Silicon-burning Process - Nuclear Fusion Sequence and The Alpha Process

Nuclear Fusion Sequence and The Alpha Process

Stars with very low mass (no greater than about 0.4 solar masses) run out of fuel after the hydrogen in their cores has been consumed and fused into helium. Stars with an intermediate mass (greater than 0.4, but less than about 8–11 solar masses) can go on to "burn" (fuse) helium into carbon by means of the triple-alpha process. These stars end their lives when the helium in their cores has been exhausted; thus they end up with carbon cores. High mass stars (more than about 8–11 solar masses) are able to burn carbon because of the extraordinarily high gravitational potential energy bound in their mass. As massive stars contract, their cores heat up to 600 MK and carbon burning begins which creates new elements as follows:

12 6C	+	4 2He	→	16 8O
16 8O	+	4 2He	→	20 10Ne
20 10Ne	+	4 2He	→	24 12Mg

The chemical elements are defined by the number of protons in their nucleus. In the elements listed above, the superscript denotes a particular isotope (form of a chemical element having a different number of neutrons) in terms of its molar mass.

After a high-mass star has burned all the carbon in its core, it contracts, gets hotter, and begins burning the oxygen, neon, and magnesium as follows:

24 12Mg

+

4 2He

→

28 14Si

After high-mass stars have nothing but sulfur and silicon in their cores, they further contract until their cores reach temperatures in the range of 2.7–3.5 GK (230–300 keV); silicon burning starts at this point. Silicon burning entails the alpha process which creates new elements by adding the equivalent of one helium nucleus (two protons plus two neutrons) per step in the following sequence:

28 14Si	+	4 2He	→	32 16S
32 16S	+	4 2He	→	36 18Ar
36 18Ar	+	4 2He	→	40 20Ca
40 20Ca	+	4 2He	→	44 22Ti
44 22Ti	+	4 2He	→	48 24Cr
48 24Cr	+	4 2He	→	52 26Fe
52 26Fe	+	4 2He	→	56 28Ni
56 28Ni	+	4 2He	→	60 30Zn		(energy is consumed and the star's core collapses)

The entire silicon-burning sequence lasts about one day and stops when nickel-56 has been produced. Nickel-56 (which has 28 protons) has a half-life of 6.02 days and decays via beta radiation (in this case, "beta-plus" decay, which is the emission of a positron) 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. At the end of the day-long silicon-burning sequence, the star can no longer release energy via nuclear fusion because a nucleus with 56 nucleons has the lowest mass per nucleon (proton and 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, the next step up in the alpha process would be zinc-60, which has slightly more mass per nucleon and thus, would actually consume energy in its production rather than release any. 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).