cinity of a black hole should appear
to have a smaller blueshift, and one
moving away from us should have
a larger redshift, than would be the
case if the law of Newtonian grav-
ity prevailed. In the most stringent
test of this prediction to date, the
team analyzed over two decades of
astrometric and spectroscopic data,
obtained using adaptive optics, on a
star known as S0-2 as it followed its
eccentric 16-year orbit around Sagit-
tarius A* (Sag A*), the supermassive
black hole at the center of our Gal-
axy. Figure 3 shows the full set of po-
sitional and velocity data.
The star reached its closest approach
to Sag A* in May 2018, when it was at
a distance of only 120 au and mov-
ing at 2.7% of the speed of light. During
the critical months surrounding pericen-
ter passage, the team used three different
spectroscopic instruments at three differ-
ent observatories, including the Near-infra-
red Integral Field Spectrometer (NIFS) on
Gemini North, the OH-Suppressing Infra-
Red Imaging Spectrograph (OSIRIS) on the
Keck II telescope, and the Infrared Camera
and Spectrograph (IRCS) on the Subaru tele-
scope. "The velocity of the star was chang-
ing quickly every night! So having all three
observatories participate was essential,"
said Tuan Do (also of UCLA), the lead author
of the study. Combining data from multiple
instruments also allowed the team to care-
fully check for instrumental biases.
As shown in Figure 4, GR provides an ac-
curate description of the star’s positional
and velocity data throughout its very large
swing in velocity near its closest approach
to Sag A*. In contrast, the observations rule
out Newton’s law of gravity with a high sta-
tistical significance. “The GR model is 43,000
times more likely than the Newtonian mod-
el in explaining the observations,” the study
October 2019
concludes. The measurements also provide
strong constraints on the black hole’s dis-
tance and mass, 8.0 kiloparsecs and 4.0 mil-
lion solar masses, respectively.
Of course, no one wins forever, and at some
point, namely the event horizon of a black
hole, GR must also fail. However, although
S0-2 plunged precipitously near Sag A*, the
minimum distance was roughly 1,000 times
larger than the radius of the event horizon.
Thus, it may be some time before observa-
tional limits encroach on the limits of GR’s
validity. Meanwhile, such observations con-
tinue to enlighten our understanding of the
dynamics and evolution of the center of our
Galaxy. The study appears in the journal
Science.
Figure 4.
Top: Zoom in on the
radial velocity data from
2018, encompassing the
maximum and minimum
of the observed radial
velocity. Measurements
from the three differ-
ent observatories are
indicated; Gemini/NIFS
and Keck/OSIRIS each
provided nine mea-
surements during this
critical period, over which
the observed velocity
changed by 6,000 km/s.
Bottom: radial velocity
residuals with respect to
the best-fitting General
Relativistic model. [Figure
from Do et al., Science,
365: 664, 2019.]
John Blakeslee is the Chief Scientist at Gemini Ob-
servatory and located at Gemini South in Chile.
He can be reached at: [email protected]
GeminiFocus
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