Free Astronomy Magazine September-October 2022

45 SEPTEMBER-OCTOBER 2022 ASTRO PUBLISHING implications of the discov- ery are still substantial, lending support to theories that the majority of mas- sive stars form and evolve as binary systems. Unlike supernovae that have a puffy shell of gas to light up, the progenitors of fully stripped-envelope supernovae have proven difficult to identify in pre- explosion images. Now that astronomers have been lucky enough to identify the surviving com- panion star, they can use it to work backward and de- termine characteristics of the star that exploded, as well as the unprecedented opportunity to watch the aftermath unfold with the survivor. As a massive star itself, SN 2013ge’s companion is also destined to undergo a su- pernova. Its former partner is now likely a compact ob- ject, such as a neutron star or black hole, and the com- panion will likely go that route as well. The closeness of the original com- panion stars will determine if they stay together. If the distance is too great, the companion star will be flung out of the system to wander alone across our galaxy, a fate that could explain many seemingly soli- tary supernovae. However, if the stars were close enough to each other pre-supernova, they will con- tinue orbiting each other as black holes or neutron stars. In that case, they would eventually spiral toward each other and merge, creating gravitational waves in the process. That is an exciting prospect for as- tronomers, as gravitational waves are a branch of astrophysics that has only begun to be explored. They are H ubble images of galaxy NGC 3287 show supernova 2013ge fading over time, revealing the steady source of ultraviolet light astronomers have identified as its binary companion star. [NASA, ESA, Ori Fox (STScI), Joseph DePasquale (STScI)] waves or ripples in the fabric of spacetime itself, predicted by Albert Einstein in the early 20 th century. Gravitational waves were first di- rectly observed by the Laser Interfer- ometer Gravitational-Wave Obser- vatory (LIGO). “With the surviving companion of SN 2013ge, we could potentially be seeing the prequel to a gravitational wave event, al- though such an event would still be about a billion years in the future,” Fox said. Fox and his collaborators will be working with Hubble to build up a larger sample of surviving companion stars to other super- novae, in effect giving SN 2013ge some company again. “There is great potential beyond just under- standing 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 under- stand the details behind binary for- mation, material-swapping, and co-evolutionary development. It’s an exciting time to be studying the stars,” Fox said. “Understanding the lifecycle of massive stars is particu- larly 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,” added co-author Alex Filippenko of the University of California at Berkeley. !