M, a large range, with the Sun again showing up as an average sort of star. At the upper end we have some true monsters, but even at the lower end we have bodies that are still far more massive than the planets.

Question 5

What is the mass of a 0.08M star, in Earth masses?

Answer

Nearly 30 000 Earth masses.

The lower the mass the greater the number of stars; the monsters are rare, and stars less massive than the Sun are more common than stars of around solar mass. These relative numbers, and the upper and lower mass limits, are all things that the stellar theories have to explain.

We can, however, throw some light on stellar evolution if we plot stellar masses on an H-R diagram. This is done in Figure 8, where a handful of representative stellar masses have been included. Note the following important features.

  • The supergiants tend to be more massive than the red giants, which in turn tend to be more massive than the white dwarfs.

  • Within each of the supergiant, red giant, and white dwarf classes, there is no correlation of mass with luminosity or photospheric temperature - the relationship is jumbled.

  • Among the main sequence stars, mass correlates closely with luminosity, and hence with temperature: as mass increases, luminosity and temperature increase. (The increase in luminosity is enormous: the 500 to 1 increase in mass along the main sequence corresponds to a 1010 increase in luminosity.)

  • In the lower part of the main sequence, the masses are comparable with the red giants, and in the upper part, with the supergiants.

Figure 8
Figure 8 Stellar mass and the H-R diagram. Masses are given in multiples of M.

Before we try to construct a model of stellar evolution based on these striking features, we have to address the question 'do stars change their mass during their evolution?' There is a good deal of observational evidence to help us to answer it. We observe main sequence stars, red giants and supergiants losing mass in the form of stellar winds streaming outwards. However, the accumulated totals of mass lost by stellar winds are estimated to be only a small fraction of the initial mass of a star. A more impressive mass loss is shown in Figure 9, where you can see shells of material that have been flung off by the central star. Such an object is misleadingly called a planetary nebula (plural: planetary nebulae), because it looks a bit like a planetary disc when viewed under low magnification. They can account for a substantial fraction of the star's mass. In passing, we note that the central star of a planetary nebula now occupies a region in the H-R diagram somewhat hotter and more luminous than the white dwarfs, and it is plausible that it could cool to become a white dwarf.

Figure 9
Figure 9 A planetary nebula: The Helix nebula is the result of a star losing its outer layers at the end of its life. The gas is really in a shell about the remnant of the star but it appears as a ring because we see through it most easily in the direction of our line of sight to the central star. (D. Malin/AAO)

Some stars end their lives more violently than by shedding a planetary nebula.

Question 6

What stars are these, and how do they end their lives?

Answer

Supergiants, which end their lives as Type II supernovae.

In fact, in a Type II supernova, most of the star's mass is blown away.

It thus seems to be the case that throughout most of the life of a star, severe mass loss occurs only when a planetary nebula is shed, with the resulting stellar remnant becoming a white dwarf, or when a massive star ends its life as a Type II supernova.

We are now in a position to suggest a plausible model for some of the stages of stellar evolution based on the features listed above, and on what we know about mass loss. During its main sequence phase, a star does not change its luminosity or photospheric temperature very much, otherwise it would move a good way along the main sequence, and this does not fit in with the large differences in mass along the main sequence (in fact, stars do drift very slightly above the main sequence, so it is a band rather than a narrow line on the H-R diagram). After the main sequence phase the less massive stars become red giants, and the more massive stars become supergiants: you can see that this is consistent with the masses in Figure 8. It is also consistent with the rarity of supergiants: there are very few main sequence precursors. Finally, red giants evolve to the point where they shed planetary nebulae, the stellar remnant evolving to become a white dwarf. Supergiants become star-destroying Type II supernovae.

We are thus continuing to unfold the story of stellar evolution. But there is one huge aspect of the story that, as yet, we have barely touched, and this is whether stars of different mass all evolve at about the same rate. Star clusters provide good observational evidence to help answer this question.

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