What is a star?
Stars are huge balls of burning gas that are scattered throughout the Universe. They burn for millions of years, giving off both light and heat. Stars produce energy by a process called nuclear fusion. The coolest stars are red and dim, while the hottest stars give off blue-white light. The temperatures on the surface range from 3,500 degree Celsius for cooler stars to over 40,000 degree Celsius for the hottest stars.
A new star is born when gas and dust are drawn together by gravity, forming a huge clump. It heats up until nuclear fusion begins, and the new star appears.
How big are stars?
Our own Sun is quite a small star, even though it measures 1,392,530 km across, which is 109 times more than the Earth’s diameter. If the Sun were the size of a football, the Earth would be only 2 mm across. Some stars are known as supergiants. The star Antares, for example, is 700 times bigger than the Sun. There is a star in the constellation of Auriga that may be 3,000 million km in diameter or 4,000 times bigger than our Sun. the neutron star that remains after the explosion of a supernova may be only 20 km in diameter, but of enormous mass. If it weighs more than two or three times that of our Sun, it begins to collapse into a black hole.
How does a star die?
Stars die when they eventually use up all their fuel and burn out, but this process takes many millions of years.
Towards the end of its life, a star starts to run out of hydrogen to power its nuclear fusion. It starts to cool, becoming a red giant. The red giant swells, and the pressure at its centre becomes so great that the star begins to absorb energy instead of emitting it. In a matter of seconds the star collapses, then explodes into a supernova. This is a huge explosion of light and energy that can be seen right across the galaxy.