Sunday, February 10, 2013

The Universe’s Last Stars

At present, the universe is in the stelliferous era where conventional star formation is an ongoing process and nuclear fusion in stars account for most of the energy generation in the universe.  However, star formation is not a perfectly efficient process and matter is continuously being lost due to incorporation into objects such as brown dwarfs, extremely long-lived red dwarfs and inert stellar remnants (white dwarfs, neutron stars and black holes). As a result, the stelliferous era is expected to end when the universe is around 1014 years old, about 103 times its current age. The universe then enters the degenerate era which is expected to last until the universe is around 1037 years old. This is the era where most of the mass in the universe is in the form of brown dwarfs, white dwarfs, neutron stars and black holes.

Brown dwarfs are objects not massive enough to become stars as their masses are below the hydrogen-burning limit. During the degenerate era, non-conventional star formation remains a possibility through 2 methods. The 1st method is when two brown dwarfs collide and merge into an object with a mass above the hydrogen-burning limit. The 2nd method occurs when a brown dwarf very slowly accretes matter from the rarefied interstellar medium until its mass is eventually pushed above the hydrogen-burning limit. At any given time, it is estimated that at least a few stars in the galaxy were created via the 1st method. As for the 2nd method, it is unobservable in the present universe because the timescale for it to happen is many orders of magnitude larger than the current age of the universe. Nevertheless, the every-increasing timescales in the far future of the universe may make the 2nd method a dominant mode of star formation in an almost starless universe.

The number of brown dwarfs in the galaxy is expected to be comparable to the number of hydrogen-burning stars. Over a very long period of time, a brown dwarf drifting through the rarefied interstellar medium can accrete enough matter to push its mass above the hydrogen-burning limit and become a red dwarf star. A more massive brown dwarf needs to accrete less material than a lower mass brown dwarf to arrive at the hydrogen-burning limit. Assuming the minimum mass necessary for hydrogen-burning is 7.7 percent the Sun’s mass and the interstellar medium has an average density of one proton for every 10 cubic centimetres, a brown dwarf with 5 percent the Sun’s mass needs to accrete for 1017 years before its mass arrives at the hydrogen-burning limit. After that, the newly born red dwarf star will shine for about 1013 years in a very dark universe. In comparison, our Sun has a lifespan of just 1010 years.

During the degenerate epoch, the accretion of matter from the interstellar medium by objects other than brown dwarfs can lead to some interesting phenomenon although such events are likely to be separated by exceedingly long periods of inactivity. For example, the accretion of matter from the interstellar medium by white dwarfs and neutron stars is expected to cause an occasional supernova via the accretion-induced collapse mechanism. On rare occasions, collisions between helium white dwarfs or between a brown dwarf and a white dwarf can create short-lived helium-burning stars and possibly even stars that burn elements heavier than helium. With fleeting lifespans of no more than a few 108 years, such stars are expected to contribute little to the overall stellar luminosity in comparison to hydrogen-burning red dwarf stars in the degenerate epoch. Nonetheless, if the universe should obey the Copernican Principle, then the current universe has no special place in time and interesting things can still happen in the seemingly inactive universe of the very far future.

1. Cirkovic, M. M. (2005), “Brown Dwarf Accretion: Non-Conventional Star Formation over Very Long Timescales”, Serbian Astronomical Journal, Vol. 171, p. 11-17.
2. Fred C. Adams and Gregory Laughlin (1997), “A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects”, arXiv:astro-ph/9701131