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.
"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
Montreal.
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.
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