Stars - twinkling suns
Stars, the little twinkling lights in the night sky, are really Suns: giant glowing balls of gas held together by the force of gravity. Long ago dust and gas were compressed by gravity, forming the spheres that became stars. The dust was vaporised, and balls of gas remained. Stars are gas right through: they have no solid parts.
Stars generate their own light and heat by nuclear fusion, the process in which hydrogen atoms join to form helium atoms at temperatures around 10 million degrees Celsius! Different stars have different brightnesses and colours. Generally the brightest are the nearest or largest. Colour is related to age and density. Stars like Aldebaran and Betelgeuse are "red giants" and are fairly old and relatively cool (as stars go). Others like Sirius and Rigel are blue-white hot young stars. Then there are intermediate yellow stars like our Sun and Capella, which are middle-aged, "warm" stars. The blue-white star Rigel in Orion gives out 62,000 times more energy than the Sun. It is a young star (a few million years old) and is using its energy up at such a rate that it will not live long (10 million years or so). Compare this with our Sun which is already 4,600 million years old.
