As we’ve seen in the previous vlog, spiral Galaxies started forming from the primordial protogalaxies about 11 billion years ago. The formation of spiral galaxies is important because it happened at the peak of population II stars formation and allowed for planets to form.
Spiral galaxies consist of a flat, rotating disc with stars, gas and dust organised in arms that wrap around a central bulge or bar.The spiral structure is given by a density wave. As cars stuck in a traffic jam move in an out of the jam, so do stars, gas and dust: they can move in and out of the spiral arm, which remain a higher density rotating region. This movement allows new stars to form.
The central region, called the bulge, is quite different. It hosts a supermassive blackhole and it is mostly composed by population II stars. Population II stars are the second generation of stars to evolve in the universe, They are metal poor, mostly composed of Hydrogen and Helium alone. They are responsible for the formation of most of the stable heavy elements now present in the Universe and it’s thanks to them that we are here. When those stars went supernovae they created the carbon in our body, the oxygen that we breathe and everything else on Earth.