Shock Breakout

Here you can see the brilliant flash of a supernova as its core blasts through its surface. This is an animated cartoon made by NASA based on observations of a red supergiant star that exploded in 2011. It has been sped up by a factor of 240. You can see a graph of brightness showing the actual timescale at lower right.

When a star like this runs out of fuel for nuclear fusion, its core cools. That makes the pressure drop—so the core collapses under the force of gravity.

When the core of a supernova collapses, the infalling matter can reach almost a quarter the speed of light. So when it hits the center, this matter becomes very hot! Indeed, the temperature can reach 100 billion kelvin. That’s 6000 times the temperature of our Sun’s core!

For a supernova less than 25 solar masses, the collapse stops only when the core is compressed into a neutron star. As this happens, lots of electrons and protons become neutrons and neutrinos. Most of the resulting energy is instantly carried away by a ten-second burst of neutrinos. This burst can have an energy of 1046 joules.

It’s hard to comprehend this. It’s what you’d get if you suddenly converted the mass of 18,000 Earths into energy! Astronomers use a specially huge unit with such energies: the foe, which stands for ten to the fifty-one ergs.

That’s 1044 joules. So, a supernova can release 100 foe in neutrinos. By comparison, only 1 or 2 foe come out as light.

Why? Neutrinos can effortlessly breeze through matter. Light cannot! So it takes longer to actually see things happen at the star’s surface—especially since a red supergiant is large. This one was about 500 times the radius of our Sun.

So what happened? A shock wave rushed upward through the star. First it broke through the star’s surface in the form of finger-like plasma jets, which you can see in the animation.

20 minutes later, the full fury of the shock wave reached the surface—and the doomed star exploded in a blinding flash! This is called the shock breakout.

Then the star expanded as a blue-hot ball of plasma.

Here’s how the star’s luminosity changed with time, measured in multiples of the Sun’s luminosity:

Note that while the shock breakout seems very bright, it’s ultimately dwarfed by the luminosity of the expanding ball of plasma. So, KSN2011d was actually one of the first two supernovae for which the shock breakout was seen! For details, read this:

• P. M. Garnavich, B. E. Tucker, A. Rest, E. J. Shaya, R. P. Olling, D. Kasen and A. Villar, Shock breakout and early light curves of Type II-P supernovae observed with Kepler.

A Type II supernova is one that shows hydrogen in its spectral lines: these are commonly formed by the collapse of a star that has run out of fuel in its core, but retains hydrogen in its outer layers. A Type II-P is one that shows a plateau in its light curve: the P is for ‘plateau’. These are more common than the Type II-L, which show a more rapid (‘linear’) decay in their luminosity:

5 Responses to Shock Breakout

  1. arch1 says:

    Thanks, fascinating. Parts of that supernova animation reminded me of the next-to-last image here (though of course a supernova is ridiculously, outrageously more powerful than a mere nuclear bomb, as memorably described in this xkcd what if?)

  2. Patrice Ayme says:

    Science is not just a knowledge of facts, but a knowledge of beauty which would not have been otherwise revealed. Science is also the uncovering of logic of previously unsuspected subtlety, for all to see. Thus subjects as esoteric as how exactly supernovae explode can reveal how explanations can go about things.

    Beauty itself, is partly a matter of logic: when god touches humanity with its finger in a famous painting, the beauty depicts a logic, and it is this uncovering of a logic what makes it, in part, beautiful… and certainly “interesting” as all great art is.

    Knowing about supernovae is not just knowing how the chemistry which made Earth possible was created. It is not just about knowing the size of the universe, and how fast it is changing. It is also knowing about analogies, metaphors, logics and possibilities we never suspected, and also about our naivety, to never have suspected they were.

    All of this to say it’s too bad the preceding post was not equipped with a “reblog” button. I had started an essay on the NASA cartoon above, but then this post was published, and it would have been easier for me to reblog it than to rehash known astrophysics. Rebloging gives full attribution, and, after a short extract, links back to the original post.

  3. domenico says:

    I am thinking that we see only a direction of supernova explosion, so that the theory cannot model the elementary interaction of the process; but if a similar process, like the inertial confinement fusion (in a lower energy without convective overturn), with different chemical elements and energies can be used to explore the explosion in many direction (with different spectroscopy and sensors, like a multiple particles collider), so that the correction to the Standard Model can be tested inside a controlled high density environment.

    • John Baez says:

      Nice! He estimates the distance from which the neutrinos released by a supernova would be fatal:

      Karam calculates that the neutrino radiation dose at a distance of one parsec [6] would be around half a nanosievert, or 1/500th the dose from eating a banana. [7]

      A fatal radiation dose is about 4 sieverts. Using the inverse-square law, we can calculate the radiation dose:

      \displaystyle{ 0.5 \textrm{ nanosieverts } \times \left(\frac{1 \textrm{ parsec }}{x}\right)^2 =5 \textrm{ sieverts }}

      x=0.00001118 \textrm{ parsecs } = 2.3 \textrm{ AU }

      2.3 AU is a little more than the distance between the Sun and Mars.

      The only mistake is to give two significant figures in the final answer; I’m sure it’s not known that accurately.

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