Stars are giant balls of gas that produce light and heat through nuclear fusion at their cores. They vary greatly in size, from dwarf stars like our Sun to massive supergiant stars over 200 times the Sun's diameter. Stars end their lives in different ways depending on their mass - smaller stars become white dwarfs, while larger stars explode as supernovae or collapse into neutron stars and other compact objects.

1. Stars...

2. Stars are gigantic balls of gas, mostly hydrogen gas. There is so much gas and other material that the gravity of this huge gas-ball holds everything together. There is so much gravity that the gas becomes very dense and hot. Our own sun is a star. Fortunately, we are far enough away from it that the gravity of the gas can’t pull us in. It would be rather unpleasant inside of a star. What Are stars?

3. The Birth Of Stars After the Big Bang, it took 300,000 years for stable hydrogen and helium atoms to form. Gradually these atoms began to clump together into gas clouds called nebulae . The Hubble telescope has sent back some beautiful images of nebulae - stunning spacescapes filled with bright gas clouds that stretch for millions of miles. These areas are the stellar nurseries of space - from deep inside these placid panoramas, stars explode into life. Learn how stars are made in Birth.

4. Types Of Stars BINARY STARS Most of the stars you look at in the sky aren't single points of light like our Sun. On closer inspection through a telescope, you can see that they are two or more stars that live very close together. These stars are known as binary stars. Anyone looking up at the stars at night could be forgiven for thinking that all stars are fairly similar. But although they might all look the same from down here on Earth, they are strikingly varied. Here are just a few of the different types of stars: Binary stars

5. PROTOSTARS This is a baby star at the earliest observable stage of formation. Although the star is forming from interstellar gas, it is not yet hot enough for nuclear reactions to start in its core. DWARFS Our Sun is a dwarf star. Throughout their lives, stars go through many phases of expansion and contraction. When they are a normal size for their weight they are called 'dwarfs'. Brown dwarfs are failed stars that never heated up enough to explode into a normal star White dwarfs are dying stars that are slowly burning away the last of their fuel. The term 'white' is a bit of a misnomer though, as they range in colour from hot white through to cool red. Eventually however, they will all become black dwarfs - non-luminous dead stars. Dwarfs stars

6. SUPERGIANTS Giant stars have luminosities reaching 1000 times that of the Sun and are up to 200 times as wide. Again, they can be all sorts of colours according to their temperature. The largest stars are called supergiants. The biggest ever found is 10 million times as bright as the Sun. If it were in the same position as the Sun it would engulf the Earth and stretch even further beyond. SUPERNOVAE When an old massive star has run out of all its fuel, it collapses amidst a catastrophic explosion called a supernova. This releases so much light that it can outshine a whole galaxy of stars put together. Supernovae star

7. NEUTRON STARS After exploding as a supernova, a star about twice as big as the Sun would form a neutron star. If we could transport a teaspoon of material from a neutron star to Earth, it would weigh as much as a mountain. It also makes them spin at amazing speeds. Some can revolve hundreds of times a second. PULSARS Pulsars are a special kind of neutron star that emit radio waves in regular bursts. They do this by spinning like the beam on the top of a lighthouse. But rather then emitting light, they sweep a beam of radio waves across the Universe as they spin. Some also emit X-rays. A pulsar has a mass similar to the Sun, but a diameter of only around 10km.

8. VARIABLE STARS Some stars also pulsate. They grow and shrinking in size periodically. These include the strange sounding Cepheid variables, RR Lyrea and Mira star types. GAMMA RAY BURSTS Gamma ray bursts were discovered by chance by military satellites monitoring nuclear testing in the late 1960s. They are intense bursts of gamma and X-ray radiation that only last for a few seconds. But these bursts are millions of times more powerful than the output of an entire galaxy.

9. HYPERNOVAE One possible explanation for the origin of Gamma ray bursts is that they are the result of extremely large explosions, or a 'hypernova'. Hypernovae are the death throes of stars up to 20 times larger than our own Sun. Such explosions emit, for just a few seconds, more radiation than everything else in the Universe combined. Yet despite their destructive power, hypernovae may also be responsible for the creation of stellar nurseries, where new stars (or protostars) are born. Hypernovae explosion

10. Inside Stars Sun Newly born stars begin by fusing their lightest atom, hydrogen. In the core of the star, these hydrogen atoms fuse to form the next heaviest element, helium. Then when the hydrogen runs out, the helium atoms fuse together to form carbon. Then when the helium runs out, the carbon atoms fuse to form oxygen. This manufacture of heavier and heavier elements continues until the star finally dies. The Sun will halt during the carbon-oxygen stage of fusion. Stars a few times bigger than the Sun will eventually have cores made entirely of iron. This is as far as nuclear fusion reactions can go inside stars. All the elements heavier than iron are created in huge supernova explosions. These are the death throes of massive stars.

11. Why Do Stars Twinkle? Twinkle twinkle little star There aren't many nursery rhymes about astronomy. But 'twinkle twinkle little star' makes a useful point. We can tell which lights in the night sky are stars because they appear to twinkle. Planets, on the other hand, don't, they shine steadily in the sky. Stars twinkle because they are very far away, and so appear as tiny points of light in our night sky. Some of this light is absorbed by moving air in the Earth's atmosphere, making the star appear to sparkle. Planets, like Saturn or Jupiter, don't sparkle. This is because they are a lot closer to the Earth and so they look bigger in our sky than stars.

12. The Death Of stars How long a star lives depends upon its mass. The bigger they are, the quicker they die. This might seem odd, but the more mass a star has, then the hotter it gets. The hotter it gets, the quicker it exhausts its fuel supply. The death of the Sun Our nearest star, the Sun, will exhaust its supply of hydrogen fuel in around 4 billion years. Then the Sun's core will collapse under its own gravity. At the same time, its atmosphere will become unstable and start to expand. This will transform the Sun into a huge red giant star. Sun

13. This is not good news for the Earth. Closer planets like Mercury will be completely engulfed by the swelling Sun. Earth will be entirely vaporised and all life on our planet will end. But there's no reason for alarm - we have a few billion years to plan our escape! Over the following billion years, the Sun will gradually die. As a star's core crashes inwards, it eventually becomes hot enough to ignite another of its constituent atoms, helium. Helium atoms fuse together to form carbon. When the helium supply runs out, the centre collapses again and the atmosphere inflates. The Sun isn't massive enough to fully re-ignite its core for a third time. So it goes on expanding, shedding its atmosphere in a series of bursts.

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