11-3 Careful observations of star clusters provide insight into how a star’s mass influences how stars change over time

Studies of star clusters reveal a curious difference between the youngest and oldest stars in our Galaxy. Stars in the youngest clusters (those with most of their main sequences still intact) are said to be metal-rich, because their spectra contain many prominent spectral lines of heavy elements. (Recall that astronomers use the term “metal” to denote any element other than hydrogen and helium, which are the two lightest elements.) Such stars are also called Population I stars. Our Sun, as an example, is a relatively young, metal-rich, Population I star.

By contrast, the spectra of stars in the oldest clusters show only weak lines of heavy elements. These ancient stars are thus said to be metal-poor, because heavy elements are only about 3% as abundant in these stars as in the Sun. They are called Population II stars. Figure 11-16 shows the difference in spectra between a metal-poor, Population II star and the Sun (a metal-rich, Population I star).

Figure 11-16: RIVUXG Spectra of a Metal-Poor Star and a Metal-rich Star The abundance of metals (elements heavier than hydrogen and helium) in a star can be inferred from its spectrum. These spectra compare (a) a metal-poor, Population II star and (b) a metal-rich, Population I star (the Sun) of the same surface temperature. We described the hydrogen absorption lines H_γ (wavelength 434 nm) and H_δ (wavelength 410 nm) in Section 2-4.

Stellar Populations and the Origin of Heavy Elements

To explain why there are two distinct populations of stars, we must go back to the explosive origin of the universe, which took place some 13.7 billion years ago. As we will discuss in Chapter 15, the early universe consisted almost exclusively of hydrogen and helium atoms, with almost no heavy elements (metals). The first stars to form were likewise metal poor. The least massive of these have survived to the present day and are now the ancient stars of Population II.

The more massive of the original stars evolved more rapidly and no longer shine. But as these stars evolved, helium fusion in their cores produced metals—carbon and oxygen. In the most massive stars, as we will learn in the next chapter, further thermonuclear reactions produce even heavier elements. As these massive original stars aged and died, they expelled their metal-enriched gases into space. (The star called HD 65750 creating the drinking mug–shaped nebula IC 2220 shown in Figure 11-17 is going through such a mass-loss phase late in its life.) This expelled material joined the interstellar medium and was eventually incorporated into a second generation of stars that have a higher concentration of heavy elements. These metal-rich members of the second stellar generation are the Population I stars, of which our Sun is an example.

Figure 11-17: RIVUXG A Mass-Loss Star As stars age and become giant stars, they expand tremendously and shed matter into space. This star, HD 65750, is losing matter at a high rate. The “toby jug” nebula shown in this picture is formed by light reflected off dust being ejected by the star.

The relatively high concentration of heavy elements in the Sun means that the solar nebula from which both the Sun and planets formed must likewise have been metal rich. Earth is composed almost entirely of heavy elements, as are our bodies. Thus, our very existence is intimately linked to the Sun’s being a Population I star. A planet like Earth probably could not have formed from the metal-poor gases that went into making Population II stars.

The concept of two stellar populations provides insight into our own Earthly origins. Helium fusion in red-giant stars produces the same isotopes of carbon (¹²C) and oxygen (¹⁶O) that are found most commonly on Earth. The reason is that Earth’s carbon and oxygen atoms, including all of those in your body, actually were produced by helium fusion. These reactions occurred billions of years ago within an earlier generation of stars that died and gave up their atoms to the interstellar medium—the same atoms that later became part of our solar system, our planet, and our bodies. We are literally children of the stars.

A Star’s Mass Determines Its Main-Sequence Lifetime

Go to Video 11-4

The main-sequence lifetime of a star depends critically on its mass. As Table 11-1 shows, massive stars have short main-sequence lifetimes because they are also very luminous. To emit energy so rapidly, these stars must be depleting the hydrogen in their cores at a prodigious rate. Hence, even though a massive O or B main-sequence star contains much more hydrogen fuel in its core than is in the entire volume of a red dwarf of spectral class M, the O or B star exhausts its hydrogen much sooner. High-mass O and B stars gobble up the available hydrogen fuel in only a few million years, while red-dwarf stars of very low mass take hundreds of billions of years to use up their hydrogen. Thus, a main-sequence star’s mass determines not only its luminosity and spectral type, but also how long it can remain a main-sequence star. In general, the more massive the star, the more rapidly it goes through all the phases of its life. Nonetheless, most of the stars we are able to detect are in their main-sequence phase, because this phase lasts so much longer than other luminous phases.

Like so many properties of stars, what happens at the end of a star’s main-sequence lifetime depends on its mass. If the star is a red dwarf of less than about 0.4 M_⊙, after hundreds of billions of years the star has converted all of its hydrogen to helium. It is possible for helium to undergo thermonuclear fusion, but this requires temperatures and pressures far higher than those found within a red dwarf. Thus, a red dwarf will end its life as an inert ball of helium that still glows due to its internal heat. As it radiates energy into space, it slowly cools and shrinks. This slow, quiet demise is the ultimate fate of the 85% of stars in the Milky Way that are red dwarfs. (As we have seen, there has not yet been time in the history of the universe for any red dwarf to reach this final stage in its evolution.)