Celestial embrace is hanging by a thread

Relative sizes of several types of low-mass objects compared to the sun, Jupiter and Earth.
Relative sizes of several types of low-mass objects compared to the sun, Jupiter and Earth. Jon Lomberg / Gemini Observatory
/ Source: Space.com

A newly spotted pair of tiny stars that holds the record for the longest-distance celestial embrace is bound by only a thread of gravity and might one day break up.

The stars are separated by 5,100 astronomical units (AU), where one AU is the distance between the Earth and Sun. Astronomers have dubbed the pair the Hang-loose Binary. The previous record holder for long-distance binary stars was the Koenigstuhl 1AB system, in which the stars are separated by about 1,800 AU.

The weak gravitational tether binding the two stars in the Hang-loose Binary results in an orbital dance so slow that one orbit takes 500,000 years to complete. On a human scale, this system would appear as two baseballs orbiting each other about 200 miles (300 kilometers) apart.

The research, published in the April 10 issue of the Astrophysical Journal Letters, throws a wrench in star-formation models, which would not predict that such a low-mass, wide binary setup could form and remain stable. Only about a half dozen very-low mass binary systems with separations greater than 50 AU are known.

Many unanswered questions still plague the finding, including the age of the objects and whether they should be classified as so-called red dwarfs or less-massive brown dwarfs. 

Spotting stars
Astronomers led by Étienne Artigau of the Gemini Observatory in Chile discovered the system by comparing the locations of the stars from archival data collected 16 years apart.

Observations with the Cerro Tololo 1.5-meter telescope confirmed the two stars were indeed traveling through space together about 200 light-years from Earth in the southern constellation Phoenix. 

Using an infrared spectrograph on the Gemini South telescope, the team estimated the objects reach temperatures of about 4,000 degrees Fahrenheit (2,200 degrees Celsius).

Each object has a mass less than 100 times that of Jupiter and could be either red dwarfs, which are the most common type of star, or brown dwarfs, which are celestial objects that are bigger than the biggest planet but smaller than the smallest star.

Stellar couples
Artigau calls the new binary discovery surprising and “disturbing.” Astronomers estimate that two out of every three stars in the Milky Way is a member of a binary or other multiple-star system.

The only other known binaries with such a lengthy separation are substantially more massive. That’s because the more mass a star packs, the greater its tug on other objects, hence the stronger gravitational bond between the couple.

Celestial duos like the recently discovered one belong to a class of very low mass binaries. How these systems form and maintain their gravitational embrace is a mystery. The same process thought to spit out lightweight binaries should also destroy them.

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“To form stars below about 0.3 or 0.2 times the mass of the Sun, you need some relatively violent phenomenon, so the core [of the protostar] doesn’t have the time to accumulate the mass to be heavier,” Artigau told SPACE.com. “If you have a phenomenon like that you don’t expect weakly bound systems to survive.”

Lasting embrace?
Whether the Hang-loose binary contains red dwarfs or brown dwarfs has bearing on the system’s age. The binary could be a billion of years, but that is uncertain because it is nestled amidst a group of much younger stars, called the Tucana/Horologium (TH) association. Its members are about 30-million-years old.

If the Hang-loose pair is a member of the TH association, the binary pair would have to be much younger, indicating that rather than red dwarfs the objects are less massive brown dwarfs.

Nailing down the binary’s age would help astronomers predict how long the embrace could last. “Unlike red dwarfs, these brown dwarfs wouldn't have enough mass to ignite hydrogen into helium at their cores, so they are destined to have their weak embrace disturbed more easily (and quickly) while they slowly cool, and fade from sight,” said study team member David Lafrenière of the University of Montréal.

In that case, all it would take is “a star that passes closer to one of these objects and just breaks the pair,” Artigau said.