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Who is a supernova's partner in explosive crime?


by Sarah Scoles

What is a Type Ia supernova?

Well, first we have to answer the question "What is a white dwarf star?" 

What is a white dwarf?

A white dwarf is what average stars like the Sun become when they get old. They have used up their hydrogen and can no longer support nuclear fusion, having already fused atoms into other atoms with atomic numbers as high as they can: carbon and oxygen.

These stars radiate light not because of nuclear energy, but because of thermal energy. And if you've ever noticed the difference between the light from a electric stove burner and the light from a nuclear blast, you'll understand why white dwarfs are only about 1/10,000 as bright as the Sun.

A white dwarf, alone, is content merely to dissipate the heat from its innards--a remnant of its former lifestyle--until the energy is all gone. It's a quiet, calm process.

However, when another star is very close by, the white dwarf perks up. Matter from that nearby star may fall onto the white dwarf, feeding it.

So what? Why does changing the white dwarf's mass change anything but its mass?

A white dwarf is subject to the Chandrasekhar limit: the dwarf can only be 1.4 times the mass of the Sun. After that, it can't get up off the couch support itself.

What makes a star a star, and what keeps a star a star, is hydrostatic equilibrium: the gravitational force toward the center of the star is exactly balanced by the pressure pushing outward.

Once a white dwarf is 1.4 times the Sun's mass, the gravitation (which gets larger as the mass gets larger) outranks the pressure.

This is one vicious cycle. Source.

At the same time, adding more material to the star increases its temperature, and increasing its temperature will eventually allow it to, once again, fuse. When atoms fuse, they release energy, which further increases the star's temperature, which further increases its ability to fuse, which further increases its temperature, which further increases its ability to fuse.

It takes approximately no time for this reaction to become runaway and blast the whole star apart in an explosion that can outshine the rest of the stars in the galaxy combined. This is a Type Ia supernova.


Thanks, Nature. This is a super-helpful
graphic. Source.
It sounds like we know a lot about Type Ia supernovae. What do we want to know that we don't yet?

What kind of stars are sidled up to white dwarfs, egging them on to explosion?

There are three possibilities:

  1. A red giant star
  2. A main sequence (regular) star
  3. Another white dwarf star

But if we only see these binary systems as Type Ia supernovae--meaning, when the supernovae explosions actually occur--how will we know what was there before?

That's the beauty of archives: We can go back and look at images taken in the same spot as the supernovae, but before the supernovae, and see what we can see. A red giant star would look big and red. A main sequence star would look boring. A white dwarf would look like barely anything. Since those three possibilities are so different, the images should allow us to tell them apart--if, that is, images exist, and if those images are "deep" enough (if the "camera shutter" was left open long enough for dim objects to be picked up).

Guess what. There was a recent supernova in "nearby" Messier 101 (~20 million light-years away). Learn anything from that?

Li, et al., and Nugent, et al., sure did, and they published some papers in that zine Nature to tell us all about it.

This supernova was discovered by the Hubble Space Telescope only 11 hours after the initial explosion. It makes sense that the closer you get to the actual boom, the closer you get to seeing what everything looked like before the boom.


SN2011fe: an adorable name for an adorable ball of light.
The arrow points to our new favorite supernova.

In this case, the astronomers were able to detect both carbon and oxygen in the exploded material. The oxygen discovery was the first, and confirms what was previously a theoretical idea without proof--that the white dwarfs that give birth to Type Ia supernovae are made of carbon and oxygen.

In images taken before the supernova, no companion star was seen. Bummer. But since we know there must be a companion, we can infer that the companion was just too faint to be detected in the particular image that was taken. However, astronomers do know that if a red giant had been there, they would have seen it. So that leaves only two possibilities, which is less than three, which is an improvement.

Why does it matter what kind of star was in orbit with a white dwarf star before it become not-a-white-dwarf-star?

Type Ia supernovae are crucial to our sense of scale in the universe. We know at what mass and temperature a white dwarf star jumps on the runaway nuclear bandwagon, which means that we know how bright that bandwagon will burn. And if we know how intrinsically bright the supernova is, and we know how bright it appears to us, we can figure out how far the light must have traveled to have dimmed that much. In short, we can know precisely how far away a Type Ia supernova is, which means we can know how far away its host galaxy is. Precise distances to galaxies, and information about how fast they're moving away from us, led to a Nobel Prize for the "accelerating expanding universe" concept, which I wrote about here.

In short, our ideas about the scale and expansion of the universe are based on systems and events whose initial conditions we don't know. I know I've said it a lot lately, and I won't say it again, but I will say that perhaps this blog should be renamed "How does anyone sleep at night?"


Nugent, P., Sullivan, M., Cenko, S., Thomas, R., Kasen, D., Howell, D., Bersier, D., Bloom, J., Kulkarni, S., Kandrashoff, M., Filippenko, A., Silverman, J., Marcy, G., Howard, A., Isaacson, H., Maguire, K., Suzuki, N., Tarlton, J., Pan, Y., Bildsten, L., Fulton, B., Parrent, J., Sand, D., Podsiadlowski, P., Bianco, F., Dilday, B., Graham, M., Lyman, J., James, P., Kasliwal, M., Law, N., Quimby, R., Hook, I., Walker, E., Mazzali, P., Pian, E., Ofek, E., Gal-Yam, A., & Poznanski, D. (2011). Supernova SN 2011fe from an exploding carbon–oxygen white dwarf star Nature, 480 (7377), 344-347 DOI: 10.1038/nature10644  

Li, W., Bloom, J., Podsiadlowski, P., Miller, A., Cenko, S., Jha, S., Sullivan, M., Howell, D., Nugent, P., Butler, N., Ofek, E., Kasliwal, M., Richards, J., Stockton, A., Shih, H., Bildsten, L., Shara, M., Bibby, J., Filippenko, A., Ganeshalingam, M., Silverman, J., Kulkarni, S., Law, N., Poznanski, D., Quimby, R., McCully, C., Patel, B., Maguire, K., & Shen, K. (2011). Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe Nature, 480 (7377), 348-350 DOI: 10.1038/nature10646

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Reader Comments (1)

I can't believe we don't have a before and after shot of this. The Andromeda is the most highly photographed galaxy after the MC.

i think it may end up being more about plasma cosmology and the latest finding of the vortices that cosmic rays come from.

December 26, 2011 | Unregistered CommenterAnonymous

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