They may be bigger than the orbit of the planet Saturn, they’re about as massive as the Sun, they’re invisible, and they’re powered by annihilating dark matter.
Meet the dark stars – the first settlers of the newborn Universe. Some of them might even still be around today.
It’s a weird idea: giant, extremely tenuous stars that don’t give off any visible light.
In fact, you might even wonder if they truly deserve ‘star status’. But American physicists Paolo Gondolo, Katherine Freese and Douglas Spolyar are pretty sure that the very first stars in the Universe were these dark monsters.
In the beginning
The birth of the first generation of stars in the young Universe is shrouded in mystery. They must, so the theory goes, have formed shortly after the Big Bang, from small density enhancements in the primordial gas.
The self-gravity of these dense blobs was able to withstand the general expansion of space. Over time, they condensed into great balls of fire – spherical clouds of gas whose interiors became dense and hot enough to ignite nuclear fusion reactions.
More about the Universe after the Big Bang
A first-generation star looked nothing like our own Sun.
Firstly, it didn’t contain any heavy elements: the Big Bang only produced hydrogen and helium, and just trace amounts of deuterium (heavy hydrogen), lithium and beryllium.
Secondly, it was probably much larger and much more massive, weighing in at a few hundred times the mass of our Sun.
As a result, it burned ferociously, and only had a very brief lifetime of a few hundred thousand years or so.
At least, that’s the theory, for no-one has ever seen a first-generation star. But Gondolo and his colleagues claim that the truth is even more bizarre.
That’s because the generally accepted ideas about the formation of the first stars in the Universe never took dark matter into account.
“This was really a new and unexpected insight,” says Gondolo, an associate professor of physics at the University of Utah in Salt Lake City.
Dark matter is a mystery in itself. Astronomers detect its gravitational influence on all possible scales, so there’s little doubt it makes up some 85% of all gravitating matter in the Universe.
But no-one knows its true nature, except it can’t consist of ‘normal’ atoms and molecules. Anyway, if a primordial blob of gas collapses under its own weight, it must contain large amounts of dark matter, too.
Heart of darkness
According to detailed calculations by Gondolo and his colleagues, the dark matter in the blob’s contracting core eventually stops the collapse, well before the core gets dense and hot enough to ignite nuclear fusion.
The reason? Dark matter particles are their own antiparticles, which means they start to annihilate one another when they’re packed tightly enough.
The annihilation energy balances gravity and prevents the gas cloud from further collapse. A dark star is born.
“They don’t shine like normal stars,” says Gondolo of dark stars. “But they’re much denser than nebulae, and they have an internal energy source that prevents them from further collapse, just like normal stars.”
If this is the case, then the properties of dark stars crucially depend on the precise nature of the dark matter particles.
Therefore, dark stars could be anything between four hundred and two hundred thousand times the size of our Sun.
In other words, slightly smaller than the orbit of Jupiter, or dozens of times larger than the orbit of Pluto.
Their masses could range between 0.5 and 10 times the mass of our own Sun.
Either way they would have very low densities, although Gondolo points out that they would still be much denser than the average galactic nebula.
That’s intriguing in its own right, but there are new mysteries, too.
If the first generation of stars never became hot and dense enough for nuclear fusion to take place, then where and when did the first heavy elements in the Universe form?
Spectroscopic fingerprints of dust particles (including heavy elements like oxygen and silicon) have been found in galaxies that existed when the Universe was less than a billion years old. So without nuclear fusion, where did these elements come from?
Can darkness shed light?
Gondolo and his colleagues suggest a variety of solutions to this problem. It could be that dark stars are just a temporary phase in the early evolution of the first generation of stars in the Universe.
Once enough dark matter has self-annihilated, gravitational contraction might take over again, eventually leading to a ‘normal’ star powered by nuclear fusion.
Or maybe dark stars are only born in regions with an excessively high dark matter density, while ‘normal’ stars form simultaneously in other regions.
On the other hand, the dark star theory might help solve a nagging cosmological riddle: the existence of massive black holes in the very early youth of the Universe.
No-one understands how supermassive black holes could have formed so quickly, but dark stars may have played a role, by rapidly attracting more and more matter from their surroundings.
In any case, says Gondolo’s team, the very early history of the Universe could have been markedly different from what astronomers have generally come to accept.
Moreover, if dark stars are still around today – it’s unclear how long they could last – they might provide a promising way to actually detect dark matter in the Universe and finally study its properties in more detail. If this is the case, then dark stars could turn out to be very illuminating after all.
