Stars are not fixed in the heavens. They are moving within the galaxies. The speed of a star moving toward or away from the Earth is indicated by a shift of its spectral lines. If a star is approaching the Earth, its lines shift towards the blue end of the spectrum. If it is moving away from the Earth, its lines shift towards the red end of the spectrum. The greater the star's speed, the more its lines shift. You have read that there are many kinds of stars-blue, yellow or red, normal or giant, pulsating or releasing excessive energy. Most stars nlove together in groups. Only one out of four stars may travel alone. Of the rest, almost a third are double stars and the rest are groups of many stars. In a double star system, known as a binary, two stars orbit one another. In a triple system; there are three stars-all three may move around each other, or two of them may move around the third. Then there are loose clusters, with a few dozen stars, to the large globular clusters containing hundreds of thousands of stars, all moving in many possible ways.