How are galaxies formed? (Intermediate)

I am sixth grader and I am doing a science project on galaxies. I wanted to know how galaxies are formed.

As you may know, galaxies are mainly of two kinds: spiral galaxies and elliptical galaxies (there are also irregular galaxies).

Spiral galaxies form from the collapse of a protogalactic cloud. Spiral galaxies consist of three components: a rotating disk, a bulge and a halo. When the protogalactic cloud collapses, the stars in the bulge and halo form first. These stars have rather random orbits around the galactic center. The remainder of the cloud settles down into a disk due to the conservation of angular momentum (the same effect as the spinning up of the dancer when she pulls her arms inside). The stars in the disk form later and hence the stars in the disk are younger than those in the bulge and the halo. Further, the stars in the disk rotate around the center of the galaxy in a collective, well defined way unlike the stars in the bulge and halo.

Elliptical galaxies are thought to be formed as a result of a merger of two disk galaxies. When two spiral galaxies merge, then the orbits of all the stars are randomized. As a result, all the stars in an elliptical galaxy have random orbits and there is not much collective motion of stars.

This page was last updated on June 27, 2015

About the Author

Jagadheep D. Pandian

Jagadheep D. Pandian

Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6.7 GHz methanol masers in our Galaxy. These masers occur at sites where massive stars are being born. He got his Ph.D from Cornell in January 2007 and was a postdoctoral fellow at the Max Planck Insitute for Radio Astronomy in Germany. After that, he worked at the Institute for Astronomy at the University of Hawaii as the Submillimeter Postdoctoral Fellow. Jagadheep is currently at the Indian Institute of Space Scence and Technology.

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