GALAXIES

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There are tens and thousands of Galaxies present in the Universe.

The Hubble classification scheme defines Galaxies in four basic types:

* Spirals

* Barred Spirals

* Ellipticals, and,

* Irregulars

Spirals: They have a central bulge with a flattened disk, normally depicting spiral arms.

Barred Spirals: Almost same as spirals but for the existence of a bar in the central region.

Ellipticals: They are roundish non-flattened galaxies. They generally have very little gas and dust when compared with spiral galaxies. They have few young stars, stars that were just formed, with no spiral structure.

Irregulars: Whatever galaxies that do not fall into the first three classifications are hereunder.

Irregular galaxies: Our own milky way is reckoned as a spiral galaxy. A spiral galaxy comprises of:

í The disk

í The central bulge, and,

í The halo

The disk contains Population I stars, i.e., young and newly formed stars with heavy elements. Disk stars normally orbit around the galactic center in almost circular orbits. Our solar system is part and parcel of the disk and is at a distance of 8KPC from the middle. The disk also has a large amount of dust and gas in the interstellar medium.

The Bulge and the Halo comprises of Population II stars, i.e., old stars. The Bulge and Halo stars rotate in random orientation around the center. No interstellar medium in the Halo.

Galaxies are generally found in clusters and groups. The local group which consists of some tens of galaxies, also has the two largest galaxies – The Milky Way – our own galaxy and the Andromeda Galaxy. Some enriched clusters have thousands of galaxies contained in them.

It is astonishing that the stars in our galaxy never collided physically. The reason attributed is that the distance between is much larger than their own size physically.

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