A hypothetical set of multiple possible universes (including our own) which exist in parallel with each other. Our universe would then be just one of an enormous number of separate and distinct parallel universes, the vast majority of which would be dead and uninteresting, not having a set of physical laws which would allow the emergence of stars, planets and life.
A sub-atomic elementary particle with no electrical charge and very small mass that travels very close to the speed of light. They are created as a result of certain types of radioactive decay or nuclear reaction, such as the decay of a free neutron(i.e. one outside of a nucleus) into a proton and electron. Being electrically neutral and unaffected by the strong nuclear force or the electromagnetic force, neutrinos are able to pass through ordinary matter almost undisturbed and are therefore extremely difficult to detect, although when created in huge numbers they are capable of blowing a star apart in a supernova.
One of the two main building blocks (along with the proton) of the nucleus at the centre of an atom. Neutrons have essentially the same mass as a proton (very slightly larger) but no electric charge, and are made up of one “up” quark and two “down”quarks. The number of neutrons in an atom determines theisotope of an element. Outside of a nucleus, they are unstable and disintegrate within about ten minutes.
Neutron Star: A star that has shrunk under its own gravity during a supernova event, so that most of its material has been compressed into neutrons only (the protons and electrons have been crushed together until they merge, leaving only neutrons). Neutron stars are very hot, quite small (typically 20 to 30 kilometres in diameter), extremely dense, have a very high surface gravity and rotate very fast. A pulsar is a kind of highly-magnetized rapidly-rotating neutron star.