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Four
pairs of what astronomers are calling "impossible stars" — stellar
twins in orbits so close they defy explanation — have been found in our Milky
Way galaxy, scientists say.
Astronomers
using the United Kingdom Infrared Telescope (UKIRT) in Hawaii discovered the
four star pairs, each of which is a binary system in which two stars circle
each other in less than four hours. Until now, scientists thought that such
twin-star setups couldn’t exist.
 |
| This artist’s impression shows two active stars — M4-type red dwarfs — that orbit each other every 2.5 hours, as they continue to spiral inwards. Eventually they will coalesce into a single star. |
Our
sun does not orbit another star, but roughly half of the stars in our Milky Way
galaxy do, as part of a binary system. These binary stars likely formed close
together, and have been orbiting one another since their birth, the researchers
said.
It
was typically thought that if a star formed too close to another, the two stars
would quickly merge into a single, bigger star. This theory seemed to agree
with observations taken over the last three decades, which reveal that binary
systems are abundant, but none of the pairs have an orbital period shorter than
five hours, the researchers said.
In
the new study, a team of astronomers monitored the brightness of hundreds of
thousands of stars in near-infrared light over the past five years, and found
several stellar binaries with surprisingly short orbits.
The
astronomers focused on binaries of red dwarfs, which are stars that are up to
ten times smaller and a thousand times dimmer than the sun. While red dwarfs
are the most common type of star in the Milky Way galaxy, they often do not
show up in astronomical surveys because they are too dim in visible light.
"To
our complete surprise, we found several red dwarf binaries with orbital periods
significantly shorter than the 5 hour cut-off found for sun-like stars,
something previously thought to be impossible," the study’s lead author
Bas Nefs, from Leiden Observatory in the Netherlands, said in a statement.
"It means that we have to rethink how these close-in binaries form and
evolve."
Early
in their lifetimes, stars shrink in size, which suggests that the orbits of
stars in these tight binary systems must have also shrunk since they were
formed, the researchers said. If not, the stars would have interacted with each
other early on, and would have likely merged.
But,
how the orbits of stars in these binaries shrunk by so much remains a mystery.
According to the new study, one possible explanation is that cool stars in
binary systems are much more active and violent than was previously thought.
As
the cool stellar companions spiral in toward each other, their magnetic field
lines may become twisted and deformed. This powerful magnetic activity may help
slow down the spinning stars, allowing them to move closer together, the
researchers explained.
"The
active nature of these stars and their apparently powerful magnetic fields has
profound implications for the environments around red dwarfs throughout our
galaxy," study co-author David Pinfield, from the University of
Hertfordshire in England, said in a statement.
Detailed
results of the new study appear in the journal Monthly Notices of the Royal
Astronomical Society.
(Space.com)
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