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Stars caught in red light hook up

Merging stars Strange red glowing lights in deep space may be caused by pairs of stars getting so close to each other they share atmospheres.

The new hypothesis, by researchers including astrophysicist Dr Natalia Ivanova from the University of Alberta could explain mysterious objects called intermediate-luminosity red transients.

These outbursts can last for a few days or many months, and appear brighter than novae explosions, but not as bright as supernovae.

A nova occurs when a dead star called a white dwarf draws too much gas from a companion star, causing a thermonuclear explosion on the white dwarf's surface.

A supernova occurs when a star blows itself apart.

But the origin of intermediate-luminosity red transients such as V838 Monocerotis has long puzzled scientists.

Theory and observation

Ivanova and colleagues modelled what would happen when two stars in a binary system get close enough to merge their atmospheres.

"These phenomena known as common-envelope events have been theorised but never observed directly," says Ivanova.

"It could happen in a range of close binary systems, such as a white dwarf and a normal star, X-ray binaries or even double-neutron stars."

According to Ivanova and colleagues, the common envelope shared by the two stars comprises super-heated ionised plasma.

"This cools down as it's ejected from the binary system, allowing electrons and protons to recombine, forming neutral gas which is really bright," says Ivanova.

"That's what creates the light we see as red transients."

Writing in the journal Science, Ivanova and colleagues say about half of these events could result in the two stars merging into a single stellar object.

Smoking gun

The team then searched for objects matching the properties theorised for common-envelope events, and were surprised to discover several red transients that fitted their predictions.

"It felt really good to find these objects which matched the theory," says Ivanova.

"It's not very often you get the chance to explain something which is unknown. It's a nice feeling."

Based on their model, the researchers suggest that many red transients result from common-envelope events, and that red transients represent their long-sought signatures.

Ivanova now wants to focus on unravelling the physics that causes the common-envelope to be ejected from the binary star system in the first place.