1 The life cycle of stars

Stars form from clouds of gas that collapse under the influence of gravity until their central temperatures and pressures become high enough for nuclear fusion to be initiated in their cores. At this point, the clouds stop contracting further and are maintained in hydrostatic equilibrium with a balance between the inward force of gravity and the outward force due to their internal pressure. Stars are typically initially composed of about 75% hydrogen and 25% helium, with small traces of other elements.

Stars are observed with a wide range of masses, ranging from around one-tenth that of the Sun to over a hundred times the Sun’s mass. Other characteristics of stars include their radii, surface temperature and luminosity. Usually it is only stars’ surface temperatures (characterised by their colour) and luminosities (characterised by their brightness) that are directly observable. When these two quantities are plotted against each other for a population of stars, on a so-called Hertzsprung–Russell diagram (see Figure 1), certain patterns emerge in the stars’ locations.

Figure 1 A schematic H–R diagram indicating the locations of four regions where most stars are found. The vertical axis shows stellar luminosity in solar units or absolute visual magnitude; the horizontal axis shows surface temperature in kelvins, spectral type, or colour index defined as the difference between B and V magnitudes. (Notice that temperature increases to the left.) Diagonal lines of constant stellar radius are also shown and some notable named stars are indicated.

Show description|Hide description

The figure illustrates a schematic H–R diagram indicating the locations of four regions where most stars are found. The horizontal axis has three ranges of information: (1) T in kelvin, (2) spectral type, and (3) colour index B minus V. The following points are marked on the horizontal axis, from left to right in logarithmic spacing at unequal distances. For T in kelvin: 40000, 30000, 20000, 10000, 5000, 3000, and 2000. For spectral type: O (about 30000), B (between 20000 and 10000), A (below 10000), F (between 7000 and 6000), G (about 5000), K (below 5000), M (above 3000), and L (above 2000). Colour index: negative 0.4 (between 40000 and 30000), 0 (at about 10000), and 1.5 (above 3000). The vertical axis is labelled M subscript V and ranges from 15 (at the bottom) to negative 10 (at the top) in decrements of 5 units, and L divided by L subscript solar and ranges from 10 raised to the negative fourth power (at the bottom) to 10 raised to the sixth power (at the top) in increments of 100 units. Diagonal parallel lines of constant stellar radius labelled 0.001 R subscript solar, 0.01 R subscript solar, 0.01 R subscript solar, 1 R subscript solar, and 10 R subscript solar from the vertical axis move toward the bottom right until the horizontal axis or the right vertical axis. The main-sequence region starts from the top left end and moves toward the bottom right end. The following stars are marked on the main sequence from the top to bottom: Zeta Puppis, Achernar, Regulus, Sirius A, Procyon A, Sun, 61 Cygni A, 61 Cygni B, Barnard’s Star, Proxima Centauri, and Wolf 359. The white dwarfs region is in the bottom left corner of the graph. Some of the white dwarfs are Sirius B (top left), Procyan B (bottom right), and Van Maanen 2 (bottom right of Procyan B). The red giants are on the top right. Some of the red giants (from bottom to top) are Pollux, Arcturus, and Aldebaran A. The supergiants region is at the top of the graph. Some of the supergiants are as follows: Rigel A (top left), Betelgeuse (top right), and Canopus (below and between Rigel A and Betelgeuse). The stars in the region between 10000 K and 7000 K are bluish-white. Moving to the right, the colours become white (around 5000 K), yellowish-white (between 5000 K and 3000 K), and orangey-white (between 3000 K and 2000 K).

Figure 1 A schematic H–R diagram indicating the locations of four regions where most stars are found. The vertical axis shows stellar ...

Most stars are found in a band running from the top left to bottom right of the H–R diagram, known as the main sequence. This is where all stars spend most of their lives as they undergo nuclear fusion in their cores, converting hydrogen into helium, and releasing energy. On the main sequence, the hottest, most luminous stars sit at the top left; these stars are the most massive and the largest. The coolest, least luminous stars sit at the bottom right; these stars are the least massive and the smallest. The most massive stars have relatively short main-sequence lifetimes of only a few tens of millions of years, whilst the least massive ones have main-sequence lifetimes of hundreds of billions of years (longer than the current age of the Universe).

When most of the hydrogen in the core of a star is depleted, its outer layers will expand and the star will move towards the right of the main sequence, such that its surface appears cooler. This is the region known as the red giant branch. To begin with, hydrogen fusion occurs in a shell around the inert helium core, but as the central temperature and pressure rise further, helium fusion is initiated in the centre of the star. The star then moves back down to somewhat lower luminosities and hotter temperatures, at the lower end of the red giant branch. This new fusion process converts helium into carbon and oxygen in the star’s core, again releasing energy. Low mass stars (< 3 M_☉) and intermediate mass stars (in the range 3–8 M_☉) ultimately begin a second ascent of the red giant branch (known as the asymptotic giant branch, or AGB) when core helium is exhausted and helium fusion only continues in a shell surrounding an inert carbon/oxygen core.

Massive stars (> 8 M_☉) leave the upper main sequence when their core hydrogen is exhausted and then track back and forth along the supergiant branch as new sources of fusion in turn become active and are then depleted. Unlike lower mass stars whose fusion ends with carbon and oxygen, they can initiate further fusion reactions, converting carbon and oxygen into elements such as neon, magnesium, silicon and ultimately iron. They will end up with an ‘onion-like’ structure with shells of less massive elements surrounding shells of heavier elements towards the core. Once the core is composed of iron, no further fusion reactions are possible as reactions to produce nuclei heavier than iron require more energy to be put in than is released.

The subsequent evolution of stars once all nuclear reactions cease is the main topic of this course, which will be explored in the following sections.