Stars
Birth
of a Star
Protostars are stars that are about to be born.
These are glowing clouds of interstellar gas and dust, which look
like dark spots in the midst of light. Gravity causes every atom and
every bit of dust to pull on every other one and all move to the
center, causing the protostar to collapse. Having begun with a diameter of perhaps 1.5
trillion km, the protostar now shrinks at a very fast rate about
1,000 years, to a diameter of about 80 million km. Because the atoms
move faster and faster as they fall toward the center, friction is
created as they rub together and the temperature rises. The protostar
starts at a temperature of about 100 K and over 1.000 years, rises to about 4,250 K. This heat
causes the protostar to glow in with its own light, giving off even
more light than our Sun even though it is not nearly as hot. After about 10,000
years, the protostar's surface temperature is up to about 4,500 K and
it is now 100 times as luminous as the Sun; after another 100,000
years the temperature is 5,000 K even though the protostar has been
shrinking the entire time and only gives off about 10 times the light
of the Sun. There is little change in temperature over the next 10
million years but the brightness continues to drop as the protostar
contracts. The next 20 million years is the last stage of the
protostar's development where it equals the Sun in luminosity and its
size becomes fixed at about 1.6 million km in diameter. 30 million
years after the pocket of gas began to form, a star is born.
Bright Stars
A star has is born when a protostar stops
shrinking in size. At this moment the core temperature reaches 10
million K and a reaction, called nuclear fusion begins in the core.
Nuclear
fusion is the joining, or fusing, of small
atomic nuclei to create larger ones. The nuclei of two hydrogen atoms
join to form the nucleus of one helium atom and there is a great release of energy.
We see and feel this energy in the form of light and heat from the
Sun; all other stars give off the same energy but they are too far
away for us to observe it. The fusion in a star continues for about
90 percent of its life, but cannot go on forever. Our Sun is expected
to continue this way for just over another 5 billion years. The life
cycle of a star can be compared to a human life: the 30 million years
as a protostar is like infancy or early childhood, from birth to
about age 4. The 10 billion year period of nuclear fusion as a bright
star is the time of youth and maturity, from age 4 to 76. Last is old
age. But while humans tend to grow smaller, the star grows larger. It
also becomes red in colour and for these reasons is called a
red
giant.
Red Giants
Without the outward
flow of energy from the star's core, gravity now takes over,
squeezing and compressing the core making it grow much smaller. The
core radiates heat until it reaches about 4 million K; it is now
releasing great amounts of energy which carries immense amounts of
hydrogen gas out to the surface. The star begins to grow larger,
however, it does not become brighter. As a result of a temperature
drop, the star now glows red instead of blue-white. Astronomers have
located a number of red giants in the universe. Alderbaran is in the
constellation Taurus, Antares in Scorpio,
Arcturus in Bootes, and Betelgeuse in Orion. Most of the red giants
are found in globular clusters, which are groups of up to one million
stars that move together through space. Astronomers believe that the
main source of carbon and oxygen in the universe is the very hot
cores of red giants and these elements make like as we know it
possible.
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Two globular clusters in the universe
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White Dwarfs
As the star cools off, a cycle begins. As a result
of the temperature drop, the outer gas layer spreads out; this causes
the temperature to drop further, and this in turn causes the gas to
spread farther. It eventually spreads to far that it actually
separates from the body of the star altogether. A cloud of glowing
atoms moves out in all directions; this is called a planetary nebula.
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Planetary Nebula: "Hubble 5"
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It is called such because early astronomers
believed it looked like a distant planet. Today we know that a
planetary nebula has nothing to do with a planet, but still the name
remains. The gas particles eventually mix with the clouds of gas
normally found between the stars and the planetary nebula disappears.
All that now remains of the star is the central core which, because
it is no longer giving off energy, begins to collapse bit by bit.
This process can be compared to squishing a large closetfull of
clothes into a small suitcase. The clothes are still the same, they
just take up much less room. Likewise, all the matter that was in the
large star is still there, only now is is packed much more tightly.
This occurs until the star is about the same size as planet Earth.
Astronomers guess that a teaspoon of this matter weighs about 907
metric tonnes! The star is still very hot, so hot that it gives off a
white light and thus is called a white
dwarf. With the passage of billions of
years the white dwarf cools off, from white hot to yellow, and then
red, eventually becoming completely cold and black. Astronomers have
located about 500 hundred white dwarfs and of the 20 stars closest to
Earth, 2 are white dwarfs.
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White dwarf stars in globular cluster M4.
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