STELLAR EVOLUTION AND THE ELEMENTS

The pioneering efforts to understand the abundance of various nuclei in nature focused on very late stages of stellar evolution, culminating in supernova explosions when the star was sufficiently massive. Continued, more detailed observations over the past 65 years have led to a realization that many different processes have contributed to nucleosynthesis in our universe. Research is still ongoing and current results may change considerably as time goes on.

One of the important contributions of the original B2FH paper was a painstaking unraveling of the processes occurring in old stars, generating the nuclei beyond helium. [Known as "metals," to astrophysicists!]

When p is depleted enough in the core, stars of sufficient mass can collapse to a core temperature of 10 to 20 keV and density of 102 to 105 grams per cubic centimeter. Under such conditions the unstable nucleus 8Be can be in equilibrium with 4He (the decay width is only 2.5 eV, half-life about 10-16 sec). The accidental fact that there is a 0+ energy level in 12C at just above (by 0.29 MeV) the zero energy state for 4He + 8Be* creates a strong resonance which allows the excited state of 12C sometimes (very rarely!) to gamma-decay before breaking back up into three 4He's.  The width for 2γ decay is only about 3 × 10-3 eV. The following  4He + 12C → 16O process is much more complex, and still an object of detailed study today.  In fact there is still a good deal of controversy as to what specific fractions of nuclei heavier than carbon were predominantly formed by helium-capture.  Routine stellar nucleosynthesis starts to get extremely tricky around A = 44, and more unusual phenomena such as neutron-star collisions become important.
Helium “sucking,” plus fusion of 12C, 16O and 28Si, and other more complex processes, can produce the abundant nuclei heavier than helium, terminating at 56Fe and 56Ni.  At that point, the star is stuck... any further fusion has a negative Q value, and is a sink rather than source of energy, no matter how much further the star collapses, no matter to what extremes of core density or temperature....






A beautiful test of stellar evolution... in a globular cluster, all stars are the same age, so the HR diagram is a perfect snapshot of star status, as a function of initial mass.


The “planetary nebula” is not only always beautiful, but a plentiful source of heavy atoms.
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The age of the universe is known to better than 1%, at 13.8 billion years, comparable to the time a single solar-mass star spends on the Main Sequence.  Yet already the rate of formation of new stars has dropped dramatically.  In fact, it has dropped so far that even if the universe continued to exist (and expand) for an infinite time, the number of new stars added during that time would be only a few percent of the number of existing stars!  We are very fortunate to live at the time we do happen to live in, when so much of the universe is still visible and wildly active.


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