Q&A: Supernova Remnants and Neutron Stars

Q:
In your article about the possibility of RX J1856.5-3754 and 3C58 being quark stars, or at least containing some strange quark matter it says that 3C58 is cooler than expected, so it is thought it could be denser than a normal neutron star. Why would something more dense be cooler?

A:
We think of dense objects as hot because of the heat generated by compression. However, once compression is over, a denser object will cool faster. The particles composing it are closer together, collide more frequently, and radiate away their energy more efficiently. An everyday example is frost, which forms on the grass at night. The grass, being denser than the air, cools faster than the air at night, so a layer of frost can form on the grass (which is an insulator and does not receive energy from the Earth) even though the temperature of the air is above freezing.