STAR CHART Armagh Planetarium（星图阿玛天文馆）.pdfVIP

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. Y 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. E • B- white, very br