STAR CHART NORTHERN HEMISPHERE

A clear night sky is a thing of beauty and wonder. Thousands of scattered stars twinkle in the darkness. What are the stars? Over the past century or so astronomers have by observation and calculation established the true nature of these gleaming specks. Now we know that stars are basically enormous balls of very hot gas, mainly hydrogen, with some helium and small amounts of a few other things. Inside the star huge quantities of energy are generated by a nuclear process called fusion. Planets, in contrast are balls of rock, metal and gas with no internal nuclear energy source. Earth is a planet, while the sun is a star.

Stars come in different brightnesses, sizes and colours. Look out at the sky tonight and you will see how stars vary in brightness. Some are bright because they are relatively close to us, while others are bright because they are thousands of times brighter than our own nearby Sun. Find the easily-spotted constellation of Orion the hunter. There you will see the red star Betelgeuse in his shoulder, the bluish white Rigel in his foot. To bring order to star catalogues, astronomers classify stars into what are called spectral types. This means stars are listed and placed in an order based on the temperature and brightness of their surfaces and the characteristics of the spectrum of light they shine out. In order of brightest to dimmest star, the sequence runs OBAFGKM. Here is the classification sequence with a rough guide to how common each type is.

• O- blue-white, brightest and hottest (30 000°C or more). About 1 star in every thirty thousand is type O. • B- white, very bright stars. About 1 star in every thousand is type B. • A- bluish white stars. About 1 star in a hundred is type A. • F- yellowish white stars. About 1 star in thirty is type F. • G- yellow stars (about 5500°C). About 1 star in fifteen is type G, eg the Sun. • K- orange stars, not as bright as the Sun. About 1 star in eight is type K. • M- red stars (about 3500°C). About four out of five stars are type M. 2009 ODYSSEY SPACE

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reddish surfaces, cool by stellar standards. In the distant future our Sun is Sun our future distant the In standards. stellar by cool surfaces, reddish

this great show above our heads. Happy star-gazing! Happy heads. our above show great this energy using other fuels, swelling enormously into huge bloated stars with stars bloated huge into enormously swelling fuels, other using energy

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converting hydrogen into helium in their hot, seething centres and producing and centres seething hot, their in helium into hydrogen converting

that not even light escapes its gravitational grip. gravitational its escapes light even not that Our Sun is a main sequence star. These are stars in their prime of life, steadily life, of prime their in stars are These star. sequence main a is Sun Our

leave behind the strangest object of all; a black hole, matter crushed so tightly so crushed matter hole, black a all; of object strangest the behind leave

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is a giant star tearing itself to pieces in nature’s most destructive detonation. destructive most nature’s in pieces to itself tearing star giant a is plot a graph of stars’ spectral type against their actual brightnesses. actual their against type spectral stars’ of graph a plot

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dwarf. White dwarf stars no longer produce light by nuclear fusion, merely fusion, nuclear by light produce longer no stars dwarf White dwarf. naked eye. The fact that the majority of stars we see in the sky are brighter are sky the in see we stars of majority the that fact The eye. naked

space. The tiny shrunken core, about the size of the Earth, remains as a white a as remains Earth, the of size the about core, shrunken tiny The space. stars to the Sun, 80 are M-type red dwarf stars, too dim be seen with the with seen be dim too stars, dwarf red M-type are 80 Sun, the to stars

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fated to become one of these red giants. A third group are the white dwarfs. white the are group third A giants. red these of one become to fated Although the vast majority of stars we see in the night sky are brighter than brighter are sky night the in see we stars of majority vast the Although

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