A companion star previously buried in the brilliance of its partner’s supernova has been discovered by NASA’s Hubble Space Telescope as a witness at the scene of a star’s cataclysmic death. The discovery is the first of its kind for a type of supernova in which the star’s outer gas envelope was stripped away before it exploded.

The result also supports the hypothesis that binary systems arise and evolve in the majority of big stars. It could also be a prelude to another cosmic drama: the surviving, huge companion star will ultimately blow supernova, and if the residual cores of both stars aren’t ejected out of the system, they would fuse and produce gravitational waves, disrupting the fabric of space itself.

The discovery sheds light on the dual nature of large stars, as well as the possibility of a prelude to the final merging of companion stars, which would send gravitational waves, vibrations in the fabric of spacetime itself, reverberating across the cosmos.

In supernova explosions, astronomers can discover the signatures of numerous components. Pre-supernova, these elements are layered like an onion. Hydrogen is located in a star’s outermost layer, thus if no hydrogen is found in the aftermath of a supernova, it suggests it was stripped away before the explosion.

The reason for the hydrogen loss was unknown, therefore astronomers used Hubble to look for clues and test theories to explain these stripped supernovae. The latest Hubble data provide the strongest evidence yet that an unseen companion star siphons off the gas envelope from its partner star before it explodes.

“This was the moment we had been waiting for, finally seeing the evidence for a binary system progenitor of a fully stripped supernova,” says astronomer Ori Fox, lead investigator on the Hubble research program. “The goal is to move this area of study from theory to working with data and seeing what these systems really look like.”

Fox’s team studied the location of supernova (SN) 2013ge in ultraviolet light with Hubble’s Wide Field Camera 3, as well as previous Hubble images in the Barbara A. Mikulski Archive for Space Telescopes. From 2016 until 2020, astronomers observed the supernova’s light diminishing, but another nearby source of ultraviolet light at the same location remained bright. The team believes that this underlying source of UV emission is the surviving binary companion to SN 2013ge.

Scientists previously thought that the tremendous winds of a giant progenitor star could sweep away its hydrogen gas envelope, but observational evidence contradicted this theory. Astronomers created theories and models in which a binary partner siphons off the hydrogen to explain the discrepancy.

“In recent years many different lines of evidence have told us that stripped supernovae are likely formed in binaries, but we had yet to actually see the companion. So much of studying cosmic explosions is like forensic science — searching for clues and seeing what theories match. Thanks to Hubble, we are able to see this directly,” adds Maria Drout, a member of the Hubble research team.

Hubble previously observed two peaks in the ultraviolet light of SN 2013ge, rather than the single peak seen in most supernovae. One explanation for the twofold brightening, according to Fox, is that the second peak shows when the supernova’s shock wave collided with a companion star, a scenario that suddenly appears far more possible. According to Hubble’s latest measurements, the companion star was jostled but not destroyed, despite the hydrogen gas it had drained from its partner. The impact is compared to a jiggling bowl of jelly that would ultimately settle back to its former form, according to Fox.

While more confirmation and similar discoveries are needed, Fox believes the discovery has significant consequences, adding support to views that the bulk of large stars start and evolve as binary systems.

The progenitors of entirely stripped-envelope supernovae have been difficult to discern in pre-explosion photos, unlike supernovae that have a puffy shell of gas to light up. Now that scientists have identified the surviving companion star, they may utilize it to work backward and establish the properties of the exploding star, as well as the once-in-a-lifetime opportunity to witness the aftermath with the survivor.

As a big star, SN 2013ge’s companion is also on the verge of exploding. Its former companion is now most likely a compact entity like a neutron star or black hole, and the companion will most likely follow suit.

The original companion stars’ closeness will determine whether they stay together. If the distance between the two stars is too vast, the companion star will be thrown out of the system, wandering alone throughout our galaxy, which could explain many seemingly isolated supernovae.

If the stars were close enough to each other before the explosion, they will continue to orbit as black holes or neutron stars. They would gradually spiral toward one other and unite in such instance, causing gravitational waves.

Gravitational waves are an area of astrophysics that has just recently been studied, thus this is an interesting potential for astronomers. Albert Einstein foresaw waves or ripples in the fabric of spacetime itself in the early twentieth century. The Laser Interferometer Gravitational-Wave Observatory was the first to directly observe gravitational waves.

“With the surviving companion of SN 2013ge, we could potentially be seeing the prequel to a gravitational wave event, although such an event would still be about a billion years in the future,” Fox says.

Fox and his colleagues will collaborate with Hubble to build up a bigger sample of surviving supernova partner stars, giving SN 2013ge some company once more.

“There is great potential beyond just understanding the supernova itself. Since we now know most massive stars in the universe form in binary pairs, observations of surviving companion stars are necessary to help understand the details behind binary formation, material-swapping, and co-evolutionary development. It’s an exciting time to be studying the stars,” Fox adds.

“Understanding the lifecycle of massive stars is particularly important to us because all heavy elements are forged in their cores and through their supernovae. Those elements make up much of the observable universe, including life as we know it,” says co-author Alex Filippenko.

Image Credit: Getty

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