Type II Supernovae Up Close and Personal

I take that title back a bit. You really don’t want to get too close to a supernova because there are so many types of lethal radiation and a shock wave that will kill you too. The neutrinos might not be lethal, but I'm not so sure since there are so many of them. But, I promise by the end of this blog to talk about the personal side.

I’m thrilled to see just today that all the types of smart telescopes are doing a good job imaging the new supernova in M101. If you haven’t imaged it yet, you still have a lot of time, but it’s well placed in the sky for nice long exposures to get rest of the details of the galaxy too. So don’t wait too long. The supernova can be imaged quickly because it’s so bright. The smoke from the Canadian fires aren't helping here in North America.... ;-(

I’m also interrupting my blogging plans to focus a bit on this one because we can learn so much about it. You know I hear from many of my amateur astronomy friends about how they like to look through telescopes and have the actual photons from their target object actually strike their own retinas. Well, I’m a smart telescope aficionado and I’m charmed by that sensibility, but really…. When we have the ability to get good color images with telescopes we can actually heft easily and set up in minutes, I say, give me a great smart telescope experience anytime!

The sources of astrophysical photons are more stimulating to my sensibilities. I like to know as much as I can about what I’m seeing. I truly believe we don't really "see" without knowing what we're looking at. So, I naturally wanted to know more about this new, Type II supernova. The usual explanations of it being due to an iron core collapse in a massive star is one I immediately think about, but you know, that’s not exactly what we are witnessing. What are we really imaging and what powers it? That’s what I want to answer here.

It’s not too hard to imagine what we are witnessing, but strap your seat belt on, it’s pretty amazing. We only need to do a little math to make it more real.

So…. This supernova is about as bright as its host galaxy. (Note that it’s in a spiral arm. That’s where we find the populations of stars that can give rise to Type II supernovas.)

What does that say about intrinsic brightness? It emits the light we see in the visible part of the spectrum, so we can use our Sun as a reference to perform a back-of-the-envelope estimation. The only way we can get the total quantity of light out in visible wavelengths is to imagine the photosphere (the emitting “surface” of the expanding supernova.) of the supernova to be large enough to emit 10^10 times the luminosity of our Sun. It’s actually a bit hotter than the Sun, so we can take advantage of that to gain energy, but only by a little.

The only way to emit nearly ten billion times more light is to make the supernova's photosphere radius larger by a factor of the Square Root of 10^10. That’s because the area of the sphere scales as the radius squared. To account for a slightly hotter surface than our Sun, the answer we get for this supernova at this point in its life is about 0.25x10^5 times the radius of the Sun! That’s right, this thing would appear in a snapshot to be like the Sun, but with nearly a million times the surface area.

Think about that. It has to be that much bigger for its surface area be emitting so much light. I have to admit I was shocked by this number too. I will also give David Arnett credit for walking me through this calculation in his definitive book, “Supernovae and Nucleosynthesis,” by Princeton U. Press. (It was actually one of my last text books on a course I audited at U. of Chicago.)

How big is this photosphere? It comes to a radius of 2x10^15 cm! (Brief aside… us astrophysicists like to use the cgs system of units. It’s absurd, I know, but it’s true.) Well, translated into units we can start to get our head around and we derive that this supernova “star” is about 2.7 times the diameter of Pluto’s orbit! In the matter of days this thing has existed since the core collapse, it has hurtled itself out this far! If you could see it up close inside M101, that’s what you’d see. A photosphere slightly hotter than our Sun’s, but big enough to swallow a dozen Solar Systems.

The next question I’m sure you’re asking yourself is… What’s keeping the energy flowing in to light this thing? That’s a good question, but first, let me describe how this thing will evolve. It’s dynamic and won’t stay in this realm for long.

It’s rapidly expanding at relativistic speeds and will become less opaque. It's slowly cooling, but not too fast because of radiation generated in the original blast. Eventually, in maybe a few hundred years it will look like the Crab Nebula we know so well in our Milky Way galaxy. But for the next few hundred days, it will continue to look like a ginormous star. So, something has to be supplying energy.

I just learned from re-reading Arnett, that this heating is due to the radioactive decay of Nickel-56 into Cobalt-56. I won’t go into the details here, but there is plenty of energy available and the time scales work for what they’ve been seeing in supernovae like this one. The nickel, of course is ginned up by the detonation of the original iron core collapse. I think we can expect it to be visible through smart scopes for a few months even though it will soon start dimming. The daughter nucleus of Cobalt decays then into Iron 56 and stays stable.

Of course astronomers talking about supernovae describe how many of the heavy elements are made in them. Life, including us, was not possible in the universe until supernovae came around. The Iron 56 in our hemoglobin started out just this way.

But, it’s a mixed bag ascribing all the heavy elements to particular types of supernovae. There are also Type I’s as well as colliding neutron stars that make up lots of elements. I remember when I used to say that the gold in your ring or tooth came from a supernova. Well…. most of that gold probably came from colliding neutron stars — those little twins are hogging the lime light!

The take away in my mind is that we can personally witness examples of the great cosmic story from our back yards with these smart telescopes. Supernovae are one of the singular events in our own evolutionary backgrounds too and we will have a chance to relive the past and observe at least one a year, I think. (I know I promised to tell you how to stay alerted. I will, but that’s too practical for this posting.)

So… Sit back and take a deep breath. All the oxygen you just inhaled came entirely from a Type II supernova just like this one. Exhale, relax and then enjoy witnessing a replay of your past and just be glad M101 is 27 million light years away.

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The attached image (including credits) shows the latest periodic table with astrophysical sources on them. A Type II supernova is considered an exploding massive star.