Friday, January 12, 2007

More reader mail: how long it takes stars to cool down

I'm in the middle of my third business trip of the year. This morning, I found I'm already 15% of the way to achieving platinum status on American Airlines for 2007 (that takes 50,000 miles traveled). Fun! At any rate, I probably won't be able to blog again until late Tuesday.

Today, I'll answer another question I received by email from a reader. Bret asks, "How long does it take a neutron star to cool off to the temperature of its surroundings?"

This is a good question, and unfortunately we don't know the answer all that well. But let's start by reviewing what a neutron star is, for those who may not know.

A neutron star is the remnant of a star that was about 8 or more times the mass of the sun. When such a massive star has used up all of its fuel (in just a few million years, as opposed to the sun, which will last for 10 billion years), it has a core made out of iron. Stars get their energy from nuclear fusion, but iron does not make energy in nuclear reactions. So, the iron core just sits there and grows. When it gets large enough (about one and a half times the mass of the sun), the iron can no longer stand up to the crushing pull of gravity. Electrons merge with the atomic nuclei to make a neutron star (essentially a giant atomic nucleus!), and the rest of the star explodes as a supernova.

A neutron star is about 1.5 times more massive than the sun, but is only about 10 or 15 miles across. Think about shrinking the sun until it was the size of Manhattan! Now, the neutron star begins its life with temperatures of a few million degrees on its surface (and over a billion degrees in its center!). When young, the neutron star has a very strong magnetic field, and the star loses energy through this magnetic field, as well as through invisible, nearly-massless particles called neutrinos.

After about a million years, the outer temperature of the neutron star has dropped to a few tens of thousands of degrees. Sometime around this point, the magnetic field begins to break down and the star stops producing neutrinos, and the neutron star will cool in a "normal" way -- by emitting photons. This is how hot things cool in our everyday life, by radiating away energy.

The time it takes a neutron star to cool to the temperatures of interstellar space is uncertain, but likely to be much longer (billions of years). The time depends on the amount of heat energy in the material (its heat capacity) and the rate at which the star loses that heat through radiation. In an ideal world, that rate of radiation is faster for higher temperatures and faster for objects with larger diameters. So, as a neutron star gets closer and closer to the ambient temperature of space (only a few degrees above absolute zero), it wil cool more and more slowly.

One problem is that we don't know how much heat the neutron star can hold, because we don't know what they look like on the inside. It is not possible to replicate the conditions of a neutron star on the earth, so there are many theories, including "soft" and "hard" equations of state, and quark and strange matter varsions of neutron stars. I won't try to explain these differences, as I don't understand them well myself.

In fact, one way of testing theories about the structure of neutron stars is to compare their observed cooling rates with those predicted by different theories. This work is just beginning, though, so it will likely be a long time before we understand neutron stars!

No comments:

Post a Comment