Most of the gas in interstellar clouds is hydrogen. And at such high temperatures, the hydrogen atoms start to combine, or fuse together. This fusion reaction produces enormous amounts of energy as light, heat and other radiation. When this happens, the collapsing cloud starts to shine as a star.
The outward "pressure" of the radiation coming from the core of the new star acts against the matter that is collapsing under gravity. Eventually the two balance each other, and the collapse ceases. The star settles down and begins to shine steadily. It takes a star the size of the Sun about 50 million years to reach this state.
The hottest stars are blue-white in colour and burn their hydrogen fuel very quickly. The Sun, a small yellow star, burns hydrogen more steadily. Proxima Centauri, the closest star to the Sun, burns its gas very slowly and is a cool, red star. The speed at which the stars burn hydrogen determines how long they will live.
A Sun-sized star shines steadily for about 10,000 million years, until the hydrogen fuel in its core is used up. The star then begins to collapse again under gravity. The heat triggers off hydrogen fusion in the gassy shell surrounding the core. The shell heats up, causing the star to expand and brighten. But the core continues to shrink and get hotter.
Blue giants have a short life, and explode dramatically. The Sun will continue to burn for another 5 billion years. Then it will expand into a large red giant and finally shrink to a white dwarf. Proxima Centauri, however, will remain unchanged for tens of billions of years.
|The first white dwarf to be recognized ( in 1915) was the faint companionof the Dog Star, Sirius, brightest star in the heavens. Properly termed Sirius B, it is about the size of Earth, but is 350,000 times more massive.|