Where do Type Ia Supernovae come from? [Starts With A Bang]

“You have to have a canon so the next generation can come along and explode it.” -Henry Louis Gates

When it comes to stars, their fates are very well known. Every single star that’s massive enough to fuse hydrogen into helium in its core will someday run out of fuel and die.

The very brightest and most massive stars — about 1-in-800 of all stars — will die in a spectacular, core-collapse supernova when their core burns fuel all the way through iron and finally runs out of room to go.

This kind of supernova explosion, a Type II supernova explosion, is the second most common in the Universe. But for the other 799-out-of-800 stars, their fate is much gentler.

When they run out of fuel, however long it takes and whatever element it stops at, the core of the star (or the entire star, in the case of an M-class star) contracts down to a degenerate mass of atoms, held up only by the Pauli Exclusion Principle, preventing the electrons in the atoms from getting any closer together.

This results in an object that’s about the physical size of planet Earth, but about the mass of an entire star, some 300,000 times denser than Earth. That’s what a white dwarf star is.

And for some of these white dwarfs, this is the end of the line. Over time, they’ll cool and radiate energy away, finally dimming out and becoming ultra-cold black dwarfs, on timescales of at least quadrillions of years.

But some of these white dwarfs will get a second chance to make their voices heard across the cosmos.

A different type of supernova — a Type Ia supernova — can happen if the circumstances are right. You see, the reason that the heaviest-mass stars become a Type II supernova is because the atoms in the core, even with the Pauli Exclusion Principle, cannot stand up against collapse.

The very atoms in the core are subject to tremendous outside pressures, and if the core itself — no longer fusing any elements and thus devoid of new radiation — is too massive, it will have no choice but to collapse even further.

In the case of a normal atoms (as in a white dwarf star), this will happen if the mass reaches or exceeds about 1.4 solar masses, known as the Chandrasekhar mass limit.

If a white dwarf star acquires enough mass that this limit is exceeded, or that some other process creates too strong of a pressure at the core, a runaway fusion reaction occurs, destroying the entire white dwarf in a catastrophic Type Ia supernova.

Above is the remnant of the Supernova first observed on Earth in 1006, the brightest ever recorded in history. This composite image (including X-ray data on the interior) shows what happens in the aftermath of such an event.

These Type Ia supernovae are the most abundant type of supernovae in the Universe. But the question remains, how do these events occur? First off, it’s pretty clear that these events are not all identical. Because the laws of physics are the same everywhere, if all Type Ia supernovae were identical, the light curves of each of them would be identical. And they’re similar, but there’s quite a bit of variety.

It used to be thought that these supernovae came about because, in binary star systems, the very dense white dwarf could siphon mass off from its companion star, doing so until it eventually exceeded this Chandrasekhar mass limit. And then, when the white dwarf got too massive, the atoms in the core would give way, there’d be a runaway fusion reaction, and a Type Ia supernova would result.

But this process would be far too rare, and also far too uniform, to explain the Type Ia supernovae that we presently see.

At this point in the Universe, white dwarfs have already become the second most abundant type of star around. So a second possibility was brought up: perhaps two white dwarf stars spiral into one another, and will eventually merge, exceeding the Chandrasekhar mass limit when they do! (Video may trigger seizures in epileptics; audio is based on the frequency of gravitational radiation.)

But this may not be the only way to do it, either. Even though we have observed inspiraling pairs of white dwarfs, they, too, may not exist with high enough frequencies to explain the supernovae rates we observe. Even more damning, however, is that the details of the spectra do not match the models!

But a new theory proposed by J. Craig Wheeler — the White Widow model — just might be the solution to where the majority of these supernovae come from.

Rather than a slow siphoning of mass or a rare white dwarf-white dwarf inspiral, these supernovae could be caused by interstellar collisions between white dwarfs and other, normal stars, including the most abundant of stars, the class-M red dwarfs.

As Wheeler himself says:

“I believe that the spectra have to be respected. The really high-order constraint [on a supernova model] is to get the spectral evolution correct. That is, you’ve got to get all the bumps and wiggles, and they’ve got to be in the right place at the right times.”

Because all you have to do is create an instability in the core of this white dwarf, and runaway fusion will undoubtedly occur all over the star, as the following simulation of a white dwarf’s temperature and density shows nicely.

So, is this the way it actually happens? It’s going to take more research to find out, but this is an outstanding possibility that illustrates the sheer variety that the Universe undertakes when creating even these “standard candles,” the very ones that were used to first discover dark energy!

I’ll likely be out of touch for the next few days while I’m at Carl Sagan Day (remember?), so for those of you who can make it, I hope to see you there, and for those who can’t, I’ll be back on Monday!

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