iBook Galaxy

David Kim

Galaxy

Galaxy is group of million or billion stars. There are three main types of galaxies in the universe. Scientists divide the galaxy into three types, which are elliptical, spiral, and irregular by Hubble sequence. The Hubble sequence is a type of Galaxy morphological classification, which was made by Edwin Hubble.

Elliptical Galaxy

An elliptical galaxy basically has elliptical and smooth shapes, and is a simply colored type of galaxy. An elliptical galaxy is not as complex as spiral galaxies. There are not many formations and it randomly follows an orbital motion. It consists of small and old planets. Elliptical galaxies have a red color due to the small and old planets that form it. Elliptical galaxies have the tendency to gather as globular clusters. It almost doesn’t have any gas and dark nebula. Elliptical galaxies have many different types of sizes. There are 1/10 kpc to more than 100 kpc size of elliptical galaxy. There are 10^7M to 10^13M types. Each of them has different masses and sizes. This range of types is wider than any other types of galaxy. In elliptical galaxies there are two physical types. One is boxy and the other is disk.

Irregular Galaxy

An Irregular galaxy is the type of galaxies which doesn’t have exact shapes. An irregular galaxy’s shape is not same, so it doesn’t include in the Hubble sequence. All spiral arms and galactic bulge are mess. Synthetically irregular galaxies occupied ¼ of all galaxies. Some of irregular galaxies were spiral galaxies, but they changed to irregular galaxies because of the gravity problem. For example there is Large Magellanic Cloud. Scientist insisted that Large Magellanic Cloud was barred spiral galaxy because we can see the spiral tracks in Large Magellanic Cloud Irregular galaxies contain a lot of interstellar medium.

Spiral Galaxy

A spiral galaxy is group of planets and stars. Spiral galaxies are formed by galactic disc, which include many stars and interstellar medium, and galactic bulge. Galactic bulge is the place which main starts gather. Galactic disc and bulge are covered my globular cluster and galactic halo. Spiral galaxies occupy 60% of near universe with irregular galaxies. In 1990 scientist figured in out that the Milky Way is barred spiral galaxy, and also Andromeda galaxy which is the closest galaxy to the Milky Way is spiral galaxy too. A spiral galaxy consists of old stars of galactic bulge and at the center of the galaxy there are super massive black hole. Spiral arm is star’s territory, which came out from the center of the galaxy. Every spiral galaxy has its own spiral arm. Spiral nebula was the word which used for spiral galaxy until 20 century because at the time scientist thought spiral galaxy and celestial bodies were just parts of nebula which located in Milky Way. In 1926 Edwin Hubble proved that Andromeda galaxy and other spiral galaxies are located outside of Milky Way.