How are galaxies and stars formed?
How is a star formed? Stars condense from giant clouds of molecular gas (mostly hydrogen). A cloud may be disturbed, perhaps by a passing star or shock waves from a distant supernova explosion. This can cause the cloud to be unstable, and regions of gas start to collapse in on themselves due to gravity. As the clouds collapse and become more dense, the gravitational pull attracts more gas, and the process continues. Pressure and density rise, and with pressure, so does the temperature of the centre of this gas cloud. Eventually the temperature becomes high enough for nuclear reactions to begin, and a star is born. In spiral galaxies, like our own, we observe star formation predominantly in spiral arm features. We believe this to be due to a density wave that causes gas to collapse to form giant molecular clouds as it propagates around the galaxy. The brightest (most massive) stars fuse their hydrogen (and then heavier elements) at such a rate that they only live for a few million years
