Things have been going well in my high-school class. I've recently taught them about supernovas, and how there are only two kinds. One occurs when the core of a single, massive star collapses (called Type II, or less commonly types Ib or Ic), and the other happens when a white dwarf in a binary system gets too fat and dense from sucking matter away from its companion (Type Ia).

Strange supernova in NGC 1032

Supernova 2005E (arrow) occurred in the halo of its host galaxy, NGC 1032, about 750,000 light-years from the nucleus.

SDSS / Lick Observatory

"Are you sure there are only two basic types, Mr. Beatty?" one student presses.

"Yep," I respond with surety. "It's very black and white. Vanilla and chocolate. Night and day.

"Type II and Type Ia."

So I beg you: Please don't show my students this week's issue of Nature, in which observers describe a recent pair of stellar explosions that don't fit the existing categories. The blasts weren't as bright as they should have been, and they didn't splatter the surrounding interstellar space with nearly enough of their innards.

The most likely reason, says astronomers, is that there must be a third type of star blast that had previously escaped our notice. But there's no agreement on what form these party-crasher take.

A team led by Hagai B. Perets (Harvard-Smithsonian Center for Astrophysics) have analyzed supernova 2005E, which blazed forth from the spiral galaxy NGC 1032 in Cetus. Technically, the blast occurred in the galaxy's halo, where new stars (massive or not) rarely form. So odds are this event involved a binary of some kind. Because the supernova ejected a mere 300 Jupiters' worth of mass, Perets' team concludes that the event involved a white dwarf that only destroyed its outer layers.

Plot of supernova brightness vs. ejected mass

Supernova 2005E (red star) ejected far less mass and was much fainter than is typical of the known types of exploding stars.

H. Perets & others / Nature

But a second team, led by Koji Kawabata (Hiroshima University), think they've stumbled on a new twist on single-star sayonaras. The blast they monitored, supernova 2005cz in the elliptical galaxy NGC 4589, proved to be helium-rich, suggesting that the core-collapse explosion scenario is more likely. The reason that SN 2005cz only became 20% as bright as it should have, they argue, is that the progenitor star was just above the minimum mass (8 Suns) needed — and all other previously observed Type II supernovas were well above that threshold.

No matter what the cause, I feel these researchers should have given me the courtesy of a head's up. At the very least, they should have directed me to the press releases here and here. Now I've got to patch up my final exam so I get the science right.


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Ken Winters

May 21, 2010 at 3:25 pm

Well, if the second hypothesis is correct, it's sill a type II, and if the first is correct, it might be considered a variant or sub-type of a Ia (or maybe an extra-strong variant of a nova, and not a supernova at all). It sounds like you're still good then.

On the other hand, if SN 2007bi was really a pair-instability supernova, then there are already three types of supernovae, and this recent discovery is raising the question of whether there is a forth type.

So I guess we get to keep looking for answers, and then asking more questions. I'm in favor of that.


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Ken Winters

May 21, 2010 at 3:35 pm

Most other comment systems I use ask for an e-mail address (not a subject) and I didn't read past "Your Name:". I'm guessing (and hoping) that the intersection of "spammers" and "readers of Sky&Telescope" is empty. At least it should be small. [crossing fingers] - - - I also note that your comment systems eats carriage returns.

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May 22, 2010 at 9:50 am

Now... what about Dark Energy ?
Dark Energy is supported by SN Type Ia.
I think spectral analisys is the key for answer about what type of SN could have taken place in NGC 1032. If the result of spectral analysis is like a SN Type Ia then it is going to unleash a conflict about Dark Energy existence.

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