Neutron:

A neutron has a mass a little greater than that of a proton. The neutrons have no electrical charge, and they are approximately as dense as protons. Though, whereas protons last for a long time all by themselves in free space, a neutron does not. The mean life of a neutron is only around 15 minutes. This means that when you gathered up a batch of, state, 1 million neutrons and let them float about in space, you would have only around 500,000 neutrons left after 15 minutes. After 30 minutes, you would have roughly 250,000 neutrons remaining; after 45 minutes, there would be only around 125,000 neutrons left.

Neutrons can last a long time whenever they are in the nuclei of atoms. This is a lucky thing since if it weren't true, matter as we know it could not exist. The neutrons can also survive for a long time whenever a huge number of them are strongly squeezed altogether. This occurs when large stars explode and then the remaining matter collapses under its own gravitation. The last product of this chain of events is a neutron star.