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A star of 15 solar masses exhausts its hydrogen in about one-thousandth the lifetime of the sun. It proceeds through the red giant phase, but when it reaches the triple-alpha process of nuclear fusion, it continues to burn for a time and expands to
Supergiant an even larger volume. The much brighter, but still reddened star is called a red supergiant (it is blue supergiants for O, B stars). Some of these supergiants are unstable and form the very important Cepheid variables (as standard candles for determining distance to galaxies). In their final stages, supergiants will explode into supernovae. The collapse of these massive stars may produce a neutron star or a black hole.

Figure 08-09 Supergiant
[view large image]

The enormous luminous energy of the stars comes from nuclear fusion processes in their centers. Depending upon the age and mass of a star, the energy may come from proton-proton fusion, helium fusion, or the carbon cycle. For brief periods near the end of their life, heavier elements up to iron may fuse, but since iron is at the peak of the binding energy curve, the fusion of elements more massive than iron would soak up energy rather than deliver it. Thus once a star's core has been converted into iron, it can no longer be supported by the thermonuclear processes and, bereft of support, it collapses almost instantaneously to trigger a supernova event.

Figure 08-09 shows the structure of a supergiant star. The element indicated in each shell denotes the nuclear burning of the heaviest elements. The time scales of the various nuclear burning are for a main-sequence star of 25 Msun. The stage is shorter for heavier element because there is less fuel available and lower efficiency in the burning process (see Nuclear Binding Energy in Figure 14-01). As the different elements are exhausted in the core, it undergoes gravitational collapse until the temperature is high enough to ignite the next available elements. The outer layers, where the lighter elements are still being burnt, are pushed outwards by the increasing radiation pressure. This leads to an onion-like shell structure. In every shell, different nuclear reactions are taking part (except in the outermost one, still made up mostly of hydrogen) and new elements are being created.

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