The Extinct Star: Quasi-Stars.
Y’know how things from the past seem to have always been bigger? Things like animals and insects and fish and plants. Let’s add stars to that list. Quasi-stars.
We should all be familiar with one slightly important star—the Sun.
Even though the Sun looks dime-sized from here on Earth, it’s actually a relatively large star. In fact, the Sun is larger than 70% of the stars we’ve observed in all our astronomical history. However, it still pales in comparison to some of the bigger stars in the universe, like UY Scuti, which is approximately 1,708 times the size of the Sun. Yet even UY Scuti is a dwarf compared to a quasi-star.
Quasi-stars don’t look much different than any other stars. They’re super bright, but also super short-lived and pretty cold for a star.
Well, it might be more accurate to say quasi-stars were all those things. They’re not around anymore, and we should be grateful for that too. Quasi-stars only formed in the early stages of the universe when there was a lot of mass in not a lot of space, but they’ve had a considerable impact on it post-extinction.
“What does ‘quasi-’ mean?”…
…is probably what you’re thinking right now. And actually, answering that question can help us understand these “stars” a bit better.
The prefix quasi- means “seemingly” or “apparently, but not really”. Essentially, it’s used to denote something that seems like another thing but really is not. In this context, it seems like quasi-stars are normal stars, and I’m sure you’ve caught on to the problem with that assumption.
The stars that we’re familiar with today are powered by nuclear fusion in their cores.
Nuclear fusion is a reaction in which two or more atomic nuclei collide with enough energy to form subatomic particles (mainly neutrons) and atomic nuclei of different elements. This reaction releases immense amounts of energy which acts as fuel for further nuclear fusion to occur and keep the star alive.
Quasi-stars—on the other hand—run on black holes at their core.
Yep. The same thing that draws in and destroys everything it can force to do so. The same thing that has a gravitational pull so strong that not even light can escape it. Counterintuitively, quasi-stars survive on their mass quite literally being continuously devoured by a black hole.
Let’s talk about how this happens.
The formation of quasi-stars.
It starts with a protostar.
Protostars are cute little baby stars. To be a little more technical, get rid of the “cute” and “little”. Protostars are stars that are still in the process of gathering fuel and matter and have not yet started to undergo nuclear fusion.
In the formation of a quasi-star, the core of a large protostar collapses in on itself to form a black hole. For this inner collapse to form a black hole instead of a supernova (the violent explosion of a star), the protostar would have to be at least 1,000 times the mass of our Sun—a number close to 2.0 x 10³³ kg.
Great. So now we’ve got our black hole at the core of the quasi-star. The black hole’s immense gravitational force will immediately begin to consume the stellar matter of the protostar around it. And though this seems like an overwhelmingly one-directional system of force and energy, that is not entirely the case.
The infall of matter into the hole generates massive amounts of radiant energy. In fact, the constant outburst of energy from the core of the star essentially counteracts the black-hole’s gravitational pull. This creates a state of equilibrium in a quasi-star’s forces system that allows it to remain as a star for longer than just a couple of seconds.
The life-span of quasi-stars.
The larger the quasi-star, the shorter its lifespan. And that should make sense as the black hole that forms at its centre will be bigger and will be able to consume mass a lot quicker than it can produce energy to keep it burning.
The Sun has a lifespan of about 8 billion years, of which about 4.5 billion years have already passed. A quasi-star has an estimated lifespan of just 7 million years, on average. This means that quasi-stars can live only about 0.0875% of the life of our sun. Or, on a more familiar scale, about 9 quasi-stars could have lived and died between the time dinosaurs went extinct on Earth and today (about 65 million years ago).
During its 7 million year lifetime, the black hole at its core would have grown to a mass of about 2.0 x 10³³ kg to 2 x 10³⁴ kg. It would have maintained a surface temperature of about 10,000K, or about 9,700°C. A quasi-star would have slowly cooled as it lost mass until it reached its limiting temperature of about 4,000K (3,730°C), indicating the end of its life. At this point, the star would have quickly disappear into its core, leaving just the black hole behind.
Now here’s the fun part. The black hole that remains after the death of a quasi-star won’t just be some ordinary black hole. It’ll eventually reach the size of a supermassive black hole, which are at the centre of most large galaxies just like the Milky Way.
It’s pretty cool to think a galaxy that we can observe today is a remnant of one of these false stars.
The size of a quasi-star.
Quasi-stars can be as luminous as a small galaxy, so expect some pretty ridiculous size.
They had a radius of approximately 10 billion kilometres. That’s 67 times the approximate distance from the Earth to the Sun (67 AU), or 14,000 times the radius of the Sun itself. To put that in context, Pluto is only about 40 AU from the Sun.
But pictures are better than words. Here’s the Sun compared to the largest known star in the universe—Stephenson 2–18.
And now here’s Stephenson 2–18 compared to a quasi-star.
Yeah. Pretty big. And to think that there’s a proportionately big black hole in there. Pretty scary too.
I don’t know about you, but I’m pretty happy quasi-stars are extinct.
In fact, you might be thinking—what would it be like if there was a quasi-star at the centre of our solar system instead of our Sun? And long story short: we wouldn’t know. Because we’d be dead.
