Stars Become Two-Faced When They Explode
This visualization of a supernova explosion shows a cross-section of the simulated density distribution ten seconds after the off-center ignition. When seen from different viewing angles, a putative observer would see different gas densities and composition, which correspond to the diversity in observed supernovae.
Credit: F. Roepke, MPA Garching

The death of a star in a supernova explosion can briefly outshine an entire galaxy in cosmic conflagrations that many astronomers thought exploded with some symmetry. But new observations suggest supernovas can be unbalanced, beginning on one side of the star to create oddball explosions.

Depending on the angle that astronomers view a supernova, it will look somewhat different, like two faces of the same coin, researchers said.

The discovery helps solve a mystery about a certain type of supernova called Type 1a. These explosions are thought to be very uniform, and are prized by astronomers for that quality.

However, observers noticed two subcategories of these supernovas that behave differently over time, with some spewing out material very quickly while others exploded with comparative leisure. Now it appears the two subcategories are actually the same thing, seen from different angles.

One supernova, two faces

According to the new study, whether a supernova appears to be exploding rapidly or slowly just depends on which side is facing us.

"Now we show this diversity in appearance is simply a consequence of this orientation," said lead researcher Keiichi Maeda of the University of Tokyo in Japan. "Now it means that these two categories are not really two populations."

The researchers resolved the two-faced nature of these supernovas by observing a group of them years after their explosions peaked. At this point, the dying stars had expelled much of their material in a translucent cloud of debris that telescopes can peer through to see into the heart of the aging supernova.

By studying the distribution and relative speed of the debris, the astronomers could deduce the original direction of the explosion.

The finding also helps astronomers understand the complicated mechanics at the heart of these stellar explosions. For one thing, models that depict supernovas as spherical explosions should be replaced with asymmetric depictions.

"This is kind of a paradigm change in the study of Type 1a supernovae," Maeda told

Maeda and his team detail their findings in the July 1 issue of the journal Nature.

Supernova candles

The finding has important ramifications for observers, because it reaffirms the usefulness of Type 1a supernovas as a cosmic distance indicator, or so-called standard candle.

"The requirement for a good standard candle is that they look basically the same in every observed characteristic," Maeda said.

All Type 1a supernovas shine at roughly the same peak brightness - about 10^36 watts. That's because all they begin when a small star, called a white dwarf, sucks up mass from a companion star.

White dwarfs have a set limit to how massive they can become before they explode, so when they do finally hit the limit, the explosions always blast out roughly the same amount of energy.

If one of these explosions appears brighter than another, it must be because it is closer to us. So by measuring how bright a Type 1a supernova appears, and comparing that with its known actual, intrinsic brightness, astronomers can calculate how far away it is.

So the idea that they might not actually be as uniform as we thought would put many distance calculations into question, including some of the fundamental calculations on the rate the universe is expanding that use Type 1a supernovas.

"Even a small systematic difference in their luminosity would potentially jeopardize the usefulness of Type 1a supernova as precision distance indicators," wrote astronomer Daniel Kasen of the University of California, Berkeley, in an accompanying essay in the same issue of Nature. Kasen was not involved in the new research.