The
Birth of the Sun
by Milu Karp
Our Sun as we see it today has been growing for some
five billion years. It is a star born of the same material as all
of the other stars in our Universe, but our Sun has pulled our unique planets
into its orbit and has given life to our planet Earth. To us it appears
as a solid glowing sphere, but at a distance of 93 million miles away,
it is a pulsating, rotating mass of hot plasma. Our Sun was born
from the remnants of past stars, a continuing process of death to life
in an ongoing cycle of stellar birth.
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The Sun is Born in a Cloud of Dust and Gas
It is believed that our Sun was born within the cloudy remains of a supernova
(Kaufmann, p.251). A supernova is the violent explosion and eventual
death of a massive star. As these remains of supernova burst through
interstellar medium, the medium glows with excited atoms.
As with the birth of our Sun, the supernova remains came into contact with
a molecular cloud and squeezed the material to a point of enough pressure
to form dense seeds of individual clouds. Within these large clouds,
gravity allows small clumps of gas and dust to form by mutual attraction
that continue to collapse to form interstellar clouds. The
temperature in each of these clouds must be cool enough to allow gravitational
collapse to continue, because if the temperature is too hot, pressure inside
the cloud will overtake gravity's pressure and stars will never form.
Inside these clouds, very compact regions called dense cores exist,
with temperatures in the cores ~10K (Kaufmann, p.252), cool enough for
gravity to take control. One molecular cloud may contain thousands
of dense cores, eventually becoming huge collections of new stars, called
open clusters. The fraction of the dust and gas that is actually
converted into stars is only 25% on average, and the more massive new stars
in the open clusters heat up the remaining gas up to a temperature of 10,000K,
which drives the excess gas and dust out of the cluster (Iben, p.37).
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A New Sun takes Shape
Because the temperature inside these dense cores is so cool, gravity continues
to control the new star's growth and the new star collapses even further
and at the same time gathering material from the surrounding area.
The outer layers of the stellar body collapse inward as well, but at a
much slower rate. The glowing object is now a protostar, and
its core develops a very large mass as accretion onto the star continues
for several thousand years. Matter falls in from the surrounding
shell, but pressure from radiation and material flowing off of the dense
core keeps this shell matter from ever reaching the center. The Sun,
as a protostar, has the same mass as the present day Sun, but its diameter
is five times larger than today (Kaufmann, p.253).
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The Adolescent Sun
After several thousand years, accretion ceases and the new star is now
called a pre-main sequence star. In this stage of the Sun's
life, contraction begins again, but much slower than before, until the
temperature in the core reaches some ten million Kelvin (Kaufmann, p.254),
at which point the core is hot enough for hydrogen fusion to begin.
The process of creating helium from hydrogen fusion exerts sufficient energy
to finally stop gravitational collapse onto the core. Hydrogen will
burn in the star's core for tens of millions of years, and eventually the
outer shell of the pre-main sequence star diffuses, and the hot core is
exposed to open space.
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The Sun as We Know It
A point is finally reached when the outside gravitational pressure of the
star is equal to the inside thermal pressure of the star's core, a state
called hydrostatic equilibrium (Kaufmann, p.259). It is at this
moment that the Sun is said to be born, now a main-sequence star.
Our Sun will remain as a main-sequence star for ten billion years.
At present day, the Sun has already been in this stage for ~five billion
years, so it has only five billion more years remaining before it begins
to die. Hydrogen continues to fuse into helium in the core during these
ten billion years, until no more hydrogen is left to burn.
Our Sun, as we see and feel it today is comprised of individual layers
(Kaufmann, p. 207-208). Above the Sun's core is the photosphere,
the layer that emits the majority of visible light that we can see from
Earth. The two layers that surround the photosphere are transparent
to visible light, so therefore we can see right through them. Above
the photosphere is the chromosphere, which is only visible to us during
a soloar eclipse as a dim pink layer around the shadow. The final
layer is the corona, so thick that it actually has a width of several million
kilometers above the chromosphere. Sunspots, seen as small, dark
spots against a bright orange surface, exist on the photosphere layer of
the Sun.
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The Sun's Death
At the point of hydrogen depletion, the star's core will no longer be able
to support the weight pushing in from all directions and the helium core
will collapse, become even hotter. The hydrogen in the shell falling
inward will begin to convert to helium, a process called shell-helium
fusion. This shell will then move outward, dropping its converted
helium onto the core that continues to contract. The Sun then becomes
a giant when the hydrogen fusing shell heats up and swells outward
into space. Our Sun will expand to the size of Earth's present orbit
(1AU) (Kaufmann, p.260).
A red supergiant is the name of the next stage of our Sun's
death (Iben, p. 39). It will be made up of almost pure helium in
the core, with an expanding shell of hydrogen and helium. It is called
a red supergiant because at this stage, the Sun will glow red. This
expansion of the hydrogen fusing shell will increase the mass of the helium
core, and this added mass will cause the core to contract and its temperature
to rise. Temperatures in the core will reach some 100 million Kelvin.
Helium fusion begins, creating carbon and oxygen (Kaufmann, p. 262):
He + He + He ---------> C + y(energy)
Carbon will then fuse with other helium atoms to make oxygen:
C + He --------> O + y(energy)
This sudden fusion of helium is called a helium flash (Iben,
p.39). After a helium flash, which will occur several times, the
star will contract as its energy output decreases. The giant is then
very hot, but because it is small in size, it is also dim. The core
will soon be converted entirely into carbon and oxygen, and it will shrink
once again, heating its shell. Then the helium shell outside the
core will undergo helium fusion. Several helium flashes will occur,
each separated into rest periods of some 300,000 years. This helium
shell will grow thicker after every flash. A single flash is then
all it will take to overcome gravity and the outer layers of the star will
be pushed off of the core and into open space. The remaining object
of glowing gas will then be a planetary nebula.
Eventually the helium in the core will be used up, and the core will
then contain oxygen and carbon nuclei. At this point, the Sun is
hundreds of times larger than its present size, with a low surface temperature.
This is called an Asymptotic Giant Branch star (AGB) (Iben, p.40).
This AGB star will pulsate. Matter is lost from its surface in winds,
which blow away in them such things as carbon and dust grains. The
winds carry in them material that will eventually form future stars.
The Sun will remain in this phase for many hundred thousand years.
Rapid collapsing raises the core temperature to some 100,000K, causing
the exterior gases to fluoresce. The burning of hydrogen in
the nebula's center will eventually end, and a white dwarf will
be all that remains of the core: a cool, carbon-oxygen rich mass of cinder.
Bibliography:
Iben, Icko Jr., "The Lives of Stars: From Birth to Death and Beyond,"
December 1997, Sky and Telescope
Cohen, Martin, "In Darkness Born," Cambridge University Press,
1988
Cooke, Donald A., "The Life and Death of Stars," New York: Crown,
1985
Shklovskii, Iosif S., "Stars: Their Birth, Life, and Death, "
San Fransisco: W.H. Freeman, 1978
Mitton, Simon, "Daytime Star, the story of our Sun," New York: Scribner,
1981
Kaufmann, William J., "Discovering the Universe," W.H. Freeman and
Company, 1996