Editing Galaxy formation and evolution (section)

==Commonly observed properties of galaxies==
[[File:Hubble Tuning Fork diagram.svg|thumb|right|[[Hubble sequence|Hubble tuning fork diagram]] of galaxy morphology]]Because of the inability to conduct experiments in outer space, the only way to “test” theories and models of galaxy evolution is to compare them with observations. Explanations for how galaxies formed and evolved must be able to predict the observed properties and types of galaxies.

[[Edwin Hubble]] created an early galaxy classification scheme, now known as the Hubble tuning-fork diagram. It partitioned galaxies into [[Elliptical galaxy|ellipticals]], normal [[Spiral galaxy|spirals]], [[Barred spiral galaxy|barred spirals]] (such as the [[Milky Way]]), and [[Irregular galaxy|irregulars]]. These galaxy types exhibit the following properties which can be explained by current galaxy evolution theories:

* Many of the properties of galaxies (including the [[galaxy color–magnitude diagram]]) indicate that there are fundamentally two types of galaxies. These groups divide into blue star-forming galaxies that are more like spiral types, and red non-star forming galaxies that are more like elliptical galaxies.
* Spiral galaxies are quite thin, dense, and rotate relatively fast, while the stars in elliptical galaxies have randomly oriented orbits.
* The majority of giant galaxies contain a [[supermassive black hole]] in their centers, ranging in mass from millions to billions of times the mass of the [[Sun]]. The black hole mass is tied to the host galaxy bulge or spheroid mass.
* [[Metallicity]] has a positive correlation with the [[luminosity]]of a galaxy and an even stronger correlation with galaxy mass.<ref>{{Cite book |last=Longair |first=Malcolm S. |url=https://link.springer.com/book/10.1007/978-3-662-65891-8 |title=Galaxy Formation |date=2023 |language=en |doi=10.1007/978-3-662-65891-8 |issn=0941-7834}}</ref>{{rp|80}}
Astronomers now believe that disk galaxies likely formed first, then evolved into elliptical galaxies through galaxy mergers.

Current models also predict that the majority of mass in galaxies is made up of [[dark matter]], a substance which is not directly observable, and might not interact through any means except gravity. This observation arises because galaxies could not have formed as they have, or rotate as they are seen to, unless they contain far more mass than can be directly observed.