What are the three main types of galaxies?

Astronomers classify galaxies into three major categories: elliptical, spiral and irregular. These galaxies span a wide range of sizes, from dwarf galaxies containing as few as 100 million stars to giant galaxies with more than a trillion stars.

Ellipticals, which account for about one-third of all galaxies, vary from nearly circular to very elongated. They possess comparatively little gas and dust, contain older stars and are not actively forming stars anymore. The largest and rarest of these, called giant ellipticals, are about 300,000 light-years across. Astronomers theorize that these are formed by the mergers of smaller galaxies. Much more common are dwarf ellipticals, which are only a few thousand light-years wide.

Spiral galaxies appear as flat, blue-white disks of stars, gas and dust with yellowish bulges in their centers. These galaxies are divided into two groups: normal spirals and barred spirals. In barred spirals, the bar of stars runs through the central bulge. The arms of barred spirals usually start at the end of the bar instead of from the bulge. Spirals are actively forming stars and comprise a large fraction of all the galaxies in the local universe.

Irregular galaxies, which have very little dust, are neither disk-like nor elliptical. Astronomers often see irregular galaxies as they peer deeply into the universe, which is equivalent to looking back in time. These galaxies are abundant in the early universe, before spirals and ellipticals developed.

 Astronomers believe that after the big bang, the explosion which began the universe 10 billion to 20 billion years ago, gravity began to compress masses of free-floating gas. Two main theories, bottom-up and top-down, explain what happened next. According to bottom-up theories, clusters began to form and assembled together into the larger units we know as galaxies. Top-down theories suggest that galaxies formed first, and the stars and other objects within them were subsequently produced.

 

Picture Credit : Google

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