Silicon burning

In astrophysics, silicon burning is a very brief[1] sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8–11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the main sequence on the Hertzsprung-Russell diagram. It follows the previous stages of hydrogen, helium, carbon, neon and oxygen burning processes.

Silicon burning begins when gravitational contraction raises the star’s core temperature to 2.7–3.5 billion kelvins (GK). The exact temperature depends on mass. When a star has completed the silicon-burning phase, no further fusion is possible. The star catastrophically collapses and may explode in what is known as a Type II supernova.

Nuclear fusion sequence and the alpha process

After a star completes the oxygen burning process, its core is composed primarily of silicon and sulfur.[2] if it has sufficiently high mass, it further contracts until its core reaches temperatures in the range of 2.7–3.5 GK (230–300 keV). At these temperatures, silicon and other elements can photodisintegrate, emitting a proton or alpha particle.[2] Silicon burning entails the alpha process, which creates new elements by adding one of these alpha particles[2] (the equivalent of a helium nucleus, two protons plus two neutrons) per step in the following sequence:

28
14
Si
 
4
2
He
 
→  32
16
S
32
16
S
 
4
2
He
 
→  36
18
Ar
36
18
Ar
 
4
2
He
 
→  40
20
Ca
40
20
Ca
 
4
2
He
 
→  44
22
Ti
44
22
Ti
 
4
2
He
 
→  48
24
Cr
48
24
Cr
 
4
2
He
 
→  52
26
Fe
52
26
Fe
 
4
2
He
 
→  56
28
Ni
56
28
Ni
 
4
2
He
 
→  60
30
Zn
   [3]

The entire silicon-burning sequence lasts about one day and stops when nickel-56 has been produced. The star can no longer release energy via nuclear fusion because a nucleus with 56 nucleons has the lowest mass per nucleon (any proton or neutron) of all the elements in the alpha process sequence. Although iron-58 and nickel-62 have slightly higher binding energies per nucleon than iron-56,[4] the next step up in the alpha process would be zinc-60, which has slightly more mass per nucleon and thus, is less thermodynamically favorable. Nickel-56 (which has 28 protons) has a half-life of 6.02 days and decays via β+ decay to cobalt-56 (27 protons), which in turn has a half-life of 77.3 days as it decays to iron-56 (26 protons). However, only minutes are available for the nickel-56 to decay within the core of a massive star. The star has run out of nuclear fuel and within minutes begins to contract. The potential energy of gravitational contraction heats the interior to 5 GK (430 keV) and this opposes and delays the contraction. However, since no additional heat energy can be generated via new fusion reactions, the contraction rapidly accelerates into a collapse lasting only a few seconds. The central portion of the star gets crushed into either a neutron star or, if the star is massive enough, a black hole. The outer layers of the star are blown off in an explosion known as a Type II supernova that lasts days to months. The supernova explosion releases a large burst of neutrons, which synthesizes in about one second roughly half the elements heavier than iron, via a neutron-capture mechanism known as the r-process (where the “r” stands for rapid neutron capture).

Binding energy

The graph above shows the binding energy of various elements. Increasing values of binding energy can be thought of in two ways: 1) it is the energy required to remove a nucleon from a nucleus, and 2) it is the energy released when a nucleon is added to a nucleus. As can be seen, light elements such as hydrogen release large amounts of energy (a big increase in binding energy) as nucleons are added—the process of fusion. Conversely, heavy elements such as uranium release energy when nucleons are removed—the process of nuclear fission. In stars, rapid nucleosynthesis proceeds by adding helium nuclei (alpha particles) to heavier nuclei. Although nuclei with 58 and 62 nucleons have the very lowest binding energy, fusing a helium nucleus into nickel-56 (14 alphas) to produce the next element—zinc-60 (15 alphas)—actually requires energy rather than releases any. Accordingly, nickel–56 is the last fusion product produced in the core of a high-mass star. Decay of nickel-56 explains the large amount of iron-56 seen in metallic meteorites and the cores of rocky planets.

See also

References

External links

  • by Arthur Holland and Mark Williams of the University of Michigan
  • by Ian Short
  • Tufts University
  • Chapter 21: Stellar Explosions, by G. Hermann
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