Dark Stars from the Big Bang

The conventional view of star formation says that the first stars formed just after the big bang (200 million years) by the fusion of hydrogen nuclei (protons) to form helium. These so-called population III stars were around 100 times the mass of the sun. As such they’d be super hot and burn through their hydrogen fuel really fast.

The trouble is where are they? We should be able to see them if we look far enough out (remember in this expanding universe, that means far enough back too). Well, where are they? 

A new theory says perhaps there are none and that instead they were “dark stars”. If you recall, most (85%) of the universe is just dark matter. We can tell this because of its gravitational effects. Dark matter emits no light but it has mass and therefore it has gravitational pull. Now, we can tell how massive a galaxy is by counting how many stars it has and how massive they are. Many galaxies, including our own, the Milky Way, are spinning so fast that the stars should have been flung out of the galaxy. So something must be pulling them back in and that something is, we postulate, dark matter. The Milky Way is about 95% dark matter! In fact, if it wasn’t for this dark matter, none of the galaxies would have formed!

We don’t know exactly what dark matter is, but maybe it’s WIMPs (weakly interacting massive particles). 

Dark stars were postulated because it was noticed that WIMPS can produce energy. Now, normal stars make energy by fusing smaller particles to make bigger one (like hydrogens to make a helium). WIMPS have two forms like matter and anti-matter and if you bang them together they annihilate each other in a burst of energy. To do this, you need a hydrogen and helium cloud with 0.1% dark matter in its center. This cloud starts off at say 1 astronomical unit (au) which is 93 million miles in radius – it’s like our sun expanded up so that we’re actually inside it (like a red giant*). At these densities the WIMPS annihilate each other and make energy, but not enough to ignite like a normal nuclear fusion reaction but enough to make the cloud glow (about 10,000 deg C). As the cloud is big and warm but not hot, it can continue to grow. It could become 10,000 times bigger and a million times more massive than the sun. Ok, they misnamed these stars as they could shine brighter than a billion suns!

But unfortunately, there isn’t much dark matter inside this cloud so eventually it would all be used up and the star would collapse. Now it’s unlikely that any such stars survive but they’d be hard to miss although they’d be very, very far away – right at the edge of the universe. Now the wonderful new  JWST telescope is almost able to see that far, so who knows?

The JWST telescope can see stars at the edge of the universe, but they’re fuzzy and faint and there are too many of them as the early universe should’ve been fairly dark. Some recent computer simulations conclude that dark star formation would reduce the polulation III star formation. So maybe these stars are the population III stars and not the dark stars. We need a better telescope :)

*Red giants – when our sun dies it starts by running out of hydrogen to convert into helium so the center of the sun will collapse and heat up. This will make the outer layers to swell up. It will get really big, engulfing the earth and inner planets. More fun stuff will happen after that! But this is 4 billion years down the road :)