Sky and Telescope - February 2016 - 18

Galactic Center Goings-On
Or perhaps it's a much more massive object: a star. If
half the Sun's mass were buried inside G2's dusty, cool
envelope, it could bind a puﬀ y atmosphere and prevent it
from being stripped as G2 passed near Sgr A*. But why
the puﬀ y shroud to begin with? One team, led by Andrea
Ghez (University of California, Los Angeles), suggested
that G2 could be the product of a stellar merger. Perhaps
two low-mass stars slammed into each other, robbed the
stars of their angular momentum, and sent the merger
product plunging toward Sgr A*. This collision would
disrupt the outer layers of the two progenitor stars and,
until they had a chance to settle, leave a merged star with
an extended, cold, dusty veil.
From the infrared images taken in 2012, as well as
archival images of G2 dating back to 2004, observers
soon concluded that the object's strongest interaction
with Sgr A* would occur in the spring of 2014. Astronomers lined up to watch the encounter.
But despite continued infrared detections, radio,
submillimeter, and X-ray observatories have come up dry
so far. G2 didn't create the shock fronts we expected as it
blasted through the hot gas around Sgr A*, nor has it yet
bumped up Sgr A*'s accretion rate or created a jet.
The lack of radio waves from a shock front means G2
is smaller than originally thought. Many think the size
limits inferred from radio observations strengthen the
stellar interpretation. The object could still be fairly big
within those limits, similar in size to some of the largest
known stars, so-called hypergiants, whose diameters can
reach up to roughly 20 a.u.
But the cloud idea isn't ruled out yet. It's just that if
the object were a cloud, some force would need to act on
it to limit its size. A strong magnetic ﬁeld could deform
the cloud, stellar winds might conﬁne it, or we might be

S0-1
S0-2
S0-5
S0-16
S0-19
S0-20
S0-38
S0-102
0.1″

UCLA GALACTIC CENTER GROUP / W.M. KECK OBSERVATORY LASER TEAM

CENTRAL STARS For about two decades, astronomers have
used adaptive optics to monitor stars' orbital motions around
the 4.3 million-solar-mass black hole sitting at the center of our
galaxy. Dots mark stars' average annual positions.

seeing the gaseous veil surrounding a merged star. The
case remains wide open.
At the moment, observers are watching G2 as it
re-emerges from behind Sgr A*. Ghez's team, using
infrared images from the Keck telescopes, has seen the
object reappear intact, which they say supports the star
scenario. However, spectroscopic data collected from
Gillessen's team suggest that the gas tail was signiﬁcantly disrupted during the close encounter, evidence
of tidal streams shorn from a cloud. Hope remains that
we could still see an increase in accretion onto Sgr A* on
long time scales, perhaps spread over several years.

February 2014

September 2014

2012

Black hole

2010
2006

WATCHING G2 Left: An archival image from 2006 reveals G2 (red), the dusty body discovered traversing the galaxy's center in
2011. (A similar object named G1 is shown in blue.) Even with adaptive optics, the Very Large Telescope (VLT) in Chile sees stars as
blobs of near-infrared light (white/gray). Dashed lines show the orbits of G2 and a close-in star named S0-2, which zips around Sgr
A* every 16 years. Right: In this composite VLT image, colors indicate G2's velocity: the object receded from us as it approached the
black hole (yellow, orange, and red), now it's rounding the bend and coming back (blue). Both images are about 0.1 light-year across.

Februar y 2016 sky & telescope

ESO / A. ECKART

O. PFUHL ET AL. / ASTROPHYSICAL JOURNAL 2015

S0-2

Sky and Telescope - February 2016

Table of Contents for the Digital Edition of Sky and Telescope - February 2016