Galaxy
Galaxy is a system of stars, dust, and gas held together by gravity. Galaxies are scattered throughout the universe. Galaxies range in diameter from a few thousand to half a million light-years. Galaxies are distributed unevenly in space. Some galaxies are found alone in space, but most are clustered in groups. Groups of galaxies range in size from a few members to several thousand. Kinds of galaxies. Scientists classify galaxies according to appearance and shape. There are two main types, spiral galaxies and elliptical galaxies.
A spiral galaxy is shaped like a disc with a bulge in the centre. The disc resembles a huge pinwheel, with bright spiral arms that coil out from the central bulge. Dark clouds of dust and gas lie throughout the disc. The Milky Way is classified as a spiral galaxy.
Elliptical galaxies range in shape from round to flattened globes. The light from an elliptical galaxy is brightest in the centre and gradually becomes fainter toward its outer region. The study of galaxies. Galaxies give off many kinds of radiation, including gamma rays, infrared rays, radio waves, ultraviolet light, visible light, and X rays. Astronomers study this radiation with optical and radio telescopes, and other instruments. They estimate the distance and motion of a galaxy by measuring its red shift. Red shift is the change in the wavelength of light coming from an object moving away from the earth in space (see RED SHIFT).
Astronomers have discovered that almost all galaxies are moving away from each other at tremendous speeds. In addition, the galaxies farthest from the earth appear to travel the fastest. Therefore, most scientists believe the universe is expanding constantly.
Scientists have proposed a theory about the origin of galaxies. The Big Bang Theory states that masses of gas formed soon after the universe began to expand billions of years ago. According to the theory, gravity slowly condensed these masses into galaxies. No new galaxies have been formed since then, and so all galaxies are about the same age.
