The Sun and stars

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The Sun is our nearest star, and much of what we know about stars comes from investigating the Sun. But there are vast numbers of stars and many are quite different from the Sun. In the night sky you can see up to 8000 stars on a clear night away from city lights. With a good light telescope this number increases dramatically. The next nearest star to Planet Earth is Proxima Centauri, which is about 40,000,000,000,000 km away. As distances between stars are vast, astronomers use units called light years (ly). A light year is the distance light travels through space in one year. Light travels at 300,000 kilometres per second, so light travels 365 x 24 x 60 x 60 x 300,000 km in a year. 1 ly ≈ 9,000,000,000,000 km Proxima Centauri is 4.3 ly away. Existing spacecraft travel at speeds of around 10 km/s. It would take 125,000 years to reach Proxima Centauri! A star is a huge ball of glowing gases. At the centre of the star, incredibly high temperatures and pressures maintain nuclear reactions. In an nuclear reaction matter is converted into vast amounts of energy. Much of this energy streams out into space as different types of radiation (light, gamma rays, X-rays, ultraviolet, infrared and radio waves). Life on Earth is dependent on the Sun as the source of light and warmth (infrared radiation). Our atmosphere protects us from other harmful types of radiation. Although stars may all look the same in the night sky, they vary in brightness, colour, size, mass, composition, temperature, and speed at which they are travelling. Astronomers learn a great deal about stars by collecting the light coming from them using powerful telescopes on Earth or on satellites (e.g. the Hubble space telescope). They also use special telescopes that detect other types of radiation coming from stars (e.g. radio and infrared waves). Stars can be classified according to their brightness, size and temperature. The diagram below shows the relative size of different stars. Stars above the horizontal line are brighter than the Sun. Stars to the left of the vertical line are hotter than the Sun.

star 8000 Proxima Centauri year gases nuclear brightness colour size