“You cannot, in human experience, rush into the light. You have to go through the twilight into the broadening day before the noon comes and the full sun is upon the landscape.” –Woodrow Wilson
Without a doubt, one of the most spectacular light shows of the cosmos happens when stars burn out — reaching the end of their normal life cycle — and die in a great supernova explosion. We’ve spoken in the past about the main ways that these stars die. Either a very massive star — something more than ten times as massive as our Sun — reaches the end of its nuclear fuel, and its core collapses, blowing off its outer layers in a massive explosion.
This is how the Crab Nebula, from a type II supernova explosion nearly 1,000 years ago, was created. These supernovae leave behind a collapsed object, either a black hole or a neutron star, at the center of the now-destroyed progenitor star.
On the other hand, less massive stars don’t collapse like this; their core merely contracts as their outer layers are blown off much more gently. This produces a planetary nebula that fades over time, and a very long-lived white dwarf at its core.
But these white dwarfs get a second chance at a supernova. When they either accrete enough additional mass or — as now seems to be the more likely scenario — merge with another compact object, they can also undergo runaway nuclear reactions. This typically destroys both stars, leaving neither a neutron star nor a black hole behind, but releasing a tremendous amount of energy in a type Ia supernova.
There is some variation among the core-collapse types of supernovae, but I’d like to remind you of what goes on inside of those stars. While nuclear fusion is occurring, the outward radiation pressure from the fusion in the core holds the rest of the star up against gravitational collapse.
But when that nuclear fusion in the core runs out, the core collapses under its own gravity, emitting tremendous amounts of light via the conservation of energy. Why’s that? You know all about gravitational potential energy; it’s why that weight you drop onto your foot hurts so much! Well, when you collapse a large mass — something hundreds of thousands to many millions of times the mass of our entire planet — into a small volume, it gives off a tremendous amount of energy.
In theory, if we made a star massive enough, like over 100 times as massive as the Sun, the energy it gave off would be so great that the individual photons could split into pairs of electrons and positrons. Electrons you know, but positrons are the anti-matter counterparts of electrons, and they’re very special.
Because a positron will run into an electron in short order, annihilating it, producing a gamma-ray. And if the rate of gamma-ray production is fast enough, you heat up the core. In other words, if you start producing these electron-positron pairs at a certain rate, but your core is collapsing, you’ll start producing them faster and faster… continuing to heat up the core! And you can’t do this indefinitely; it eventually causes the most spectacular supernova explosion of all: a pair instability supernova, where the entire, 100+ Solar Mass star is blown apart!
At least, that was the theory. Was, I say, because in 2007, that’s exactly what was observed!
This supernova, known as Sn 2007bi, is exactly this type of pair-instability supernova that was only theorized. What’s extra remarkable about it is that, when we peer deep into the Universe to look at where it came from in depth, we literally see practically nothing!
I say practically nothing, of course, because there really is something there. Far away is a tiny, faint, and distant dwarf galaxy, just barely visible with the right image manipulation.
Dwarf galaxies, it turns out, form stars only on very rare occasions. But when they do, they form them in great bursts, with often the entire galaxy becoming a great star-forming region! When this happens — much like the great star-forming region at the center of our galaxy — we get large, 100+ solar-mass stars as part of the deal: the only candidate for forming these pair-instability supernovae!
As was just reported, this supernova, Sn 2007bi, is the first confirmed pair-instability supernova ever, and it needs this relatively pristine environment present only in young, dwarf galaxies (and not in our metal-rich Milky Way’s center) to do it!
Which means, of course, that if we want to see one close by, we need to know where to look. Ladies and gentlemen, may I present to you your neighbor, NGC 1569!
A dwarf galaxy so close to us it’s actually blueshifted towards us, this old, low-metal galaxy has been forming stars as recently as within the last 5 million years! (Look in the X-ray if you don’t believe me!)
So supernovae formed from massive stars will leave you a neutron star, or, if they’re bigger, a black hole, or, if they’re really bigger, absolutely nothing, except a much richer, heavy-element-filled Universe, perfect for creating things like you and me. And aren’t we fortunate for that!