Saturday, February 13, 2010

Dark Stars

The first stars that form in the early universe can be powered by dark matter instead of conventional nuclear fusion and these stars are conveniently termed dark stars. These hypothesized dark stars still remain to be discovered and future telescopes such as the James Webb Space Telescope (JWST) may have the sensitivities required to detect such stars. The term ‘dark’ does not mean that these stars appear dark, but rather it means that the energy from the annihilation of dark matter particles is the primary mechanism keeping these stars shining in hydrostatic equilibrium. A paper by Katherine Freese, et al. (2010) entitled “Supermassive Dark Stars - Detectable in JWST” basically describes such stars and their possible detection by the JWST.

Weakly Interacting Massive Particles (WIMPs), an excellent candidate for dark matter, may be their own antiparticles and this allows them to annihilate among themselves. Wherever the density is sufficiently high, the annihilations of WIMPs can generate enough energy to power such dark stars for millions to billions of years. The WIMP annihilation products thermalize within the star which provides the power required to keep the star shining and prevents the star from gravitationally collapsing. Dark stars are composed primarily of hydrogen and helium, with only a fraction of a percent of their mass in the form of dark matter. Due to the high efficiency of energy production via the annihilation of WIMPs, a small proportion of it is sufficient to sustain the star against its own gravity over astronomical timescales.

The much cooler surface temperatures of dark stars is the primary reason which allows darks stars to grow to become so much more massive than ordinary fusion-powered stars, as long as the annihilation of dark matter persists. Ordinary fusion powered stars produce ionizing photons that provide a variety of feedback mechanisms that cut off further accretion of baryonic matter. On the other hand, dark stars with their cooler surface temperatures allow the continued accretion of baryonic matter all the way up to colossal stellar masses. Dark stars can grow up to millions of times the mass of the Sun and exceed billions of times the luminosity of the Sun. Such massive stars can lead to the formation of supermassive black holes when they run out of dark matter fuel to sustain them. When the production of energy from dark matter annihilation dwindles, it causes the star to contract, heat up and undergo a brief phase as a fusion powered star before collapsing into a massive black hole.