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> Star Formation
> Life and Death of a star
> Physical Description
> Star Catalogues
> Classification of Stellar Spectra
> Double Stars
> Variable Stars
> Neutron Star
> Black Holes
 
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After millions to
billions of years, depending on its initial mass, a star has exhausted all
the hydrogen in its core. Larger and hotter stars consume their hydrogen
much more rapidly than cooler and less massive ones. Once the core's ready
supply of hydrogen is gone, nuclear processes there cease.
Without the outward pressure generated by these reactions to counteract
the force of gravity, the outer layers of the star begin to collapse
inward on the core. The temperature and pressure increase as during
formation of the protostar, but now to even higher levels, until helium
fusion begins at core temperatures of around 100 million kelvin.
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The Last years of a Star
Eventually the energy supply is exhausted.
Stars the size of the sun end their lives as white dwarfs, which are
extremely small, dense and hot. Larger stars end in spectacular
explosions called supernovae, caused by the abrupt collapse of the
stars. One is shown here right in the Large Magellanic Cloud. More
energy is emitted by the dying star in a few seconds than is
produced by the sun in millions of years. |
The very hot core causes the outer layers of the star to expand
enormously; the star becomes as much as 100 times larger than it was
during its main sequence lifetime. It is now a red giant, and the helium
burning phase lasts for a few million years. Almost all red giants are
variable.
What happens next depends, once more, on the star's mass.
Geriatric low-mass stars
What happens after a low-mass star exhausts its hydrogen is not
directly known: the universe is around 13.7 billion years old, which is
less time (by several orders of magnitude, in some cases) than it takes
for the fuel to be exhausted. Current theory is based on computer
modelling.
Some stars may fuse helium in core hot-spots, causing an unstable and
uneven reaction as well as a heavy solar wind. In this case, the star will
form no planetary nebula but simply evaporate, leaving little more than a
brown dwarf.
But a star of less than about 0.5 solar mass will never be able to fuse
helium even after the core ceases hydrogen fusion. There simply isn't a
stellar envelope massive enough to bear down enough pressure on the core.
These are the red dwarfs, such as Proxima Centauri, which live for
hundreds of billions of years. When nuclear reactions eventually cease in
their cores, they will continue to glow weakly in the infrared and
microwave part of the electromagnetic spectrum for many billions of years.
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Mid-sized stars
Once a medium-size star (between 0.4 and 3.4 solar masses)
has reached the red giant phase, its outer layers continue to expand, the
core contracts inward, and helium begins to fuse into carbon. The fusion
releases energy, granting the star a temporary reprieve. In a Sun-sized
star, this process will take approximately one billion years.
Helium burning reactions are extremely sensitive to temperature, which
causes great instability. Huge pulsations build up, which eventually give
the outer layers of the star enough kinetic energy to be ejected as a
planetary nebula. At the center of the nebula remains the core of the
star, which cools down to become a small but dense white dwarf, typically
weighing about 0.6 solar masses, but only the volume of the Earth.
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White Dwarf
White dwarfs are stable because the inward pull of gravity
is balanced by the degeneracy pressure of the star's electrons. (This
should not be confused with the electrical repulsion of electrons, which
maintains the volume of normal matter, but is a consequence of the Pauli
exclusion principle.) With no fuel left to burn, the star radiates its
remaining heat into space for many millions of years.
In the end, all that remains is a cold dark mass sometimes called a black
dwarf. However, the universe is not old enough for any black dwarf stars
to exist.
If the white dwarf's mass increases above the Chandrasekhar limit of 1.4
solar masses, then electron degeneracy pressure fails and the star
collapses. Mass transfer in a binary system may cause such an increase in
mass. This causes the white dwarf to be blasted apart in a type Ia
supernova. These supernovae may be many times more powerful than the type
II supernova marking the death of a massive star. Hence, no white dwarf
more massive than 1.4 solar masses can exist; electron degeneracy pressure
isn't strong enough.
If a white dwarf forms a close binary system with another star, hydrogen
from the larger companion may accrete around and onto a white dwarf until
it gets hot enough to fuse in a runaway reaction, although the white dwarf
remains below the Chandrasekhar limit. Such an explosion is termed a nova.
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Supermassive stars
After the outer layers of a star greater than five solar
masses have swollen into a gigantic red supergiant, the core begins to
yield to gravity and starts to shrink. As it shrinks, it grows hotter and
denser, and a new series of nuclear reactions begin to occur. These
reactions fuse progressively heavier elements, temporarily halting the
collapse of the core.
Eventually, as the star progresses through heavier elements on the
periodic table, silicon fuses to iron-56. Until now, the star has been
maintained by these energy-liberating fusion reactions, but iron cannot
release energy through fusion; instead, iron fusion absorbs energy. Once
this occurs, there is no further energy outflow to counteract the enormous
force of gravity, and the interior of the star collapses nearly instantly.
What happens next is not clearly understood. But whatever it is can cause
a tremendous supernova explosion in a fraction of a second.
The accompanying surge of neutrinos starts a shock wave while the
continuing jets of neutrinos blast much of the star's accumulated
material—the so-called seed elements, lighter than and including iron—into
space. As some of the escaping mass is bombarded by the neutrinos, its
atoms capture them, creating a spectrum of heavier-than-iron material
including the radioactive elements up to uranium. Without supernovae, no
elements heavier than iron would exist.
The shock wave and jets of neutrinos continue to propel the material away
from the dying star and off into interstellar space. Then, streaming
through space, the material from the supernova may collide with other
cosmic debris, perhaps to form new stars, planets or moons, or to serve as
raw materials for a vast variety of living things.
Modern science does not have a clear understanding of the actual supernova
explosion mechanism, nor what exactly remains of the original star. There
are, however, two possibilities:
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Neutron stars
Detailed Article: Neutron
Stars
It is known that in some supernovae, the intense gravity
inside the supergiant forces the electrons into the atomic nuclei, where
they combine with the protons to form neutrons. The electromagnetic forces
keeping separate nuclei apart are gone (proportionally, if nuclei were the
size of dust motes, atoms would be as large as football stadiums), and the
entire core of the star becomes nothing but a dense ball of contiguous
neutrons or a single atomic nucleus.
These stars, known as neutron stars, are extremely small—no bigger than
the size of a large city—and are phenomenally dense. Their period of
revolution can be extremely rapid, with some spinning at over 600
revolutions per second. When these rapidly rotating stars' northern or
southern magnetic poles are aligned with the Earth, a pulse of radiation
is received each revolution. Such neutron stars are called pulsars, and
were the first neutron stars to be discovered.
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Black Holes
Detailed Article: Black Holes
It is widely believed that not all supernovae form neutron
stars. If the stellar mass is high enough, the neutrons themselves will be
crushed and the star will collapse until its radius is smaller than the
Schwarzschild radius. The star has then become a black hole.
Black holes are predicted by the theory of general relativity. According
to classical general relativity, no matter or information can flow from
the interior of a black hole to an outside observer, although quantum
effects may allow deviations from this strict rule. The existence of black
holes in the universe is well supported, both theoretically and by
astronomical observation.
However, questions still remain. Current understanding of stellar collapse
is not good enough to tell whether it is possible to collapse directly to
a black hole without a supernova, if there are supernovae which then form
black holes, or what the exact relationship is between the initial mass of
the star and the final object that remains.
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