Sky and Telescope - September 2018 - 28

Galactic Center

Even if the OB disk isn't related to the current Fermi
bubbles, its stars might produce a very similar structure in
the future. Because the disk stars live such brief, brilliant
existences, "quite a number of them are expected to explode
more or less tomorrow," Genzel says. (By tomorrow, he means
in the next few thousand years.) When they do, they'll throw
out a lot of gas. "That could lead to a phenomenon like the
Fermi bubbles."

1"
1"

S2
0'
0'

Sgr A*

Close Encounters of the Stellar Kind

28

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

1'''
1'

2""

1"
1"

0""

1"
1"

p CROWDED Adaptive optics reveals dozens of stars near Sgr A* -
including stellar interlopers that aren't physically near the black hole but
invade the field of view. As stars move, they can overlap other ones from
our perspective, as S2 does in this image. The region of sky shown spans
roughly 4 arcseconds, about the size of Uranus in the sky. The light at the
position of Sgr A* could be from its gas or from an unrecognized star.

4,000

0.0125
0.01
0.075
0.05
0.025

3,000

2,000

1,000

0

-1,000

0
-2,000
0.05

0.025

0

-0.025 -0.05 -0.025

Change in right ascension
(arcseconds)

2000

2005

2010

Year

2015

S2 ORBIT A ND G A L ACTIC CEN TER: S. GILLESSEN E T A L. / ASTR OPHYSICAL JOUR NAL 2017 (837:30)

well look basically as they would in Newtonian physics, but
farther in, relativity creates a distorted pit. When it enters
this distorted region, S2's light loses more energy in its escape
than it would otherwise, causing an additional redshift of
about 250 km/s.
The redshift only shows up in the star's light right before
and after its pass, appearing as two "turning points" in a
plot of the star's radial velocity. One of these turning points
happened at the beginning of May; the second should come
at the end of September. At its peak the radial velocity will
be changing by 100 km/s per night, Ghez says, due to both
classical and relativistic effects. Both teams should have the

Line-of-sight velocity (km/s)

u S2'S ORBIT Left: Observations by the UCLA
(purple) and Max Planck (blue) teams of S2's
position over two decades trace out the star's
orbit in the plane of the sky. The black circle
marks Sgr A*; gray crosshatches mark recorded flares. (No relativistic precession appears
yet in these data; instead, the drawn orbit
doesn't close due to motion with respect to the
reference frame.) Right: When S2 completed its
closest approach in 2002, two "turning points"
appeared in a plot of its radial velocity: As the
star swung around, its velocity along our line
of sight quickly changed from a hasty retreat
(positive values) to a headlong approach. The
black line is the same orbit as on the left.

Change in declination (arcseconds)

The brightest member of the S cluster is poised to reveal a lot
about gravity in the galactic center. The star, called S2 by the
Max Planck team and S02 by the UCLA team, is a 7-millionyear-old B-type star that weighs about as much as a dozen
Suns combined. It finishes its racetrack course around Sgr A*
every 16 years. During its closest approach it comes closer to
the black hole than any other star yet detected: about 120 a.u.
The latest pass was in May 2018.
Ghez and Genzel each lead teams that have been watching
S2 and its fellow S stars for more than two decades - Ghez
with the W. M. Keck Observatory on Mauna Kea in Hawai'i,
Genzel with the Very Large Telescope on Cerro Paranal in
Chile. These stars scout out the gravity landscape around a
supermassive black hole in a way that's never been probed
before, Ghez explains.
"You're in unexplored turf," she says. "We know that
ultimately we don't have the complete theory of gravity, so
any of these tests [that] push the frontier . . . forward is very
important. So everyone is extremely excited."
S2's flyby enables two tests of our current theory of gravity,
general relativity. The first is gravitational redshift. As S2 bears
down on the black hole, it dives further into the broad, deep
well the black hole creates in spacetime. Its photons have to
climb out of the well in order to reach us. This climb robs
them of energy, shifting them to longer, redder wavelengths,
manifesting as a dramatic swing of
several thousand km/s in the star's
0.0175
velocity along our line of sight.
This shift happens in Newtonian
0.015
gravity, too. But relativity adds something extra. The outer reaches of the



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