6 mins read 28 May 2020

Finding the Signatures of Exploding Stars

Astronomers are analysing the gravitational wave signatures of simulated supernovae to help find their real-life counterparts.

An illustration of an energetic supernova explosion. Credit: NASA/CXC/M.Weiss

About 50 years ago in the Burzahama region in Kashmir, India, archaeologists uncovered a stone carving of two hunters, a bull, and two beaming disks in the sky. The carving was presumed to depict a hunting scene under a pair of bright stars at the local zenith, but a team of astrophysicists in India and Germany had other ideas. One by one, they assessed and ruled out various theories about the origin of the scene; that it was an illustration of the Sun and Moon, a pair of stars, or comets or asteroids in the sky. Only one theory seemed to fit the evidence. The object depicted on the stone carving appears to be supernova HB9, an exploding star that shone 100 times as brightly as Venus around 6,500 years ago. And if that is true, then it is the oldest recorded observation of a supernova by humans.

The Burzahama stone carving. The carving is of two hunters, a bull, and two beaming disks in the sky. Credit: Vahia/Tata Research Institute

The Milky Way is home to around 250 billion stars, but astronomers expect maybe two or three supernovae to occur in our galaxy every 100 years. Chinese astronomers have an illustrious history of making supernovae observations, having recorded SN 185, and then SN 393, SN 1006, SN 1054, and SN 1181, all of which are named for the years they were first observed. Tycho Brahe, famous for making some of the most accurate observations of planetary positions at the time, observed SN 1572, and the most recent supernova seen in the Milky Way galaxy was SN 1604. Some may suggest that we are overdue for one of the greatest light shows in the universe.

Of course, astronomers have powerful telescopes combing the observable universe for these very events. Supernovae are an important step on the cosmic distance ladder, a measurement tool by which astronomers determine the distances to celestial objects, and understanding them is key to calibrating many of our most important astrophysical models. Later this year the Vera C. Rubin Observatory, a massive telescope observatory situated in Chile, is expected to see first light, and astronomers are hopeful of discovering upwards of three million supernovae during its ten-year survey. But to gain insights into the death rattles of massive stars, they don’t need to wait.

When massive stars end their lives as core-collapse supernovae, the explosions are so powerful that they cause tiny ripples in space and time. These ripples, or gravitational waves, travel across the universe, squeezing and stretching anything in their path. Our current generation of gravitational wave detectors have been detecting the collisions between pairs of black holes (and neutron stars) for a few years now, but the next, more sensitive, generation of detectors may even be able to detect a core-collapse supernova. Before that can happen though, astronomers need to understand the gravitational wave signature of core-collapse supernovae, and they are turning to computer simulations to help them out.

A 3D-volume render of a core-collapse supernova. Credit: Dr. Bernhard Mueller

Recently, researchers Dr. Jade Powell and Dr. Bernhard Mueller from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) ran some of the most comprehensive supernova simulations to date. They evolved three stars, two Wolf-Rayet stars, and a red supergiant, to the point of collapse, and then assessed the gravitational wave emissions from the resulting explosions. Their models included the effects of stellar rotation, neutrinos, and general relativity, and were run on the OzSTAR supercomputer at Swinburne University of Technology in Victoria, as well as on the Raijin supercomputer at Australia’s National Computational Infrastructure. Underlining the complexity of the simulations, Dr. Powell told me that about 600 hours were required on a single CPU just to simulate one millisecond of one supernova.

Science Check: Core-Collapse Supernovae

A star is a giant nuclear reactor. It fuses atoms together to form other atoms, and in the process, releases enough energy to create an outward pressure that counteracts its own gravity. As the star uses up its fuel of lighter elements, the reactions slow down, and it contracts. This heats the core enough for heavier elements to begin fusing, and, if the star has more than about 10 times the mass of the Sun, this cycle continues until it begins fusing silicon into iron. At this point fusion slows dramatically because iron must absorb energy to fuse into heavier elements, and without the energy to support the star against gravity it collapses in on itself. The final collapse happens quickly and releases an enormous amount of energy in a burst of neutrinos that can be seen across the universe.

The supernovae studied by Dr. Powell and Dr. Mueller were neutrino-driven supernovae in which neutrinos play an active role in powering the explosion. Dr. Powell explains. “When a star starts to explode, a powerful shock wave gets launched outwards from the core of the star. After a while the shock wave starts to lose energy and stalls.” This is where the neutrinos come in; they leak out from the stellar core, causing violent motions in the gases and re-energising the shock wave. “A neutrino-driven supernova occurs when the shock wave is revived by absorbing energy from neutrinos.”

Detecting Supernovae with Gravitational Waves

The researchers included the effects of progenitor rotation on the dynamics of the explosions to see whether this had a significant impact on the observable gravitational wave emissions. What they found was that the supernovae could be detected at much greater distances than had been expected. Of the three stars they simulated, the two massive Wolf-Rayet stars ended with a neutrino-driven supernova and the formation of a neutron star and could be detected up to 6 million light-years away depending on the properties of the progenitors. Even the red supergiant could be detected by next-generation gravitational wave detectors if it were no more than about half a million light-years away. Not only that, but current generation detectors could conceivably pick up these explosive events too, at least if they occurred within our own galaxy.

Dr. Powell explains, “for the first time, we showed that rotation changes the relationship between the gravitational wave frequency and the properties of the newly forming neutron star. Our long-duration supernova gravitational wave signal predictions help improve gravitational wave searches, bringing us a step closer to the first detection of a gravitational wave supernova signal”. While more research is needed to confirm the study’s optimistic findings, it is certain that gravitational wave astronomy will be providing us with insights into some of the most mysterious phenomena in the universe for some time yet.

The paper appears in the MNRAS