This type of supernova is unusual because most, but not all, of the hydrogen is gone prior to the explosion

Washington: NASA's Hubble Space Telescope has captured the first image of a surviving companion to a supernova, a compelling evidence that some supernovas originate in double-star systems, the US space agency said. This first-ever documented binary star system was discovered by Hubble in “a galaxy far, far away,” lying at a distance of 40 million light-years from Earth, in the Grus constellation, also known as the Crane. NASA provides the whole back story in a recent news release, explaining that the supernova, called SN 2001ig, was first sighted 17 years ago, when the light from its explosion eventually reached Earth. SN 2001ig is a rare type of supernova, categorized as a Type IIb stripped-envelope supernova, which means that most of the hydrogen in is “stellar envelope” — the region of the star which transports materials from its core to its atmosphere — had already been depleted before the explosion. This type of supernova is unusual because most, but not all, of the hydrogen is gone prior to the explosion. This is the first time anyone has found actual proof that some massive stars, just like the one that was killed off in the supernova, can occur in binary star systems, where they coexist next to a companion star. Millions of years before its companion exploded a fire death, the star was creating instability in the primary star by siphoning off hydrogen from its stellar envelope, the layer that shuttles energy from the core to the star's atmosphere. Millions of years before the primary star went supernova, the companion’s thievery created an instability in the primary star, causing it to episodically blow off a cocoon and shells of hydrogen gas before the catastrophe. Most scientists thought Type IIb stripped-envelope supernovas originated in single star systems, their out envelopes blown away by strong stellar winds. The problem was that when astronomers started looking for the primary stars from which supernovas were spawned, they couldn’t find them for many stripped-envelope supernovas. “That was especially bizarre, because astronomers expected that they would be the most massive and the brightest progenitor stars,” said Ori Fox, an astronomer at the Space Telescope Science Institute in Baltimore, US. “Also, the sheer number of stripped-envelope supernovas is greater than predicted.”

This fact led astronomers to hypothesise that Type IIb stripped-envelope supernovae may instead be a result of co-orbiting pairs of binary stars. Many of these binary pairs will interact and transfer gas from one star to the other when their orbits bring them close together,” Stuart Ryder from the Australian Astronomical Observatory in Sydney, and lead author of a recent study on the Hubble observation, said in a statement Thursday. “Many of these binary pairs will interact and transfer gas from one star to the other when their orbits bring them close together.”

The companion to the supernova’s progenitor star was no innocent bystander to the explosion.
A photograph from the Hubble Space Telescope reveals a companion star that survived the explosion of a supernova in galaxy NGC 7424. This brings strong evidence to a theory according to which Type IIb "stripped-envelope" supernovae are due to stellar companions capturing hydrogen from the progenitor star's envelope before its explosion.