Pedagoguery

The first stars to form after the Big Bang are a matter of great interest and some speculation to astronomers. Typically referred to as Population III stars for obscure historical reasons, these stars were massive, in excess of 200 times the mass of our sun. However, there is the possibility that a different type of star was the first type to form, one that got its energy from a very different source than a typical star.

During the early universe, dark matter dominated. Its gravitational influence dictated where regular baryonic matter collected. Dark matter also potentially has an interesting property. According to some theories, dark matter is its own antiparticle. Therefore, if two dark matter particles collide, they annihilate, releasing large amounts of energy since they are so massive. If this is an accurate property of dark matter, then it is possible that the very first stars would have been powered by the annihilation of dark matter, rather than nuclear fusion. How would such stars have looked?

Typical Population III stars would have had somewhere between 200 and 1000 solar masses, surface temperatures higher than 100,000K, and luminosities in tens of millions times that of our sun. Even so, they would have been relatively compact – with radii about one tenth of an astronomical unit (a.u.). This is still quite large by the standards of main sequence stars today. The sun, for example, has a radius of about 0.009 a.u. A red supergiant like Betelguese, by contrast, has a mass of about 15 suns, a radius of about 10 a.u., a surface temperature of about 3500K, and a luminosity 100,000 times that of our sun.

Contrast this with a star powered by the annihilation of dark matter. Such a power source is much more efficient than nuclear fusion. When fusing four hydrogen nuclei into one helium nucleus, about 4% of the mass is converted into energy. By contrast, in dark matter annihilation, nearly 100% of the matter is converted to energy. (Some of the annihilation products could include standard particles like electrons and such.) As such, a dark matter star would contain between 1000 and 10,000 times the mass of our sun. The evolution would be somewhat arrested since the efficient energy source would stop the star from further contracting at a very early stage. Thus, they would be huge, ranging between 1 and 30 a.u. in diameter. At the largest, that would extend out to the orbit of Neptune from the sun's location. Because they are so large, however, they would not be as hot as a Population III star, a mere 5000K, about the same temperature as our sun. However, because they are so large, they would have tremendous luminosities – as much as a billion suns.

Dark matter powered stars would potentially last for millions of years, depending on the local density of dark matter. Eventually, however, the local density would fall below a critical threshold, and depending on how large the star is, its evolution would take one of two paths. For the lighter stars, normal stellar evolution would resume, and nuclear fusion would resume and they would take the normal path of a Population III star – that is, a brief, bright life ending in a supernova and a black hole. More massive dark stars would skip the nuclear fusion step and collapse straight into supermassive black holes.

This theory solves a puzzle. Quasars, which are the ultra bright centers of active galaxies, exist very early in the life of the universe. In order to be so luminous, they must be powered by supermassive black holes, and the standard way for such holes to form through Population III stars takes too long. The most massive dark stars provide a way for such massive black holes to form earlier.

Dark stars are quite speculative, but there could be evidence of their existence. Such stars would be quite noticeable provided we have a telescope sensitive enough in the far infrared, where their light would be shifted. Unfortunately, the James Webb Telescope, which will be launched in a few years, will not quite be sensitive enough, but some future telescope will probably be. Secondly, experiments at the Large Hadron Collider in Geneva will hopefully provide us with clues as to the nature of dark matter. Such clues can potentially rule out the types of dark matter that would allow such stars. Finally, there is a small, but non-zero chance that such a star could exist to this very day. Provided it had enough dark matter, there is no limit to how long such a star could shine. Clearly there isn't any such star close by – we would have noticed, but the telltale properties of such a star would stand out if one were found.

Next time, how to deflect an asteroid.