What Exactly are Supernovae?


This is one topic I bet you guys have been looking forward to since I first started posting about stellar evolution. Well, I won’t disappoint you!

In my last post, we covered how a massive star gets to the point of supernova. When it exhausts all the nuclear fuel in its core, iron ash is left behind—which can’t be fused or split for energy. That’s a dead end for the star, and the core begins to freely collapse…

Until a shockwave, originating in the center of the star, pushes outward. It’s stalled at first, but convection as in-falling material bounces off the dense core gives it a boost, and the star bursts apart.

Now, we’ll cover all the ins and outs of these spectacular explosions.

And when I say they’re spectacular…I mean spectacular.

Supernovae are so powerful that most of the iron ash in the core of the star is destroyed. So…where the heck does the iron in the interstellar medium come from, if not from the cores of supergiant stars?

Turns out, it actually comes from supernovae, which are powerful enough even to trigger nuclear fusion in the star’s outer layers as they blow the star apart. Iron forms from these reactions.

There are probably many different types of supernovae, only a few of which astronomers have been able to observe in human lifetimes. Those that we’ve studied are called Type I and Type II.

I know. Imaginative, right?

First, let’s talk about Type I supernovae. Remember how in my post on the evolution of binary systems, I said that matter transferred from a main sequence star that fills its Roche lobe to a companion white dwarf can cause a nova?

It’s not quite the same as a supernova and it’s not one of the types shown above. But the thing with novas is that they involve ready nuclear fuel accumulating on the surface of a white dwarf, becoming degenerate, and getting so hot that nuclear reactions, unhindered, go out of control and result in an explosion.

That explosion, called a nova, blasts most of the accumulated hydrogen off the white dwarf’s surface—but not all of it. And as I described in my earlier post, novas can reoccur again and again as more material from the white dwarf’s companion falls onto its surface.

Here’s what can happen. Since novas don’t blast away all the accumulated material, it can build up over time, eventually increasing the white dwarf’s total mass until it exceeds the Chandrasekhar limit of about 1.4 solar masses (M).

At that point, the white dwarf collapses. Unlike a collapsing massive star, a white dwarf has usable nuclear fuel—it still has an inert carbon core, it just has never been massive enough to ignite carbon fusion.

The problem is, the white dwarf is still made of degenerate material—that’s material that doesn’t follow the laws of physics we know and love, and is ridiculously dense. It follows the laws of the quantum mechanical world. The consequence: it can’t regulate the speed of its nuclear reactions.

The result: the carbon in the white dwarf’s core fuses suddenly and completely in a violent explosion called carbon deflagration, which completely destroys the star and leaves only an expanding cloud of hot gas.

This produces a Type Ia supernova.

Wait a second—type Ia? There’s multiple type I’s?

Yup—and now I’ll introduce you to the other type I supernova, a type Ib supernova.

I know. Astronomers name things so creatively.

Type Ib supernovae are caused when a massive star loses its hydrogen, either to a strong superwind or to a binary companion (by filling its Roche lobe).

Type Ia and Ib supernovae have very different causes, but are also similar in that they generally involve binary systems…and that they have no hydrogen lines in their spectra.

You might remember from posts I published eons ago that spectra are how we know just about anything about a star, from what it’s made of to its temperature to its lifespan. Having hydrogen lines simply means that the star contains hydrogen in its atmosphere. Having no hydrogen lines means…well…there’s no hydrogen in that star.

That’s what makes type I supernovae so weird at first glance. Astronomers expect stars to have hydrogen in their atmospheres. Even though all the hydrogen in their cores gets fused into helium and then carbon, they never use the hydrogen fuel in their atmospheres.

The spectra of type Ia supernovae make sense, however, when you think about the composition of a white dwarf. A white dwarf is a stellar remnant, not a true star. The star’s atmosphere has already been blown away to form a planetary nebula, and there is little to no hydrogen left in the white dwarf itself.

The spectra of type Ib supernovae also make sense. Even if a massive star expands to fill its Roche lobe and loses its atmosphere to its companion, the giant will continue to evolve. It’ll exhaust its nuclear fuel, develop an iron core, and collapse into a supernova—but it will have lost all its hydrogen.

So…what’s a type II supernova?

Well, put simply, it’s the “normal” kind—what happens when there’s hydrogen involved.

Type II supernovae are just the “plain-ol'” explosion of a massive star whose iron core collapses.

These three types of supernovae can generally be grouped into a “type I” and “type II” category not only because of their spectra, but because of their light curves.

It’s clear from observations that these aren’t the only two supernovae light curves in existence. For example, a supernova in 1987 produced a very strange light curve, its luminosity stalling for a few months before reaching its peak and then falling.

That means that, especially given how rare supernovae are, it’s very probable that there are other types of supernovae besides type I and type II, and not just outliers, exceptions to the rule. Supernovae are a subject of ongoing study, and they are so rare that we depend on observations of them in other galaxies.

Most of what we know about them comes from supernova remnants, the clouds of gas that they leave behind. And here’s where the story comes full-circle, all the way back to the star formation we explored many moons ago.

You know how we can say living things have a life cycle because they reproduce before they die?

Well, stars don’t reproduce, exactly…but when they go supernova, they return their materials to the interstellar medium, the stuff between the stars. And the shockwave that produces a supernova can and often does trigger new star formation.

So, now you know the ins and outs of supernovae—how stars reach that point in their lives, how they explode, and how they return their materials to space for the formation of new stars.

Next up, we’ll move on to one of my favorite subjects in astronomy: the compact objects that are left behind when stars meet their end.

Questions? Or just want to talk?

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