Scientists have found the most compelling evidence till date that that some supernovas originate in double-star systems.
The base of the study and its findings can be said to be a supernova that went off 40 million light-years away in the galaxy called NGC 7424, located in the southern constellation Grus, the Crane. In the fading afterglow of that explosion, NASA’s Hubble has captured the first image of a surviving companion to a supernova.
The companion to the supernova’s progenitor star was no innocent bystander to the explosion. It siphoned off almost all of the hydrogen from the doomed star’s stellar envelope, the region that transports energy from the star’s core to its 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.
The supernova, called SN 2001ig, is categorized as a Type IIb stripped-envelope supernova. This type of supernova is unusual because most, but not all, of the hydrogen is gone prior to the explosion. This type of exploding star was first identified in 1987 by team member Alex Filippenko of the University of California, Berkeley.
Looking for a binary companion after a supernova explosion is no easy task. First, it has to be at a relatively close distance to Earth for Hubble to see such a faint star. SN 2001ig and its companion are about at that limit. Within that distance range, not many supernovas go off. Even more importantly, astronomers have to know the exact position through very precise measurements.
In 2002, shortly after SN 2001ig exploded, scientists pinpointed the precise location of the supernova with the European Southern Observatory’s Very Large Telescope (VLT) in Cerro Paranal, Chile. In 2004, they then followed up with the Gemini South Observatory in Cerro Pachón, Chile. This observation first hinted at the presence of a surviving binary companion.
Knowing the exact coordinates, researchers were able to focus Hubble on that location 12 years later, as the supernova’s glow faded. With Hubble’s exquisite resolution and ultraviolet capability, they were able to find and photograph the surviving companion–something only Hubble could do.
Prior to the supernova explosion, the orbit of the two stars around each other took about a year.
When the primary star exploded, it had far less impact on the surviving companion than might be thought. Imagine an avocado pit–representing the dense core of the companion star–embedded in a gelatin dessert–representing the star’s gaseous envelope. As a shock wave passes through, the gelatin might temporarily stretch and wobble, but the avocado pit would remain intact.
In 2014, researchers used Hubble to detect the companion of another Type IIb supernova, SN 1993J. However, they captured a spectrum, not an image. The case of SN 2001ig is the first time a surviving companion has been photographed.