Sky and Telescope - February 2016 - 17

ofthe

In our galaxy's busy center,
stellar nurseries exist
amongst aging and exploding stars and spinning stellar
corpses. A supermassive
black hole, Sgr A*, lurks in
near (but not perfect) silence
at the center of it all.

Milky Way
NASA / JPL-CALTECH / ESA / CXC / STSCI

nearly suicidal slingshot orbit around Sgr A* (S&T: June
2013, p. 23). Already, astronomers could see that its path
would take it within a couple hundred astronomical units
(a.u., the distance between Earth and the Sun) of the
black hole and deep into a hot, gas-filled environment.
Controversy ensued: was G2 a gas cloud, a gasshrouded exoplanet, a star, or something altogether new?
Normally, Sgr A* is a notoriously quiet black hole,
accreting only about a thousandth of an Earth mass
every year. Early models for G2 estimated about 3 Earths'
worth of mass in gas, assuming a simple spherical
geometry. If a substantial fraction of this gas fell onto
the black hole over the course of a few years, Sgr A*
would morph into something akin to an active galactic
nucleus (AGN), similar to those we observe in galaxies
in the far reaches of the universe. To witness this transition would be spectacular.
But G2 might not be wholly gas. Other interpretations yield equally compelling possibilities. G2 could be a
gas-shrouded exoplanet torn from its parent star during a
close encounter in one of Sgr A*'s stellar disks, tossed into
a plunging, Interstellar-style orbit around the black hole.

What is Sgr A*?
At a distance of 26,000 light-years, Sgr A* is
our nearest supermassive black hole. Thanks to
observations that monitor stars' orbits around
this unseen dark object, we know its mass to
excellent accuracy: between 3.8 and 4.8 million
times the mass of the Sun.
Sgr A* feeds on winds blown out from massive
stars in its vicinity. But for reasons we still don't
understand, Sgr A* is an inefficient gas-guzzler.
It swallows only a small fraction of an Earth mass
per year (somewhere between 0.06 and 6 times
the Moon's mass per year, as estimated from
radio polarization measurements), blowing the
rest back out into the galaxy. Its low-calorie diet
leads to the surprising conclusion that the largest
black hole in our galaxy is not a roaring lion but a
cosmic pussycat, emitting about one-billionth of
its maximum theoretical radiation.

Sk yandTelescope.com Februar y 2016

17



http://www.SkyandTelescope.com

Sky and Telescope - February 2016

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

Contents
Sky and Telescope - February 2016 - Cover1
Sky and Telescope - February 2016 - Cover2
Sky and Telescope - February 2016 - 1
Sky and Telescope - February 2016 - Contents
Sky and Telescope - February 2016 - 3
Sky and Telescope - February 2016 - A
Sky and Telescope - February 2016 - B
Sky and Telescope - February 2016 - 4
Sky and Telescope - February 2016 - 5
Sky and Telescope - February 2016 - 6
Sky and Telescope - February 2016 - 7
Sky and Telescope - February 2016 - 8
Sky and Telescope - February 2016 - 9
Sky and Telescope - February 2016 - 10
Sky and Telescope - February 2016 - 11
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Sky and Telescope - February 2016 - 15
Sky and Telescope - February 2016 - 16
Sky and Telescope - February 2016 - 17
Sky and Telescope - February 2016 - 18
Sky and Telescope - February 2016 - 19
Sky and Telescope - February 2016 - 20
Sky and Telescope - February 2016 - 21
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Sky and Telescope - February 2016 - Cover3
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