The Solar System allows us to investigate a number of diferent gravitational efects. Many of them can be measured to high accuracy, because we have easy access to the nearby planets and satellites. They are, however, quite weak gravitational fields. This is because all of the objects in the Solar System are, relatively speaking, rather slow moving and not very dense. If we set our sights a little further though, we can find objects that are much more extreme than anything we have available nearby.
Let’s start by considering the life of a star. First-generation stars are thought to form when the clouds of hydrogen gas that emerged from the Big Bang collapse under their own gravitational field, and become hotter and denser. Eventually nuclear reactions start occurring, and the outward pressure from these reactions becomes strong enough to balance the inward attraction of gravity.
This results in a star: a hot ball of collapsed gas undergoing violent nuclear reactions. This process of collapsing gas and nuclear reactions is, of course, also a rough description of what happens in our own Sun.
But this isn’t the end of the story. A star such as the Sun can only ever have a finite lifetime. Eventually the hydrogen required for nuclear fusion will run out, and the star will have to start burning other materials. This makes it swell into a red giant. In turn, even these alternative fuels must eventually run out, and the gravitational collapse of the star will resume. What stops the collapse at this point depends on the size of the star. A small star will settle down to become a white dwarf. This is a state in which the quantum mechanical properties of the electron prevent it from becoming any smaller. At this point it’s just not possible to fit any more electrons in the space that the star takes up.
If a star is a bit bigger, then instead of becoming a white dwarf it will end up as a neutron star. In a star of this type, the end of nuclear fusion leads to a collapse of its core. The star then collapses, which in turn causes a colossal explosion, known as a supernova. During this process the gravitational force is strong enough to force the electrons and protons to combine into neutrons. The electron pressure is then absent, and the star collapses until the neutrons are so dense that no more can fit into the same space. The end result is a star that has a density that’s comparable to the nucleus of an atom. In a sense, a neutron star can be thought of as a giant atomic nucleus (but without any protons and without electrons orbiting it). Neutron stars are very small and very dense. They are denser than anything that exists in the Solar System, and tend to move at extremely high velocities.A neutron star isn’t the most extreme object that can result from a collapsing star though. That title goes to a type of object called a black hole. If a star is so large that even the pressure from neutrons can’t support it, then it will collapse to a black hole.