Science & Universe: Types of galaxy

Types of galaxy

Elliptical Galaxies

Elliptical galaxies are shaped like a elongated sphere. In the sky, where we can only see two of their three dimensions, these galaxies look like elliptical, or oval, shaped disks. The light is smooth, with the surface brightness decreasing as you go farther out from the center. Elliptical galaxies are given a classification that corresponds to their elongation from a perfect circle, otherwise known as their ellipticity. The larger the number, the more elliptical the galaxy is. So, for example a galaxy of classification of E0 appears to be perfectly circular, while a classification of E7 is very flattened. The elliptical scale varies from E0 to E7. Elliptical galaxies have no particular axis of rotation.

Spiral Galaxies

Spiral galaxies have three main components: a bulge, disk, and halo (see right). The bulge is a spherical structure found in the center of the galaxy. This feature mostly contains older stars. The disk is made up of dust, gas, and younger stars. The disk forms arm structures. Our Sun is located in an arm of our galaxy, the Milky Way. The halo of a galaxy is a loose, spherical structure located around the bulge and some of the disk. The halo contains old clusters of stars, known as globular clusters.

Spiral galaxies are classified into two groups, ordinary and barred. The ordinary group is designated by S or SA, and the barred group by SB. In normal spirals (as seen at above left) the arms originate directly from the nucleus, or bulge, where in the barred spirals (see right) there is a bar of material that runs through the nucleus that the arms emerge from. Both of these types are given a classification according to how tightly their arms are wound. The classifications are a, b, c, d ... with "a" having the tightest arms. In type "a", the arms are usually not well defined and form almost a circular pattern. Sometimes you will see the classification of a galaxy with two lower case letters. This means that the tightness of the spiral structure is halfway between those two letters.

Irregular galaxies

Irregular galaxies does not have a distinct regular shape. The shape of an irregular galaxy does not fall into any of the regular classes of the Hubble sequence and is often chaotic in appearance, lacking a bulge, disk, or arms. This type is thought to make up 25% of all galaxies. Most irregulars were once spiral or elliptical galaxies that were deformed by distortions in gravitational pull.

Spiral galaxies make up about 35% of the galaxies in the known universe. They consist of a flat, rotating disk that contains stars, gas, and dust along with a bulge full of stars. The bulge and disk are surrounded by a halo. They are named from the spiral appearing arms jutting from the center into the disk. Nearly half of the spiral galaxies have an additional component: a bar-like structure, extending from the central bulge where the spiral arms begin.