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Astronomers get a chance to watch a rare red nova in its opening strains

Advance warning of big astronomical events is something astronomers really love. That makes the behavior of one binary pair of stars in Cygnus a pleasant surprise. The pair is called a contact binary, which means that the two main sequence stars orbit each other so closely they’re touching. If our sun was part of a contact binary, at the distance from which we orbit, we’d see it as a luminous peanut-shaped light source in the sky. (Never mind the habitability consequences of doubling the solar radiation that hits our planet.) This doomed dynamic duo, named KIC9832227, is just clearing its throat before it really starts to make a scene.

As you might suspect, a contact binary is an unstable state of affairs. Big things tend to be pulled together by mutual gravity. This contact binary is spinning faster and faster, and scientists believe it’s about to finally snap together. When that finally happens, the cores of the two stars will merge and then explode as a luminous red nova. Since we have advance warning based on the stars’ behavior, this time we don’t even have to choose between getting popcorn and watching the previews. The cataclysmic explosion will increase the contact binary’s brightness by about ten visible magnitudes, starting from something totally invisible to the naked eye and flaring up as bright as the brightest stars in the Big Dipper, and we get to watch it the whole way through.

Hertzsprung-Russell Diagram

This is a Hertzsprung-Russell diagram, which plots size against luminosity and color for known stars. The big streak is called the main sequence. Image via Wikipedia

The big tip-off was the stars’ orbital dynamic, which scientists are watching and measuring using the hilariously named OGLE gravitational-lensing monitor. The two stars are circling each other ever faster as their orbits tighten and tighten. Soon, much like the merging black holes that LIGO caught, they’ll slam together and explode asymmetrically, creating telltale gravitational waves.

Red novae are a particular kind of stellar explosion: bigger than a nova, but not yet as big as a supernova. Normally a nova forms out of a white dwarf and some other larger star that together form a binary pair. As the larger star starts approaching its end of life, it expands and expands until its gas envelope starts getting siphoned away by the white dwarf. Sort of like a stellar astrophysics version of gavage or cannibalism or something, the white dwarf gluts itself on the larger star’s gas until the white dwarf can handle no more, at which point it explodes. Often the other star is blasted away into space.

This time, though, the explosion won’t be on the surface of the white dwarf. Instead it should originate from the plane of contact between the two stars. Pressure and temperature will hit a critical value, at which point the whole contact binary goes kabang: red nova.

Once the fireworks really begin, we should see a sharp, characteristic flare in brightness that declines over time. The flare is because at the core of the exploding star, the pressure is enough that it’s reached critical mass and is now undergoing fission: Matter is actually moving down its radioactive decay chain. When this particular star goes nova, it will give off light from the decay of nickel-56 into cobalt, then iron. According to the authors, who are already planning a series of careful observations of the nova as it develops, this should all start “sometime between summer 2019 and summer 2022.”

Now read: What is a supernova — or why stars explode, creating the universe as we know it

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