Teardrop-Shaped Star Reveals A Supernova In The Making

How the star system HD265435 is expected to look in 30 million years time when it is almost half way from its current state to becoming a Type Ia supernova. Image Credit: University of Warwick/Mark Garlick

Astronomers have spotted a star whose shape indicates it will one day be consumed as fuel so a neighbor can become a supernova. The pair are only 1,500 light-years away, so the view from Earth will be spectacular when the explosion occurs. However, at an estimated 70 million years to ignition, there is no guarantee there will be anyone here to watch it.

Astronomers find proto-supernovas all the time. Any star with a mass of more than eight times the Sun will end its life in an enormous explosion, with Betelgeuse being the most famous example. However, these are core collapse supernovas. It's much harder to find Type Ia supernovas before they go off because they originate from white dwarfs, whose faint light is hard to spot unless very nearby.

However, white dwarfs do not explode on their own – to become supernovas they need a neighbor to feed them extra mass, and these stars can be considerably brighter. In Nature Astronomy University of Warwick researchers announced the discovery of one of these companion stars which looks a certainty to be consumed this way.

The stellar pair are named HD265435. We see a subdwarf star with a mass 60 percent that of the Sun twirling around in an orbit lasting just 100 minutes. Although we cannot see the object it is dancing with, the speed of the orbit allows us to calculate that it must have a mass one percent greater than the Sun. Something that massive, but invisible to our telescopes, must be a white dwarf.

White dwarfs are the ultradense remnants of stars that have ceased fusion. Most live out their second lives quietly, slowly cooling. However, those with a close binary companion can meet a different fate. They can either draw material off a neighboring star or merge with it entirely. In either case, on passing 1.4 solar masses, they become unstable and explode as Type Ia supernovas. Besides the spectacular show these give anyone with eyes within hundreds of thousands of light-years, these serve as standard candles, revealing the distance to their home galaxies. This formed the basis of this century's revolution in cosmology, revealing the existence of dark energy accelerating the expansion of the universe.

Despite their crucial importance to science, much about Type Ia supernova remains mysterious – largely because astronomers have not had the chance to study one close-up since the development of modern equipment. "We don't know exactly how these supernovae explode, but we know it has to happen because we see it happening elsewhere in the universe,” lead study author Dr Ingrid Pelisoli said in a statement

Provided the whole system exceeds 1.4 solar masses, as HD265435 does, an explosion is only a matter of time. Based on the rate they are moving together, Pelisoli and co-authors estimate this will be in 72 million years. By the time this occurs the white dwarf may have captured so much material its companion may no longer be able to achieve fusion, and will be a second white dwarf.

"The more we understand how supernovae work, the better we can calibrate our standard candles,” Pelisoli said. “This is very important at the moment because there's a discrepancy between what we get from [Type Ia supernova], and what we get through other methods,” including measurements of the cosmic background radiation.

Astronomers noticed HD265435 because of the way the subdwarf star was being pulled out of shape by the white dwarf's gravitational field. Theory suggests we should see a lot more pairs like this than we have actually found – Pelisoli noted that even with this discovery, there is a puzzling lack of such known systems, but the discrepancy is now a little smaller.



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