Stars in the Milky Way come in two main types, known as populations. The stars of Population I are found in the spiral arms and the galactic disc between them. They are generally brighter and hotter and contain greater amounts of heavy elements. Like all stars, Population I are dominated by the lightweight gases hydrogen and helium, but these heavy elements boost the rate of nuclear fusion in their cores – this is why Population I stars tend to shine hotter and brighter than Population II stars of a similar mass, but it also means they have shorter life spans.
Population II stars are mostly crowded together in the central bulge, but they also form globular clusters – huge balls of hundreds of thousands of stars that follow their own orbits around the galactic centre and are often found in the apparently empty halo region above and below the disc. Population II stars tend to be smaller and fainter than the Sun, with cool red, orange or yellow surfaces. A combination of lower mass and a lack of heavy elements causes them to burn fuel at a reduced rate, giving them extremely long life spans – they are the oldest stars in our galaxy.