Supernova Burning Its Nearby Companion Star Observed By Astronomers For The First Time

The blue light here is coming from supernova 2012cg, which is burning its nearby companion star. Peter Challis/Harvard-Smithsonian CfA

A burst of energy from the depths of the darkest reaches of space is always a sign of something exciting happening. The fireworks could be originating from an exploding supermassive star; on the other hand, it could be a gamma ray burst jettisoned from the merger of two black holes.

Now, in a new study due to be published in the Astrophysical Journal, a team of astronomers has described another mysterious burst of light, this time coming from a star orbiting around another exploding one. This is known as a Type 1a supernova, and although they’ve been observed before, this research is the first time in history that astronomers have directly witnessed the impact of a supernova on its celestial neighbor.

“Regular” supernovae happen when a star above eight solar masses runs out of lighter elements, like hydrogen and helium, to burn. Burning heavier elements still generates heat, but not enough to push against the immense gravitational field compressing the star. Eventually, the core collapses, generating a titanic explosion.  

Type 1a supernovae are different, in that they require two stars to occur, one of which is a white dwarf. These dense, small stellar remnants cannot undergo fusion in order to produce heat. They sometimes have a companion star, which many have thought to be an elderly red giant.

Over time, as the red giant burns heavier elements (like carbon and oxygen) and increases in size, its outer layers of gas are stripped away by the white dwarf. This increases the mass of the white dwarf until it becomes massive enough to exert huge pressures on its core. This starts a runaway fusion reaction, and within seconds, the white dwarf catastrophically explodes.

This new study notes that this companion star theory is decades old but is not yet supported by any direct evidence. Now, after observing light being emitted from a supernova named 2012cg, located 50 million light-years away in the constellation of Virgo, the researchers think they have it.

SN 1994D, the bright spot in the lower left of the image, is another example of a type 1a supernova. The galaxy NGC 4526 takes up the rest of the image. NASA/ESA

In the immediate aftermath of this particular white dwarf’s destruction, a prolonged period of unexpected brightness with a blue tint was observed. Ruling out any additional outbursts from the white dwarf’s supernova itself, the team concluded that blue emission could only have one explanation: The intense heat of the supernova was cooking a nearby companion star.

The wavelength of light being emitted from a star gives an indication as to what is being combusted at the time. Blue light emissions in this particular case indicate that the supernova was superheating a neighboring, hydrogen-burning, main sequence star with about six solar masses.

Significantly, this means the leading type 1a supernova theory – that the companion star involved in each one is a massive red giant – may not be as strong as previously thought. No red giant of any kind was observed.

“Supernova 2012cg is the smoking – actually glowing – gun… that some Type 1a supernovae come from white dwarfs doing a do-si-do with ordinary stars,” said contributing study author Robert P. Kirshner, an astrophysicist at Harvard University, in a statement.

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