Galaxies by Nandita Subbiah 5/24/13

A galaxy is a system of billions of stars, along with gas and dust that is held together by gravity. There are three types of galaxies: spiral, elliptical and irregular. Each of these types of galaxies are formed differently. Spiral galaxies are formed when a protogalactic cloud collapses (a protogalactic cloud is a denser area of gas, that originally expanded with the rest of universe, but then stopped.) The momentum of the protogalactic cloud will cause it to spin, creating a spiral galaxy. Spiral galaxies have three parts: a rotating disk, a bulge and a halo. Our galaxy, the Milky Way galaxy, is an example of a spiral galaxy. Elliptical galaxies form when two spiral galaxies merge. It is unclear whether all irregular galaxies form the same way. Right now, scientists believe that irregular galaxies start as either a spiral or elliptical galaxy. They then turn into irregular galaxies through gravitational interactions and merging with other galaxies.

A spiral galaxy
(Source:http://www.nasa.gov/images/content/611262main_hubble_holidaywreath1200_946-710.jpg)

Elliptical Galaxy

(Source:http://apod.nasa.gov/apod/image/0406/m87_cfht.jpg)

Irregular Galaxy (Source:http://scitechdaily.com/images/dwarf-irregular-galaxy-DDO-190.jpg)

Galaxies are arranged in clusters in the universe. These clusters are held together by gravity. Rich clusters are clusters that contain more than 1,000 galaxies. Poor clusters are clusters that contain less than 1,000 galaxies. The Milky Way and Andromeda galaxies are part of a poor cluster called the Local Group.

Picture of the Earth in the Solar System, Solar Interstellar Neighborhood, Milky Way Galaxy, Local Galactic Group, Virgo Supercluster, Local Supercluster and Observable Universe.

(Source:http://lcogt.net/spacebook/distribution-galaxies)

Map of the clusters.

(Source:http://smithsonianscience.org/wordpress/w)

Sometimes, galaxies that are near each other will collide. This process take hundreds of millions of years. This creates new galaxies. Galaxy collisions are very common, because galaxies tend to be relatively close to each other. Stars rarely collide because they are spaced further apart. Because the process takes such a long time, scientists use computer simulations to simulate what would happen when two galaxies collide. They are predicting that about four billion years from now, the Milky Way galaxy will collide with the Andromeda galaxy.

Prediction of how it will look when the Milky Way and Andromeda galaxies collide.

(Source:http://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html)

The Milky Way galaxy is about 100,000 light years in diameter. Our solar system’s location in the Milky Way is estimated to be 28,000 light years away or ⅔ of the way to the edge of the galaxy away from the center. It is about 20 light years above the Milky Way galaxy’s equatorial plane in the Orion spiral arm.

Our solar system’s position in the Milky Way galaxy

(Source:http://www2.mcdaniel.edu/slm/student/sheehyr/fieldtrip_files/image002.jpg)

At the center of the Milky Way galaxy, the stars become tightly packed together. About 10 million stars orbit within 1 light year. This is called the galactic bulge. Most parts of the Milky Way aren’t crowded. The closest star to the Sun is 4.2 light years away. An interesting fact is that the Sun and our solar system is actually orbiting around the center of the galaxy. The solar system is moving at an average velocity of 828,000 km/hr. It takes the solar system 230 million years to complete its’ orbit around the center of the Milky Way.