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What are stars?

An illustration of a star 40 light years away, with a planet (an exoplanet—a planet from outside our own Solar System) orbiting...Read More >>An illustration of a star 40 light years away, with a planet (an exoplanet—a planet from outside our own Solar System) orbiting in front of itStars are giant, hot spinning balls of gas. Our Sun is a star. Stars give off massive amounts of energy, including light and heat. They vary enormously in size and by the amount of light they give off. Stars vary in colour according to how old they are. Blue stars are young and hot; red stars are old and cool. Orange and yellow stars (like our Sun) are middle-aged and moderately hot.




Stars are of widely varying sizes and colours. The oldest stars are swollen, red giants. Betelgeuse is a red supergiant. Old...Read More >>Stars are of widely varying sizes and colours. The oldest stars are swollen, red giants. Betelgeuse is a red supergiant. Old stars are much bigger—and cooler—than young, blue ones like Rigel, even though Rigel is 40 times bigger than the middle-aged Sun. A white dwarf, the collapsed core of an old star, is, however, tiny when compared to the Sun.

Giants and dwarfs

Stars come in many colours and sizes. The largest star in this scale illustration is the red supergiant Betelgeuse in the constellation of Orion. Next are the blue giants Rigel and Polaris (the Pole Star). Our Sun, known as a "yellow dwarf" is the small yellow dot. Betelgeuse is 800 times the size of the Sun.

A neutron star, the remnant of a supernova, is no more than 25 km (15 miles) across. A teaspoonful of matter from a neutron star would weigh a billion tonnes.

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