The Life and Death of Stars

Stars

Notes from a presentation by Christian Ready at
Windycon 38 on Saturday, November 12, 2011. I have added web links.

A lot has been learned about stars from new telescopes, both in space (e.g. Hubble) and on Earth (e.g. Keck).

Coming out of the Big Bang the normal (baryonic) matter of the universe was about 3/4 Hydrogen
and 1/4 Helium. See Found:
The First Atoms In The Universe!!!
. Technical details can be found in Big
Bang Nucleosynthesis (BBN)
.

Stars come from gas
clouds
, such as the Orion nebula. In the middle is the Trapezium,
a collection of hot and bright (spectral types O and B) stars.

Red dwarfs in regions of cooler gas. They are very common. They are so many of them that
our sun is brighter than about 85% of the stars in our galaxy.

Star formation begins when some of the gas cloud begins to contract into a
protostar. The gas cloud may be ablated by
radiation from larger nearby stars, sometimes with spectacular results, such as the
Mystic Mountain of the Carina nebula. A protostar shines just by the radiation
from gravitational contraction, not by nuclear fusion. As it contracts it is likely
to also form a protoplanetary
disk
. The gas cloud may be ablated by radiation from larger nearby stars, as seen in the
Mystic Mountain of the Carina nebula.

HerbigHaro
jets
are created as the protostar contracts into a star. The changes in some of these have been
observed in real time.

If the protostar is sufficiently massive, then the heat from the gravitational contraction will eventually
make it hot enough for nuclear fusion to start. At that point the protostar becomes a star.

Planets may in turn be created out of the protoplanetary disk. Many exoplanets (planets orbiting a star
other than our sun) have been discovered. When an exoplanet passes in from of its star (as seen from
Earth), there is a small but measurable drop in the amount of light that reaches us. The observed
light curve can give a lot of information.
A couple interesting exoplanets are HAT-P-7b
and Kepler 16b, informally
called “Tatooine.”

Inside a star, fusion takes place in the core, which is much hotter than the surface. The energy
from fusion balances the inward force of gravity, preventing further gravitational collapse. The
star is in hydrostatic equilibrium. This phase of a star’s existence is called main sequence burning.
Our Sun has been in that state for about five billion years, and will remain there for about another
five billion.

Eventually the star will run out of Hydrogen in the core. Gravitation, no longer opposed by
fusion, will make the core contract. This will warm the core some more and ignite Hydrogen fusion
in a shell around it. The extra energy will cause the outer layers to greaty expand, and the star will
become a red giant. See Stellar
Evolution I – Solar Type Stars

The contraction of the core will raise its temperature, perhaps to the point where three Helium nuclei
can fuse into a carbon nucleus (the triple alpha process. This yields more energy, but is much less efficient than Hydrogen
fusion. Some of those new Carbon nuclei may fuse with other Helium nuclei, yielding Oxygen. Large
amounts of matter will be ejected from the star. When the Helium at the core is finally gone
there will be a particularly large ejection, resulting in a planetary nebula. Beautiful examples are the
Helix
Nebula
, the Bug
Nebula
(from a binary star system) and the Cat’s
eye nebula
.

For most stars this is the end of the line. The core cannot contract enough to ignite further fusion.
Complete graviational collapse will be prevented by electron
degeneracy
, producting a white dwarf. As the dwarf age it will cool down as energy is radiated
away.

In larger stars the contraction of the Carbon-Oxygen core may raise the temperature to ignite further
short-lived fusion reactions, yielding Neon, Sodium, Silicon, Sulfur, and eventually Iron. However this
is the end. You cannot gain energy from Iron fusion, rather the opposite. At this point the core finally
collapses. The outer layer explodes in a supernova. The core ends up as a neutron star=pulsar. The
Crab
Nebula
is what remains from a supernova in 1054 AD, with a pulsar in the middle.
Cassiopeia
A
is another supernova remant.

The energy of the supernova allows nuclei heavier than iron to be formed by fusion, and it is the only way that can happen. Furthermore, the explosion disperses all those elements from the star into the galaxy. They join the interstellar gas, some of which gets recycled into new stars, planets, and people.

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