Sky and Telescope - September 2018 - 29

data they need to see the relativistic shift
definitively by the end of 2018.
The second test S2 permits is that of
peribothron shift, the movement of the star's
closest point to the black hole. In general
relativity, there are no closed orbits. Just as
Mercury precesses around the Sun, drawing
out cosmic Spirograph stencil patterns, so
too do the S stars spiral around Sgr A*. In
S2's case, the precession is about 12 arcminutes per orbit.
"Now don't think of the precession in this
case as a sort of continuous motion of the
orbit," Genzel warns. General relativity only
matters close to the black hole, and S2 follows a very elongated orbit that has it spending most of its time beyond this region. "But
when you are within half a year to a year of
the peri, then that's when the 12-arcminute
precession happens, almost all of it."
This measurement is immensely challenging. Images of the galactic center look
like a polka-dot fabric. "Essentially everything is filled with stars," he says.
"And in this field of thousands of stars,
everything is moving," Ghez says. Not only
moving - orbiting. That adds multiple directions of motion to the problem, and because
there's no clear coordinate system, it's all a
big mess. Even with adaptive optics to combat the turbulence from Earth's atmosphere,
astronomers are working at the limits of
what 8- and 10-meter telescopes can do.
The German group is tackling this
problem with a new interferometry instrument called Gravity. The instrument
enables them to combine the four 8.2meter telescopes of the Very Large Telescope
in Chile into a single superscope, with an
equivalent diameter of a whopping 130
meters. At near-infrared wavelengths, that
boosts resolution by more than a factor of
10, enabling them to clearly see the star's
motion from night to night.
The instrument has taken a decade and
more than $10 million to build, requiring
all of the different telescopes' components
to be aligned to about 5 nanometers. "That's
something that does not come free, I can tell
you that," Genzel laughs. But the payoff is
worth it: They could see S2 moving night by
night as it approached the black hole.
Gravity came online just in time for the
researchers to watch the year or so leading
up to S2's pass. They'll need a total of three

Implications
for LIGO?
Research into how the
S stars formed might
also help astronomers
understand the origin of
some of the black holes
found with gravitational
waves. The Milky Way's
center contains the
galaxy's most intense
concentration of stars,
and the same holds
true for other galaxies.
Since extremely massive stars turn into black
holes when they die,
astronomers expect a
cadre of little black holes
in galactic cores - and
recent work by Charles
Hailey (Columbia University) and colleagues
suggests they're right:
The team uncovered
a dozen objects that
might be black holes
paired up with stars in
the Milky Way's center,
very close to where the
OB disk lies (S&T: July
2018, p. 10).
Such small black holes
could fall victim to the
same eccentric KozaiLidov effect that might
have created some of
the S stars. Calculations
by Naoz's student BaoMinh Hoang (University of California, Los
Angeles) and colleagues
show that, if a pair of
star-scale black holes
loops around a galaxy's
central supermassive
one, the big black hole's
gravity could force the
binary members to
merge. The process
might create a fair
number of the events
gravitational-wave observations uncover.

years, with the peribothron in the middle, in
order to triangulate the precession.

Up Close and Personal
The S stars bring us within about 100 a.u.
of Sgr A*, some 1,000 times the radius of
the event horizon. To get closer to the black
hole, astronomers need to find more stars.
Right now, they can see old, swollen red
giants that are the Sun's mass or larger, but
they can't see main-sequence stars that are
much less than 10 solar masses, Ghez says.
They know more stars are hiding beyond
these detection limits.
A recent study by Idel Waisberg (Max
Planck Institute for Extraterrestrial Physics,
Germany), Genzel, and colleagues suggests
that observers would need to find a star that
approaches 10 times closer to Sgr A* than S2
does in order to probe the way the black hole
drags spacetime around as it spins. Given
the odds of such a star existing (not high),
they estimate that finding it would take
several years of searching.
So to come closer, astronomers are
placing their hopes in the Event Horizon
Telescope, a worldwide network of radio
telescopes that work together to create the
equivalent of a planet-size dish (S&T: Feb.
2012, p. 20). In April 2017 the EHT integrated ALMA into its array and stared at
Sgr A* over several days, peering through the
gas and dust hiding the black hole. The team
is still working on the observations - it took
the better part of a year to retrieve the data
from the South Pole - but the image captured might show hints of the black hole's
silhouette on the surrounding gas, delineated by a ring of light.
As astronomers dive deeper into the
galactic center, they're finding connections
to areas of study they never expected, not
only to binary stars but also to how some of
the stellar-mass black holes caught merging
by gravitational-wave detectors might have
formed (see sidebar). Coupled with the EHT
and gravitational waves, the S stars are carrying us into a new regime of astrophysics,
Ghez says. "We're in the era of gravity."

¢ Much like the stars in this article, Science
Editor CAMILLE M. CARLISLE is caught by
the inexorable gravity of black holes. Find
more adventures in her new blog, The Black
Hole Files: https://is.gd/bhfiles.
sk yandtelescope.com * S E P TE M B E R 2 018

29


https://www.is.gd/bhfiles http://www.skyandtelescope.com

Table of Contents for the Digital Edition of Sky and Telescope - September 2018

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