The barred spiral galaxy NGC 1300.<\/figcaption><\/figure>\nEach galaxy carries a record of how it formed and its evolutionary history is encoded in its shape, colour, and features.<\/p>\n
\u00a0If a galaxy has a central bulge then it\u2019s probably the result of a merger that has had time to re-organise itself into a spherical shape. If it\u2019s a thin disc then it\u2019s grown by slowly swallowing gas.<\/p>\n
\u201cA lot of galaxies we see have a central bulge and a thin disk. Which suggests that the galaxy first formed by merging and then later on it formed a disc through gas accretion,\u201d says Driver.<\/p>\n
If a galaxy looks messy or irregular then it\u2019s undergoing a significant evolutionary event – either merging with another galaxy or accreting gas very fast. These galaxies aren\u2019t in equilibrium.<\/p>\n
Alternatively, if a galaxy is left alone and hasn\u2019t undergone any major mergers for a long time then a bar may start forming. This begins if there\u2019s a region where there are slightly more stars. With time, these stars tend to pull others towards them.<\/p>\n
\u201cAn\u00a0over-density of stars rotating round \u00a0a galaxy\u2019s centre will pull on the ones in front, slowing them down, and accelerate the ones behind,\u201d says Driver.<\/p>\n
\u201cAnd over time, you go from a flat Frisbee-like structure to a galaxy with a bar. But it only happens if a galaxy is left alone. If another galaxy goes by, it gives enough of a kick that disrupts that bar process.\u201d<\/p>\n
When it comes to spiral arms, the process that forms them is not fully understood.<\/p>\n
“They\u2019re believed to be a shock-wave or\u00a0density-wave\u00a0that permeates out from the centre of the galaxy,\u201d says Driver.<\/p>\n
Many galaxies have all three features \u2013 bulges, bars and spiral arms \u2013 telling a complex story of evolution.<\/p>\n