A neutron star is the collapsed remains of a massive star, between 8 and 40 times more massive than the sun. They are comprised, as their name implies, almost entirely of neurons. Neutron stars are extremely small, but contain between about 1.5 to 3 solar masses, so they are extremely dense. Neutron stars are so dense that one cubic millimeter of neutron star material can weigh upwards of 80,000 metric tons, or about as much as the population of Helsinki, Finland. There are also many varieties of neutron star, such as magnetars, which have magnetic fields strong enough to destroy human tissue (due to the magnetic properties of water) from 1000km away. Finally there are the hypothetical quark stars, which are made mostly of a quark plasma created from neutron matter under heavy pressure.