Supernovae are in the news this week, as two papers in the latest release from Nature provide fresh perspectives on stellar explosions old and new. The old one is Supernova 1987A, the closest one in the age of modern astronomy, which has recently undergone a brightening that indicates a key transition in its evolution has taken place. The new one is actually an entirely new category of supernova, represented by four examples. The output of this new category is heavily biased towards the blue end of the spectrum, it's ten times brighter than a Type Ia supernova, and we aren't sure what could possibly be powering it.

Some basic background on supernovae will make it easier to understand both stories. The initial collapse of a star creates a tremendous explosion, one heralded by the arrival of neutrinos and high-energy photons. But the fusion events that accompany the explosion also produce some unstable radioactive isotopes, such as nickel-56 and -57 and titanium-44. Both immediately after the explosion and for the following several years, the remnant of the supernova is primarily lit by energy released as these isotopes decay. Only after a few decades do other processes, primarily the interaction between the expanding shell of the explosion and the stellar environment, begin to dominate the light seen at the site of the supernova.

Supernova 1987A occurred during my second year in college, and my physics professor was so excited that he cancelled our expected lecture on Newtonian mechanics to spend an hour and a half describing why it was so exciting. That excitement was largely based on its proximity in the Large Magellanic Cloud, only 160,000 light years away, which was already close enough for detailed observations. Since then, observatories have gotten bigger and we got the Hubble Space Telescope up and working, so the situation has only improved, making SN1987A one of the best-studied supernovae around.

After about 1,500 days from the initial explosion, the supernova is expected to go into a steady decline as the half-life of 44Ti ensures that there's less and less energy being input into the remnant via radioactive decay. And observations show a decline in luminosity of the debris, which centered on the former site of the star. At about 5,000 days post-supernova (November of 2000), however, the predicted decline stopped, and the total luminosity started to rise again. By 8,000 days (April of 2009), the total luminosity was at or above where it was at 3,000 days, depending on the wavelength.

What's going on? Observations with the Hubble indicate that it's not the debris itself that is brightening. Instead, the added light comes from a ring of gas that was pushed out of the star's equator about 20,000 years ago (the ring is now about 1.3 light years across). The authors conclude that the brightening is the result of the first remnants of the supernova plowing into this ring, with the resulting collision generating X-rays that are lighting up more of the gas. This sort of behavior has been predicted for some time, but this represents the first time it has been observed.

Not everything in space is going quite according to predictions though. The fact that most supernovae are initially powered by the decay of some specific radioactive isotopes allows us to make very specific predictions about what the timing of luminosity should be. A survey of supernovae run at the Palomar Observatory has now identified four objects that don't fit the expected pattern. A study of the light that came off these events show that they all share common properties.

One of them is that their luminosity is heavily biased to the UV portion of the spectrum, and they're unusually bright. To get the luminosity from the decay of 56Ni, the explosion would have had to produce several solar masses of that alone. The decline in luminosity is also much faster than we'd expect based on half lives of the common isotopes. "These are therefore not radioactively powered events," the authors conclude. To be a product of the explosion itself would require what they term "an unrealistic total explosion energy."

Finally, the spectrum of the light seems to indicate the light is coming from material that is extremely hydrogen-poor, which rules out some form of a rapid collision analogous to the one observed in SN1987A.

So, just about all of our common expectations about supernovae aren't working out. The authors suggest two possible explanations. One is the explosion of a star of over 90 solar masses, which will often expel a hydrogen-poor shell shortly before dying. If the time difference between this expulsion and the explosion is short enough, debris will impact the shell days after the explosion itself, creating the effect seen here. The alternative is that the remains of the star are injecting energy into the debris. This could happen if these supernovae produced a rapidly spinning magnetar.

The Palomar team has identified four of these items in about two years of observations, so it's possible that we'll be able to spot a few more and get a better sense of what's happening. Otherwise, we may have to wait a bit to get lucky enough to observe one from a shorter distance.

Nature, 2011. DOI: 10.1038/nature10090, 10.1038/nature10095 (About DOIs).