How do brown dwarfs die?

Brown dwarfs can't die, because they are, so to speak, already dead.

As you move down the Main Sequence, the density of stars increases, because the lower mass stars have to contract more, in order to achieve a given temperature, than the higher mass stars do. This means that when they begin nuclear burning, they are much fainter, and therefore, don't need to produce as much heat in order to stop their contraction. As a result, they are not only much denser, but also, much cooler (in relative terms) than higher mass stars.

As an example, a massive star may have central temperatures in excess of 30 million Kelvins, and central densities only a little greater than that of water, whereas the Sun has a central temperature of less than 15 million Kelvins, and a central density more than 100 times that of water. By the time you get to the bottom of the Main Sequence, the lowest mass stars have central densities of thousands of times that of water, and central temperatures of only 6 or 7 million Kelvins.

The higher densities of low mass stars mean that their electrons are pushed closer together, and their lower temperatures mean that the electrons act bigger, because of the Uncertainty Principle, than in high mass stars. As you go to lower and lower masses, the decrease in distance between the electrons, and the increase in electron "size" means that the space between the electrons rapidly decreases.

Once the mass gets low enough, it becomes so hard for the faint, dense, cool protostar to increase its temperature that, as it contracts, the electrons begin to fill up its entire interior, and to act like a liquid, or, as it is technically known, an electron- degenerate gas. This is exactly the same situation which white dwarfs achieve, but at much higher densities, because they are much hotter, so that their electrons "act" smaller.

So, if the mass of a would-be star is small enough (somewhere below l/10th the mass of the Sun), as it contracts toward the Main Sequence, the electron gas begins to act like a liquid. At first, the electron degeneracy is small, and the protostar continues to shrink more or less as it previously did. But as the protostar continues to shrink, and the electron degeneracy grows, any further loss of heat becomes less and less important, and it shrinks more and more slowly. Eventually, once the electrons are completely degenerate, and the electron gas behaves like a liquid, the protostar stops shrinking.

At this point, the protostar has no heat source save the heat stored inside it during its contraction. As that heat is slowly radiated away, the star cools off, and becomes gradually fainter and fainter. Such stars are already so cool that most of their heat is in the infrared, and as they cool off further, they become almost impossible to observe at interstellar distances, save through their infrared radiation. Once that happens, they are called brown dwarfs.

So, in a sense, brown dwarfs never have a life, in the sense that the Sun has, as a Main Sequence star. Instead, they are "stillborn", and "die" without ever having lived.

Дата добавления: 2015-09-04