Actually, a type Ia supernova is pretty nasty. The star that will eventually perish has basically done everything right and ended its long, modest life as a white dwarf. As such, it could continue to watch its fellow stars burn up for many billions of years – longer than the universe is old – if it didn’t have a younger partner that was still in the prime of its life. Because if material flows from this other star to our white dwarf, an overshooting reaction likes to happen. The (former) white dwarf can no longer cope with the inflowing material and perishes in a supernova, which astronomers call “type Ia”.

In Nature Astronomy, astronomers now report a pair of stars that still face this fate. The pair – a binary star system named HD265435 – is located about 1,500 light-years away; it consists of a hot dwarf star and a white dwarf star that closely orbit each other every 100 minutes. White dwarfs are “dead” stars that have burned all their fuel and collapsed in on themselves, making them small but extremely dense.

A Type Ia supernova is thought to occur when the core of a white dwarf star re-ignites and a thermonuclear explosion occurs. There are two scenarios in which this can happen. In the first case, the white dwarf gains enough mass to reach 1.4 times the mass of our Sun, which is known as the Chandrasekhar limit. HD265435 fits the second scenario, in which the total mass of a nearby stellar system of multiple stars is close to or above this limit. So far, only a handful of other stellar systems have been discovered to reach this limit and lead to a type Ia supernova.

Lead author Ingrid Pelisoli of the University of Warwick’s Department of Physics explains, “We don’t know exactly how these supernovae explode, but we know it must happen because we see it happening elsewhere in the universe.” Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), the team was able to observe the hot subdwarf. While they did not detect the white dwarf, the researchers observed that the brightness of the hot subdwarf varied over time; suggesting that a nearby massive object was distorting the star into a teardrop shape.

Astronomers then used the Echellette Spectrograph and Imager (ESI) at Palomar Observatory and Keck Observatory to measure the radial and rotational velocity of the hot subdwarf, which allowed them to confirm that the hidden white dwarf is as heavy as our Sun, but only slightly smaller than the radius of Earth. Together with the mass of the hot dwarf star, which is slightly more than 0.6 times the mass of our Sun, both stars have the necessary mass to cause a type Ia supernova.

Since the two stars have already converged to the point where they are spiraling toward each other, the white dwarf will inevitably go supernova in about 70 million years. Theoretical models created specifically for this study also predict that the hot subdwarf will shrink to a white dwarf star before merging with its companion.

Type Ia supernovae are important to cosmology as “standard candles.” Their brightness is always the same, so astronomers can infer distance from measured brightness. “The more we understand how supernovae work, the better we can calibrate our standard candles. That’s very important right now, because in determining the expansion of the universe, there’s a discrepancy between what we get from these kinds of standard candles and what other methods give,” Pelisoli says.