What would happen to us on the blue marble if our closest stellar neighbours ran into each other? And I don’t mean bumping into each other while picking up groceries. Head-on collisions between stars are rare (this is mainly due to space being pretty big) but they do happen in dense globular clusters every once in an astronomical while. We don’t have a dense globular cluster nearby to work with, nor a lot of time, so instead we’ll find a way to make it happen.
The neighbours in question are the close pair of stars in the Alpha Centauri system, each roughly the sun’s mass. One is slightly larger and a good bit brighter than the other, and they are close enough to each other to combine to form the third brightest object in the sky. I mean, they’re still about 30 times farther apart than the distance between the Earth and the sun, but that’s really close in the bigger picture. There’s actually a third star in the Alpha Centauri system as well — it’s the closest star in the sky to us, and the aptly named Proxima Centauri is rather interesting, despite being quite dim and small. There is an Earth-sized planet in the habitable zone orbiting Proxima Centauri, and it’s such a promising planet that there is a plan (“Project Starshot”) to send a scattering of tiny probes there to try and see, among other things, if there’s any life.
Anyhow, we’ll say that to cause our collision, a pretty white dwarf star passes right by the Alpha Centauri pair, miraculously slingshotting the lighter one smack dab into his beefier brother. This should not be an event to miss.
Back at home, 4.37 light years away, I’m sitting on the edge of my seat. Reading. The light from the event will take those four years to reach me. But eventually …
The two sun-sized stars collide. Hmm. It might be more appropriate to say they merge. It’s simply less exciting than I had hoped for. Even if they were to collide at nearly a thousand kilometers ***per second, the relative velocity of the stars is pretty similar to the thermal velocity of the stellar matter already there. (Because remember — hot stuff jiggles really fast. It’s a law of nature.) So they merge, and become one star that is quite a bit brighter than either was before, but there is no grand explosion. I guess I’m not going to be reading by starlight. Disappointing.
Okay, instead of having the white dwarf being the cause of the stellar collision, let’s have it smash right into both stars. Consecutively. And after that, Proxima Centauri as well. It is extremely unlikely that all three stars are in such a perfect alignment for this to be able to happen, but who cares? I want to see some fireworks. Let’s also assume the white dwarf is pretty heavy for one of its kind, say about 1.3 solar masses. Just for fun.
Well the whole perfect alignment thing turns out not to matter much. Almost as soon as the dense white dwarf hits the first star in the line, it surpasses the Chandrasekhar mass. The what-mass, you ask? Well, this essentially means that all of a sudden, a good portion of the matter inside the white dwarf fuses all at once, the runaway fusion reaction putting out about 10^44 joules of energy. A good rule of thumb is that if you see anything to the power of 44, stay away from it. It’s probably dangerous. On top of everything, this reaction all happens in a few short seconds. The white dwarf is shredded in a supernova that is four billion times more luminous than the sun. If I’m still sitting on the edge of my seat, it’s time to put on some shades.
As it turns out, the shades would be nice to have, but not a necessity. Because space is big and everything is far away, the supernova only appears to be six per cent as bright as the sun, which it turns out is still amazingly bright. The moon is only 0.0003 per cent as bright as the sun; our eyes are just kinda really good at adjusting to different levels of brightness. So yeah, reading by the starlight? Not a problem.
But also, someone had better call those guys trying to send probes to that little promising planet around Proxima Centauri. If there was life before, well, there’s not now.