Sky and Telescope - September 2018 - 26

tance from its companion. The other is ejected
Furthermore, binaries are common. A star
peribothron, n.
An orbit's closest
at enormous speed - not only out of the galacof the Sun's mass has about a 50/50 chance
approach to a black
tic center but out of the Milky Way entirely.
of being born as a binary, and the chances
hole. From the preﬁx
Astronomers have found more than a dozen
are higher for massive stars like those in the
peri- for closest and
stars that might be these slingshotted suns. At
galactic center. "So if you think that stars form
the Greek bothros,
hundreds of thousands of light-years from the
the same way near the black hole - which of
translations for which
galactic center, they don't have an obvious concourse is a big IF - then it's quite possible that
include "a hole or pit
nection to the black hole. But like those in the S
the massive stars would form as binary stars,"
in the ground" and
cluster they're B stars, and many are ﬂying fast
Ghez says. "And those binaries, when they're
"cesspool."
enough through space that our galaxy can't hold
close to the black hole, can merge."
onto them. In fact, using Gaia data, Warren
Naoz's student Alexander Stephan (UniverBrown (Smithsonian Astrophysical Observatory) and others
sity of California, Los Angeles), along with Naoz, Ghez, and
recently determined that seven of the more than three dozen
their colleagues, decided to see how common mergers might
hypervelocity stars they studied are moving so fast that the
be in the galactic center. Simulating more than a thousand
only reasonable explanation is that they've been ejected from
binaries in the innermost 0.3 light-year, the team found that
the galactic center by the supermassive black hole. The team
13% of the binaries merged after a few million years. For
suspects another nine stars might be members of same club.
those stars speciﬁcally in the S cluster region, the rate was
Recent calculations by Smadar Naoz (University of Calieven higher: About 40% had fused in the same time period.
fornia, Los Angeles) and colleagues suggest a different kind of
This three-body merging process, called the eccentric
binary-black hole encounter might be at work in the S clusKozai-Lidov (EKL) mechanism, might explain why there are
ter's creation. When three bodies interact, the gravitational
far more ﬂedgling stars around Sgr A* than expected. "If it's
inﬂuences become complicated. For two stars in a binary
correct, [then] young stars in the S cluster actually belong to
that's already orbiting the supermassive black hole, the black
a way older population, as old as a billion years or so," Naoz
hole's inﬂuence will make the stars' orbits around each other
says. The team is now looking at speciﬁc S stars to see if it can
elongate with time. This change moves their closest approach
spot signs that indicate they're merger products.
to each other even closer, potentially close enough that they
merge - either by colliding or by getting stuck in a different
A Major Event
orbit such that, when one or both stars ages and swells, the
The disk of massive stars outside the S cluster is a different
stars kiss and combine.
kettle of ﬁsh. The stars orbit too far from the black hole to be
When stars merge, they reset their clocks. "It's like a total
survivors of split-up binaries. Furthermore, out beyond oneface-lift," she says. "They rejuvenate themselves."
third light-year, other things besides the black hole inﬂuence
Astronomers have seen at least one likely stellar merger
stars' motions.
unfold elsewhere in the galaxy, the star V1309 Scorpii.
Astronomers increasingly suspect that, despite the odds,
They've also detected mysteriously youthful stars known as
these stars actually did form in place. If a big enough cloud of
blue stragglers in the ancient stellar balls called globular clusdense gas fell into the galactic center, the black hole's gravity
ters, where starbirth died out eons ago. In some cases these
would stretch the cloud into a long streamer and wrap the gas
blue stragglers are likely merger products.
around itself like a pool noodle. As the gas came back around

The Mysterious G Objects

Simulation of
G2's orbit around
Sgr A*, with 2011
positions

S E P TE M B E R 2 018 * SK Y & TELESCOPE

The three-body interaction that Naoz and others are exploring might explain more than just the youthful S stars.
The mechanism might also have created some strange
sources near the black hole, called the G objects. These
are red, dusty thingamajigs that might be stars ensconced in gas. The most famous is G2, which swung
around Sgr A* in 2013 and 2014, coming within 130 a.u.
of it. Members of the UCLA group and their collaborators recently discovered three more, for a total of ﬁve.
Genzel, whose team discovered G2 in 2012, pleads
agnosticism about these objects' nature. But Ghez is
more willing to speculate. When two stars ﬁrst merge,
she says, the result is a red, dusty cocoon. This state
lasts roughly a million years. It also happens to be a

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Sky and Telescope - September 2018

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