Galaxies

Formation & Evolution

15.14 - Understand the main theories for the formation and evolution of galaxies

Possible Origins

We know galaxies formed over a billion years after the Big Bang becasue of observational evidence form the Hubble Space Telescope. Two theories exist about their formation:

1. As with star formation gas and dust gathered, collapsed, forming stars in large groups
2. ‘Lumps’ of matter left over from the Big Bang grouped together to form galaxies

Continual Evolution

Galaxies continue to merge and collide with each other, creating larger galaxies and increasing star formation.

These mergers take place over millions of years, there is evidence the Milky Way and Magellanic Clouds are merging together. The Milky Way and Andromeda Galaxies will collide in the future as they are gravitationally bound and moving toward each other, it is likely they will produce an elliptical galaxy over billions of years.

Galaxies are large and have strong gravity. Although the distances between stars in them are typically enormous, the distance between galaxies is not that dramatic compared to a galaxies size.    

It is believed that spiral galaxies formed first while elliptical galaxies formed as a result of mergers. Stars along the disk developed later than the stars in the bulge and halo. Stars along the disk have more stable orbits compared to ellipticals where stars have more random orbits.

Galaxies are categorised by how much star formation is ongoing. Star Formation in galaxies depends on how much gas is available for stars to form and if the conditions are cool enough for gas to collapse.  

Stars with high populations of red stars are older and contain less gas and dust. Galaxies with a ‘blue cloud’ tend to have large amounts of dust to form stars. Typically, these are spiral galaxies. It is presumed. When a galaxy appears to contain gas to form stars it is said to be ‘quenched’.

Other Galaxies

A Lenticular galaxy is a combination of elliptical and spiral, it lacks long spiral arms and a prominent halo due to lack of interstellar gas. Older textbooks may refer to these as ‘peculiar’ galaxies.