In our neighborhood of the Milky Way, we see a region surrounding the solar system that is far less dense than average. But that space, that cavity, is a very irregular, elongated shape. What little material is left inside of this cavity is insanely hot, as it has a temperature of around a million Kelvin.
What’s going on?
We call this region surrounding the solar system the Local Bubble, which is a bit unimaginative, but it gets the job done. This bubble is carved out of the interstellar medium and is about 1,000 light years wide and has a density about one tenth the average density of the Milky Way.
At the edge of the Bubble is a shell of gas with slightly higher density than the surroundings. And in that shell there are a bunch of young hot star-forming regions. Based on the sun’s current position, our velocity, the size of the Bubble, and our location in the Bubble, we know that we were not born here. We entered one end of the Local Bubble somewhere between five and ten million years ago, and we will continue traveling through the Bubble for tens of millions of years to come.
What could have the energy to create this enormous cavity, this giant bubble?
A single supernova doesn’t have nearly the kick required.
Active galactic nuclei have more than enough energy to blow great voids like these, but those affect entire galaxies. If the Milky Way were active, very likely we wouldn’t even be here to enjoy the view.
So it has to be something in between. Something strong than a single supernova, but much weaker than an active galactic nucleus.
How about instead of a single supernova, we have a bunch going off at once? Maybe a hundred, or even a thousand, supernova all triggering in a short amount of time. That would definitely provide enough energy to carve out a Bubble, but is it possible?
Well it helps that stars are never born alone.
When you have a giant molecular cloud, a big cloud of gas and dust, it’s…big. A single giant molecular cloud has enough material to form up to tens of thousands of stars at once. Most of that gas does not get to participate in star formation for various complicated physics reasons. But once the process of star formation gets going, you will see an entire cluster of stars appearing out of that gas cloud because it will compress, it will fragment, and then all of a sudden, you’ll get a whole population of stars. Now, when these gas clouds make populations of stars, they make all different kinds of sizes of stars.
They make small ones. They make medium ones. And they make big ones. And then because physics is physics, and it’s easier to make small stuff than it is to make big stuff, you get a whole bunch of small ones, a fair number of medium ones, and then a handful of giant ones. But if the molecular cloud is big enough, it can produce hundreds, or even thousands, of giant stars at once, alongside millions of smaller ones.
And all of these giant stars will have very similar lifetimes because the lifetime of a star is dictated by its mass. Which means they are all born at roughly the same time, go through their life stages in parallel, and die in roughly the same time.
We already know what a single supernova can do. Look at the Crab Nebula, where you have this expanding shock wave that’s been expanding for a thousand years. And then inside it has evacuated this cavity, and we can see all the lacy tendrils of leftover gas. We know that a single supernova can affect its local environment and blow out cavities a hundred light-years wide.
They’re able to do this because they pour out so much raw radiation. The light itself pushes on the surrounding material – the hydrogen and helium atoms, the dust grains, everything – and literally pushes them out.
It takes a lot of light to do it, but that’s exactly what large stars have going for them. So we know that individual supernova can carve out pretty decent sized cavities in the interstellar medium.
And we know that clusters of supernovae can go off at relatively similar times. And we think the Local Bubble is a product of it. We think that millions of years ago, multiple supernovae went off and worked together to carve it out.
A scene of absolute carnage and destruction. We fit right in.