9.11. NUCLEOSYNTHESIS AND EVOLUTION OF THE ISM 137
e most massive stars can make either a black hole or a neutron star, but I have not said
which do one and which do the other. e reason is that it is not known with any clarity. All of
the stars with initial masses greater than about 10 M
@
undergo supernova explosions, but it is
difficult to calculate exactly what percentage of the mass of the star is lost in the explosion. And
that is the key point; it is the mass of the contracting core left over that decides whether it will
be a neutron star or a black hole.
It has, however, become increasingly clear that while neutron stars are relatively common,
black holes formed by collapsing stars are rather rare. is could mean that only the very most
massive (and thus rarest) stars eventually form black holes. But it could also mean that only a
very narrow range of initial masses eventually lead to a black hole.
9.11 NUCLEOSYNTHESIS AND EVOLUTION OF THE ISM
Nuclear fusion in the cores of stars makes heavier elements out of lighter ones. In a supernova
explosion, these fusion reactions generate all of the elements on the periodic table, as they are
blasted into space to mix with, and become part of, the interstellar medium (ISM). is pro-
cess of creating heavier elements from lighter elements by fusion in the cores of stars is called
nucleosynthesis.
Recall from Part II of e Big Picture that the Big Bang produced mostly hydrogen and
helium, with only minuscule traces of everything else—what astronomers refer to as “metals.”
And so the presence of what Earth—and we—are made of, is due to nucleosynthesis. e uni-
verse would be a far less interesting place without it, if hydrogen and helium were the only two
elements. Helium does not combine chemically with anything, and so there would be only indi-
vidual helium atoms, individual hydrogen atoms, and H
2
molecules. No bobolinks,
1
barley malt
distillates, or ill-tempered felines in such a universe!
Since stars form from the ISM wherever it is dense enough, the process of nucleosynthesis
continuously alters these parts of our Galaxy. In particular, the metallicity gradually increases over
time, as successive generations of stars form, generate heavy elements in their cores, and blast
these synthesized metals out into space to rejoin the ISM. And so a region of the ISM that
shows a high metallicity betrays a billions-of-years history of continuous star formation.
1
ere are, unfortunately, fewer of these every year in the lovely universe we do have.
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