Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Supernovae’s multiple origins (Image: Romano Corradi and the Instituto de Astrofísica de Canarias)

Object: Supernova PTF 11kx

Main characters: A gassy giant and a vengeful dwarf

Once upon a time, in a faraway place, a white-bearded dwarf lived next to a red-haired giant.

Every so often the giant would belch a great wind through the land, which lifted his stray hairs into the air and plastered them all over the dwarf’s small hovel. Blaming the giant for poor hygiene, the dwarf vowed revenge and set about brewing the hair into a potion, hoping that downing the draught would give him the giant’s great stature.


Sure enough it worked – over time, the dwarf grew larger and sprouted red hair all over his body. But one day the excited dwarf drank so much that he exploded, killing the giant and leaving behind only a candle burning in the debris.

This cosmic fairy tale seems to be the best explanation for supernova PTF 11kx, an explosion spotted 600 million light-years away, says a team led by Benjamin Dilday at the University of California, Santa Barbara.

The researchers think that a white dwarf, the ultra-dense core of a dead sun-like star, exploded after gaining too much mass from a nearby red giant, a dying star that has puffed up to many times its original size.

Figuring out what spawned the blast is important to unlocking the mysteries of the universe, because it produced what’s known as a type Ia supernova.

All stellar explosions in this class have roughly the same brightness, so astronomers have been using them as “standard candles” for measuring cosmological distances. But scientists have collected different versions of how type Ias originated, which could have implications for how accurate their measurements can get.

Brotherly feud

In most tales, the process starts with a white dwarf interacting with a binary companion. One theory says the white dwarf siphons material from a larger star. Another idea states that intense stellar winds blow material from the companion onto the dwarf.

In either case, the added material causes the white dwarf to grow so massive that it collapses, triggering a violent outburst.

There is also evidence that type Ias involve two white dwarfs, with a larger star pulling apart a smaller one until the pair collide, like a punch-up between two angry brothers.

Dilday and colleagues used spectral data from the Keck Observatory in Hawaii to examine the composition of supernova PTF 11kx, which was first observed on 16 January 2011.

The team discovered that the supernova was surrounded by layers of hydrogen gas moving at different speeds, with the outer layers drifting slower than the inner ones. This ruled out a merger of two white dwarfs, as they would have burned off most of their hydrogen as they formed.

Then, 60 days after the supernova, Dilday and colleagues saw material from the explosion crash into surrounding gas. This suggests there was an empty gap around the white dwarf, which would not have been present if it had been constantly stealing matter from a binary partner.

The best theory for their data is that mass from a red giant built up on the surface of the white dwarf. The surface material periodically generated a nova eruption, creating the hydrogen layers, until the entire star exploded (Science, DOI: 10.1126/science.1219164).

Setting standards

Although the characteristics of supernova PTF 11kx line up with those of other type Ias, previous research ruled out the tale of the windy giant for others in this class. According to Dilday, the combined evidence means that type Ia bodies don’t all start under the same conditions.

But does that cast doubt on their usefulness as standard candles? Not at all, says Dilday.

“The question is more one of how systems with different progenitors end up as very similar supernovae.” In fact, it would be a boon to know whether there are subtle differences in brightness between type Ias caused by alchemy gone wrong and those resulting from brotherly fisticuffs.

“If we can determine that there is a small difference, that would just improve our measurements,” agrees Ryan Foley, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who also studies supernova origins.

“If you are making measurements with a yard stick and a metre stick and assuming they are the same distance, you’re going to be pretty close. But if you can separate out the yards and the metres, then you’ll be even better.”