Astronomers think that a star begins to form as a dense cloud of gas in the arms of spiral galaxies. Individual hydrogen atoms fall with increasing speed and energy toward the center of the cloud under the force of the star's gravity. The increase in energy heats the gas. When this process has continued for some millions of years, the temperature reaches about 20 million degrees Fahrenheit. At this temperature, the hydrogen within the star ignites and burns in a continuing series of nuclear reactions. The onset of these reactions marks the birth of a star. When a star begins to exhaust its hydrogen supply, its life nears an end. The first sign of a star's old age is a swelling and reddening of its outer regions. Such an aging, swollen star is called a red giant. The Sun, a middle-aged star, will probably swell to a red giant in 5 billion years, vaporizing Earth and any creatures that may be on its surface. When all its fuel has been exhausted, a star cannot generate sufficient pressure at its center to balance the crushing force of gravity. The star collapses under the force of its own weight; if it is a small star, it collapses gently and remains collapsed. Such a collapsed star, at its life's end, is called a white dwarf. The Sun will probably end its life in this way. A different fate awaits a large star. Its final collapse generates a violent explosion, blowing the innards of the star out into space. There, the materials of the exploded star mix with the primeval hydrogen of the universe. Later in the history of the galaxy, other stars are formed out of this mixture. The Sun is one of these stars. It contains the debris of countless other stars that exploded before the Sun was born. In 2006, astronomers were excited about star formation in Arp 220, a super galaxy created by the collision of two other galaxies 250 million light-years from Earth. The Hubble Space Telescope has observed more than 200 huge star clusters, giving scientists a glimpse of what occurred when the universe was young. A surprising find was that the mix of gas and dust in this new galaxy is very similar to our own older Milky Way Before considering how stars die it is worth considering how stars are born. How do we know what stage a star has reached in its life? The answer comes from a detailed theory of star life, which is based on the laws of physics. This theory of "stellar evolution" is one of the great achievements of science in the 20th century. The first question to ask is, ‘what is a star’ ? A star is just a cloud of gas. Stars are born from the tenuous gas which fills the whole of space. This gas is composed mainly of hydrogen atoms, with a sprinkling of helium and some dust. In some places, the gas is disturbed by nearby stars and clumps together in rather more dense interstellar gas clouds. According to gravitational theory, the gas cloud's own gravity makes it attract itself. This causes the gas to pull it n on itself, compressing the cloud to ever higher densities. The centre of the cloud becomes the most compressed region. Here, astronomers expect some of the gas to condense into individual blobs, each held together by its own gravity. When a gas is compressed, it becomes hotter. So, the temperature at the centre of each blob rises to 10 million degrees Celsius hot enough to start nuclear reactions. These reactions turn hydrogen to helium and create vast amounts of energy. As a result, the blob begins to shine: a star is born. Using infrared telescopes astronomers have found that the blobs of gas collapse in rather an odd way. The central parts of a blob fall inwards rather quickly, while the outer parts follow at a more leisurely rate. The blobs are also rotating, quite slowly, but as the outer parts fall inwards, they begin to spin more rapidly-just as ice-skaters spin more quickly when they draw in their arms. As a result, the in falling gas forms a disc around the newly born star at the centre, where the gas is compressed enough for nuclear reactions to start. Within this disc, the gas and the dust that is mixed in it eventually form into a set of planets orbiting the new star. Once the star is shining, it produces a powerful ‘wind’ of hot gas that forces its way outwards in opposite directions, above and below the disc. This wind drives away most of the original gas cloud that hides the star from view. Now we can see the young stars with an ordinary telescope. They light up the final tatters of gas from the original cloud, making it glow as a bright nebula. Nebulae, each surrounding a "nursery" of young stars, form some of the most beautiful sights in the sky. Most famous is the Orion Nebula M42 which you can spot from Europe with the naked eye during the winter months, as a misty patch below the stars of the belt in the constellation of the mighty hunter.

• published: 26 Jul 2015
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