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Scientists spot rare binary star for the first time, solve cosmic puzzle

This breakthrough could fill a gap in the stellar evolutionary timeline.

White dwarf star being consumed by companion star
An artist's depiction of a soon-to-be extremely low mass white dwarf (left) being consumed by its companion star. 
M.Weiss/Center for Astrophysics | Harvard & Smithsonian

The universe is a star museum. Up close, we have our beloved sun in its middle age. Farther out, we'll find white dwarfs nearing the end of their days. Peer deeper, and you'll stumble upon hyper-dense neutron stars on the brink of collapsing into black holes. Now, for the first time, scientists have uncovered a star exhibit that, for years, has only been a hypothesis.

Technically, the team's discovery is of a binary star system, or one star orbiting another. In this case, the smaller companion is eating the larger star, and very soon, the sparkly orb being chomped on will become a rare form of white dwarf, or late-stage star, called an extremely low mass, or ELM, white dwarf. 

They've published a paper explaining their findings in this month's issue of Monthly Notices of the Royal Astronomical Society.

Though fascinating in itself, the sighting of the deteriorating binary star could also fill an empty slot in a long-standing cosmic puzzle.

"This is exciting; it's a missing evolutionary link in binary star formation models that we've been looking for," Kareem El-Badry, lead author of the paper and a researcher from the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, said in a statement. 

Zooming out from the binary star itself, El-Badry's discovery fills a gap in our knowledge of how ELM white dwarfs are generated in the first place. We know ELM white dwarfs exist, as scientists have already pinpointed them, but these stellar bodies pose a conundrum. 

According to astronomical calculations, experts say, for a star to naturally reach the mass of an ELM white dwarf, it would need to have been born over 13.8 billion years ago. The issue is, the universe itself is only 13.8 billion years old -- so that doesn't really make sense with what we know about the cosmos so far. 

Therefore, the only way these stars could possibly be formed within the universe's timeline is if an external force created them. That force, theorists say, could be the gravitational pull of a companion in a binary star system that's pulling matter away from the other until the latter object becomes an ELM white dwarf. Seems intuitive, but for a long time, that was just a hypothesis. 

Scientists have come upon binary star systems called cataclysmic variables where regular, massive stars are being consumed by a companion star. They've even recorded ELM white dwarfs with companions no longer consuming them. It's easy to extrapolate that an intermediate step would involve an almost-ELM white dwarf with a companion star eating away at it, but no one had seen such a sight. 

Thanks to El-Badry's revelatory documentation, the transitional step appears to be proven at last.

Star hunting

El-Badry used a wide array of star data, several astronomical surveys and his own firsthand observations with the Shane Telescope at the Lick Observatory in California to locate the unique binary star. He'd come up with 50 potential candidates of transitional ELM white dwarfs and ultimately zeroed in on 21 options.

"100 percent of the candidates were these pre-ELMs we'd been looking for," he explained in the statement. "They were more puffed up and bloated than ELMs. They also were egg-shaped because the gravitational pull of the other star distorts their spherical shape."

Aptly, he compares the importance of this breakthrough to biological studies of organisms. "You go out into the jungle and find an organism. You describe how big it is, how much it weighs -- and then you go on to some other organism," he said. "You see all these different types of objects and need to piece together how they are all connected." 

Like the ever-advancing tree of life, El-Badry's new evidence of ELM white dwarfs helps to unveil the entirety of our cosmic star gallery. "We found the evolutionary link between two classes of binary stars -- cataclysmic variables and ELM white dwarfs -- and we found a decent number of them," he said.

Next, he hopes to delve into the other 29 pre-ELM white dwarf star candidates to enrich the universe's star museum even further.