Astronomy-The Galaxies
Stars are found in huge groups called galaxies. Scientists estimate that the larger galaxies may contain as many as a trillion stars, while the smallest may have fewer than a million. Galaxies can be up to 100,000 light-years in diameter.
Galaxies may have any of four general shapes. Elliptical galaxies show little or no structure and vary from moderately flat to spherical in general shape. Spiral galaxies have a small, bright central region, or nucleus, and arms that come out of the nucleus and wind around, trailing off life a giant pinwheel. In barred spiral galaxies, the arms extend sideways in a short straight line before turning off into the spiral shape. Both kinds of spiral systems are flat. Irregular galaxies are usually rather small and have no particular shape or form.
Galaxies were long thought to be more or less passive objects, containing stars and interstellar gas and dust and shining by the radiation that their stars give off. When astronomers became able to make accurate observations of radio frequencies coming from space, they were surprised to find that a number of galaxies emit large amounts of energy in the radio region. Ordinary stars are so hot that most of their energy is emitted in invisible light, with little energy emitted at radio frequencies. Furthermore, astronomers were able to deduce that this radiation had been given off by charged particles of extremely high energy moving in magnetic fields.
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