MSSL Astrophysics Group Beginner's Guides

The formation of the Universe is believed to have begun with the Big Bang, a colossal explosion of matter and energy. The galaxies condensed out of this matter and continued to move away from the point of the explosion, thus the Universe has been expanding since the beginning of time. If you imagine yourself standing in any one of these galaxies and watch the motion of the other galaxies relative to you, you will notice that all the other galaxies appear to be moving away from you. This is the cosmological redshift and is due to the expansion of the Universe.

The galaxies are different colours in this animation to help you visualize their relative motions. In fact, galaxies do have different colours according to the types of stars which each contains; every galaxy is an individual. In spiral galaxies (the type shown here), the central region (the bulge) looks red because it has mostly old red, stars, whereas the arms have many more younger, hotter stars and appear blue. The other main types of galaxy are ellipticals (which look like the bulge regions of spirals) and irregulars.

• Back to redshifts for more information about the cosmological redshift and Hubble's Law
• See the beginner's guide for the essential facts about the monsters in the centres of galaxies - active galactic nuclei