Teardrop star reveals hidden supernova doom

Astronomers have spotted a teardrop star that suggests two dwarfs are spiralling towards supernova doom.

The team, led by Ingrid Pelisoli of the University of Warwick in the UK, recently found the stars, which are 1,500 light years away; one is a hot subdwarf and the other a white dwarf that orbit each other closely at a rate of around 100 minutes.

The teardrop shape was caused by the bigger white dwarf distorting the subdwarf’s shape because of its intense gravitational pull, which will eventually be a catalyst for a white dwarf supernova in approximately 70 million years.

According to the paper, published in Nature Astronomy, this is one of very few systems discovered that will see a white dwarf star reignite its core.

“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,” says Pelisoli.

“One way is if the white dwarf accretes enough mass from the hot subdwarf, so as the two of them are orbiting each other and getting closer, matter will start to escape the hot subdwarf and fall onto the white dwarf.

“Another way is that because they are losing energy to gravitational wave emissions, they will get closer until they merge. Once the white dwarf gains enough mass from either method, it will go supernova.”

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Using NASA’s Transiting Exoplanet Survey Satellite (TESS), the team were able to see the subdwarf, but it was so bright that it obscured the dimmer white dwarf. However, the brightness varied over time, which suggested that the star’s shape was being distorted into a teardrop by another celestial body.

Using velocity measurements and modelling, the team concluded that the second star in the system was a hidden white dwarf that was as heavy as the Sunbut smaller than Earth. They predicted that the subdwarf will become a white dwarf before its demise, too.

Supernovae are used in cosmology as “standard candles”, which is a reference point to measure the luminosity of different stars in the area. They are used as standard candles because the supernova has a specific type of light and the brightness is constant, so other stars can be measured relative to them. It also helps astronomers measure which stars are moving where and how fast, to calculate the expansion of the universe.

“The more we understand how supernovae work, the better we can calibrate our standard candles,” says Pelisoli. “This is very important at the moment because there’s a discrepancy between what we get from this kind of standard candle, and what we get through other methods.

“The more we understand about how supernovae form, the better we can understand whether this discrepancy we are seeing is because of new physics that we’re unaware of and not taking into account, or simply because we’re underestimating the uncertainties in those distances.”