Fingerprinting the Stars:
Astronomers use various methods to measure the distances to stars. For determining the distances to nearby stars, the method of stellar parallax is used (see Fig. 9.6). For stars farther away, more sophisticated methods are used. We will not go into their details. The distance to astronomical objects situated very far away is found by measuring the 'red shift' of their spectral lines. As far away objects, such as galaxies and quasars, move away from us, the lines in their optical spectra are shifted towards the red end. This shift can be measured and their distances calculated by using appropriate formulae.
Maximum information about starlight can be derived from its spectrum. When a lens-sized prism is put over the front (or objective) end of a telescope, each star can beseen as a colourful spectrum. We can place a photographic film at the focal plane of the lens-sized prism. Then it becomes possible to register the spectrum of starlight. Ironically, the astronomer sees the spectra, not as brilliant rainbows, but as black and white patterns shown in Fig. Each star has its own characteristic spectrum-a fingerprint of its individual personality. From its spectrum, we can learn what elementsa star is made up of, what its temperature is, how bright it is, how fast it is moving etc. Stellar Motion 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 itis moving away from the Earth, its lines shift towards the red end of the spectrum. Thegreater 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.