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R. N. Mohapatra
neon and finally silicon and then more silicon and sulfur. Finally, silicon fuses
with helium to give argon, calcium, titanium, chromium, iron, and nickel. In
we give the required conditions for various stages of stellar nucleosynthesis to
proceed. With iron, the fusion process stops because unlike the other less heavy
elements, when iron fuses, it does not generate extra heat; rather, it requires
heat to fuse (such processes are called endothermic) Fig. 19.3. Thus production
of heavier elements requires a different site in the universe to be produced. If
a star is more than eight times more massive than the sun, after it reaches the
stage of iron in the core, no further fusion can generate radiation and the star
will undergo a big explosion. At that stage, its core will collapse, giving out
more than 90% of its energy in the form of neutrinos and a flare-up of its
outer layers producing a big light show called Supernova. It will distribute the
new elements and whatever other heavy elements formed prior to that, into
the environment around it. The core, on the other hand collapses to form
either a neutron star or a black hole depending again what the original mass of
the star was prior to explosion. The first supernova was observed by Chinese
observers in 185 AD and may also have been observed by Italian astronomers.
Then in 393 AD, Chinese astronomers may have seen another supernova in
the constellation Scorpius. Another widely observed supernova was in 1054
AD in the constellation Taurus. Supernovae tend to occur every half-century
in a galaxy of the size of our own Milky Way Galaxy. In the whole universe,
pretty much every second, there is a supernova explosion. It is just that they
are not always close to us to be observable.
So far, we have covered up to iron in the periodic table, whose mass number
(i.e. the number of protons and neutrons together) is near 56 and which
has 26 protons in the nucleus, much less than half the proton number and
mass number of the last element known to date. How the remaining elements
formed is a matter of great debate right now, although there are some ideas
and possibly some clues to what really happened. Just to close the section,
another type of supernova occurs if a white dwarf star whose mass is less than
1.4 solar masses (the Chandrasekhar limit) pairs up with a red giant. In this
case, the white dwarf will suck in mass from the red giant until its mass exceeds
1.4 solar masses or the Chandrasekhar limit, in which case it will go unstable
and explode, leaving interstellar dust that contains carbon, oxygen, and nickel.
Thus, again some new elements have been formed in the process. All these
are collectively called stellar nucleosynthesis that follows the primordial BBN
process (Fig. 19.3).
R. N. Mohapatra
neon and finally silicon and then more silicon and sulfur. Finally, silicon fuses
with helium to give argon, calcium, titanium, chromium, iron, and nickel. In
we give the required conditions for various stages of stellar nucleosynthesis to
proceed. With iron, the fusion process stops because unlike the other less heavy
elements, when iron fuses, it does not generate extra heat; rather, it requires
heat to fuse (such processes are called endothermic) Fig. 19.3. Thus production
of heavier elements requires a different site in the universe to be produced. If
a star is more than eight times more massive than the sun, after it reaches the
stage of iron in the core, no further fusion can generate radiation and the star
will undergo a big explosion. At that stage, its core will collapse, giving out
more than 90% of its energy in the form of neutrinos and a flare-up of its
outer layers producing a big light show called Supernova. It will distribute the
new elements and whatever other heavy elements formed prior to that, into
the environment around it. The core, on the other hand collapses to form
either a neutron star or a black hole depending again what the original mass of
the star was prior to explosion. The first supernova was observed by Chinese
observers in 185 AD and may also have been observed by Italian astronomers.
Then in 393 AD, Chinese astronomers may have seen another supernova in
the constellation Scorpius. Another widely observed supernova was in 1054
AD in the constellation Taurus. Supernovae tend to occur every half-century
in a galaxy of the size of our own Milky Way Galaxy. In the whole universe,
pretty much every second, there is a supernova explosion. It is just that they
are not always close to us to be observable.
So far, we have covered up to iron in the periodic table, whose mass number
(i.e. the number of protons and neutrons together) is near 56 and which
has 26 protons in the nucleus, much less than half the proton number and
mass number of the last element known to date. How the remaining elements
formed is a matter of great debate right now, although there are some ideas
and possibly some clues to what really happened. Just to close the section,
another type of supernova occurs if a white dwarf star whose mass is less than
1.4 solar masses (the Chandrasekhar limit) pairs up with a red giant. In this
case, the white dwarf will suck in mass from the red giant until its mass exceeds
1.4 solar masses or the Chandrasekhar limit, in which case it will go unstable
and explode, leaving interstellar dust that contains carbon, oxygen, and nickel.
Thus, again some new elements have been formed in the process. All these
are collectively called stellar nucleosynthesis that follows the primordial BBN
process (Fig. 19.3).