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All stars spend a significant
amount of their lifetime fusing hydrogen to helium.
This phase of the star's life is called the main
sequence. Examples of main sequence stars are the
Sun, Vega, Sirius and Spica. When the hydrogen in
the core of the star is depleted, the envelope of
the star expands tremendously and the star becomes a
red giant. Examples of red giants are Betelguese,
Arcturus, Aldebaran and Antares. |
The
star constellations are totally imaginary things that poets,
farmers and astronomers have made up over the past 6,000
years. In the second century BC, the Greek astronomer
Hipparchus divided the stars into six brightness groups called
magnitudes, first magnitude the brightest, sixth the faintest.
The system is still used today. |
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Telescopes |
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A star begins as a collapsing cloud of
material composed primarily of hydrogen, along with helium and
trace amounts of heavier elements. Once the stellar core is
sufficiently dense, some of the hydrogen is steadily converted
into helium through the
process of nuclear fusion.
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Stars and gas |
Sky and telescopes |
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Stars and gas exist in enormous collections called Galaxies.
The Milky way can be seen to be the unresolved light from numerous stars.
Stars like Omega Centauri or h and Chi Persei can be seen to be clusters of
many stars. The largest telescopes in the world can record the images of
stars as faint as magnitude 24. Some double stars are recognized because
over the course of time the motion on the sky of one around the other is
noticed. Sirius is a binary system consisting of a visible star, Sirius A
and another invisible star Sirius B. Stars are not all the same age, but
that isn't immediately apparent from looking at the night sky.Astronomers
realize this from careful examination of stellar spectra and from an
increasing understanding of the origin evolution and age of the entire
Universe. By human or geological standards the universe is still in a
certain sence newborn. The sun will become a full fledged red giant 7
billion years from now. Then in a 100 million years more it will exaust its
nuclear fuel, and at last to become a white dwarf star. |
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Astronomy picture of the day's
Through
the telescope we see much fainter, to near 30th magnitude 4 billion times
fainter than the human eye can see alone. A star has is born when a protostar stops shrinking in size this moment the core temperature reaches
10 million K and a reaction, called nuclear fusion begins in the core.Once
the protostar starts burning hydrogen in its core, it quickly passes through
the T-Tauri stage (in a few million years) and becomes a main sequence star
where its total mass determines all its structural properties. |
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