Can a couple of stars have a toxic relationship? Keep this in mind as you learn about the newly discovered star system ZTF J1813+4251, a close-knit pair of stars with such tyrannical control over each other that they complete a full orbit around each other once every 51 minutes — the shortest orbit ever discovered for any binary star system to date, according to research published today (Oct. 5) in the journal Nature (opens in new tab).
Astronomers spotted the clingy star system about 3,000 light-years away The sun, in the constellation Hercules, while combing through a database of more than 1 billion stars. There a bright sun-like star of about the same mass as Jupiter is spending its last healthy years in the company of a white dwarf — the shriveled shell of a once-mighty star that is technically already dead and no longer burning fuel. But beyond the star grave, the white dwarf’s gravity continues to suck hydrogen from the Sun-like star’s atmosphere, causing the larger star to slowly shrink and hasten its inevitable demise.
This is not uncommon in astronomy; more than 50% of all stars in the Milky Way are binary pairs of two or more stars that share a common center of gravity and rob each other of valuable fuel. However, astronomers have found few binary systems with orbital times less than an hour — particularly systems with large, Sun-like stars that take many hours to complete a single orbit. When astronomers saw the two stars in Hercules eclipse each other every 51 minutes, they knew they had encountered something strange.
“This one star looked like the Sun, but the Sun can’t fit into an orbit shorter than eight hours — what’s going on here?” The study’s lead author Kevin Burdge, a physicist at MIT, said in a statement.
In their new work, the researchers give this oddity a name. According to the study’s authors, these stars belong to a rare class of binary star systems called “catastrophic variables.” These systems form when a white dwarf and a Sun-like star move closer together over billions of years, allowing the white dwarf to engulf material from its companion. As the dwarf star cavorts, it can unleash enormous flashes of light that appear from afar, like supernova explosions or some other type of cosmic catastrophe, the authors said.
Researchers have long suspected that stars like these are capable of incredibly short orbital periods given enough time. This is only possible when the star system shifts to a new diet, so to speak; Once the hungry white dwarf finishes scavenging the hydrogen from its companion star’s atmosphere, it begins to gobble up helium from the exposed core of the companion star. Because helium is denser and heavier than hydrogen, the core of the Sun-like star should be sufficiently massive to remain in superdense binary orbit with its white dwarf companion.
According to the study’s authors, this strange star system in Hercules appears to be in the midst of such a transition — and the system’s orbital period should only get tighter and shorter. Using computer simulations, the team calculated that in about 70 million years, the two stars will come so close that their orbits will drop to just 18 minutes – far shorter than any star system ever seen.
From there, the loss of mass will cause the Sun-like star to expand rather than contract, and the two stars will eventually drift apart for the next several hundred million years, the team said. At this point, the pair’s orbital period will settle to around 30 minutes, giving the binary pair much-needed space in their final years together before the larger star burns out.
While this type of binary relationship has been theorized, this new research is the first time it has been seen in action, according to the study’s authors. This answers a “big open question” about the orbital limits of cataclysmic variable star systems, Burdge said, and should provide much material for further study in the years to come.
Is the bigger star in a toxic relationship? Maybe it’s best to ask again in a few hundred million years.
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