As the Universe expands, galaxies change. Galaxies are constantly evolving, growing larger through mergers and acquiring new stars through accretion. Over time, they also change in appearance; their star content and structure evolve as gas is used up or lost, and stellar evolution changes the mix of stars present. In this article, we will explore how galaxies change over time, and how our understanding of galaxy evolution has developed.
Galaxies are gravitationally bound systems of gas, dust, dark matter, and stars. They come in a wide range of sizes and masses; from dwarf galaxies with just a few million stars to giant elliptical galaxies with hundreds of trillions of stars. Our own Milky Way Galaxy is thought to contain between 200 and 400 billion stars.
Most galaxies contain a mix of different types of stars – young hot blue O and B type stars, cooler yellowish F type main sequence stars like our Sun, red giant branch (RGB)stars, horizontal branch (HB)stars, white dwarf (WD)stars ,and neutron Stars(NS)/pulsars/black holes (BH). The relative proportions of these different types of stars depends on the age of the galaxy; younger galaxies have a higher proportion of hot blue O and B type stars while older galaxies have a higher proportion Of red giants branch (RGB)Stars , horizontal branch (HB)stars , white dwarf (WD)stars ,and neutron Stars(NS)/pulsars/black holes .
All these various types Of star occur In all three broad morphological classes Of galaxy: spirals ,ellipticals And irregulars . However each class shows some typical features which help us to understand how they formed And evolved .