| MadSci Network: Astronomy |
A rough estimate of the density of a neutron star can be found by taking its mass, 1.4 times the mass of the Sun, and dividing by its volume, that of a sphere about 10 km in diameter. The result is that the average density of a NS is about 7 x 1014 g/cm-3. (For thos unfamiliar with scientific notation, that's a seven followed by 14 zeros or 700000000000000 g/cm-3; a human has a density of about 1 g/cm-3 and the Earth's density is about 5 g/cm-3.)
Densities this high are comparable to those measured in atomic nuclei (leading NSs to be described occasionally as the largest atomic nuclei). A more detailed analysis, that uses information obtained from terrestrial accelerators, suggests the following. At the center of a NS the density is something like 1015 g/cm-3. Toward the edge of the NS, the density might drop to something like 106 g/cm-3.
The high central density of a NS means that its core might be something quite strange. It might be just composed of neutrons, or the pressure might be high enough to break the neutrons down into their component quarks. Outside of the core, there would be a large fraction of the NS that would be composed of neutrons. The surface itself would be composed of dense atoms like iron and nickel (just their nuclei really).
The sound speed within a NS is somewhat uncertain. The sound speed is probably about 10% of the speed of light (or 30 000 km/s). The actual sound speed is determined by how the pressure varies with density inside the NS. Astronomers and physicists continue to try to determine this exact quantity better.
(The sound speed measures how fast information can be transmitted through an object. For instance, imagine hitting an NS with a large hammer to cause an NS-quake. A person on the other side of the NS wouldn't know you had hit the NS until the vibration travelled through the NS, at the sound speed within the neutrons. Note that the sound speed does not mean that the NS is composed of air; air certainly does have a sound speed, but so does water, Earth, and the Sun.)
As for the cooling of NSs, they are actually very inefficient at cooling. There are three ways an object can cool: conduction, convection, and radiation. Conduction and convection both require that the object be in contact with something else (like an ice cube or cold water). However, NSs are in outer space, so there's almost nothing around them. Their only means of cooling is by radiation. The amount of radiation from an object depends, in part, on its radius. The radius of a NS is only 10 km (smaller than many cities!). (Compare that to the radii of most stars which are about 700 000 km.) This tiny radius means that an NS has a miniscule amount of surface area from which it can radiate, and so it cools quite slowly.
These questions are quite interesting, and at the forefront of some of modern astronomy. If you'd like to do more reading, you'll probably have to start with a college-level introductory astronomy textbook. A good discussion of NS is found in a book by Shapiro & Teukolsky, Black Holes, White Dwarfs, and Neutron, but be warned, it is quite technical.
Try the links in the MadSci Library for more information on Astronomy.