19-5 Stellar evolution has produced two distinct populations of stars

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 from Section 17-5 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. The Sun 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 also called Population II stars. The stars in globular clusters are metal-poor, Population II stars. Figure 19-15 shows the difference in spectra between a metal-poor, Population II star and the Sun (a metal-rich, Population I star).

Figure 19-15: R I V U X G
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 5-8.

Stellar Populations and the Origin of Heavy Elements

To explain why there are two distinct populations of stars, we must go back to the Big Bang, the explosive origin of the universe that took place some 13.7 billion years ago. As we will discuss in Chapter 26, the early universe consisted almost exclusively of hydrogen and helium, with almost no heavy elements (metals). The first stars to form were likewise metal poor. The least massive of these stars 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 Chapter 20, further thermonuclear reactions produced even heavier elements. As these massive original stars aged and died, they expelled their metal-enriched gases into space. (The star shown in Figure 19-5 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.

The relatively high concentration of heavy elements in the Sun means that the solar nebula, from which both the Sun and planets formed (see Section 8-4), 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 origins. Recall from Section 19-3 that 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.