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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 .

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.

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.
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 dwarfstar.
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