Sky and Telescope - September 2018 - 24

Galactic Center

5 li
gh
t-y
ea
rs

S cluster
1½ light-years
3 li g t - y e a
h

OB disk

Molecular
clumps

Blue giants
B dwarfs
Red giants

Compact remnants
Faint low-mass stars

Not to scale

p GALACTIC CENTER Within 5 light-years of Sgr A* lie several stellar
populations, split into structures and shown here as a cutaway. The older
stars are randomly distributed, whereas the young ones preferentially
appear in either the warped, rotating disk of OB stars or in the S cluster
inside the disk. Stars are color-coded by type, but the dots are representative of detections or inferences, not exact counts and coordinates.

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

this past May, it entered gravitational territory we've never
been able to explore before. These perplexing stars thus have a
lot to teach us not only about how stars and black holes make
peace, but about gravity itself.

Close Quarters
The galactic center is a crowded place. Within four light-years
of Sgr A* snuggle more than a million stars. Average distances
between them range from a few hundred astronomical units
(in the innermost sanctum) to a few thousand a.u. - roughly
a hundredth the distance between us and the nearest star
system, Alpha Centauri.
These stars split into several populations, forming what
essentially looks like a ball surrounded by a disk surrounded
by a bigger ball. It's a familiar picture for astronomers. "The
galactic center looks just like a solar system," says Andrea
Ghez (University of California, Los Angeles), comparing the
region to our planar system of planets and the vast sphere of
icy bodies around it, called the Oort Cloud. "The physics is
really just scaled-up solar system physics."
The central ball in the galactic center, called the S cluster,
is a cloud of some three dozen massive, bluish-white B-type
stars. Each star follows an elongated loop around the black
hole. The cluster cuddles close to Sgr A*, the stars not straying more than 0.13 light-year away, or about 8,000 a.u. That's
roughly where the outer comet reservoir begins in our solar
system. Imagine having 30-something beefy stars scuttling
around the solar system.
Just beyond the S stars lies a disk of about 120 O- and
B-type stars, the youngest, hottest, and most massive mainsequence stars. This disk extends out to 1½ light-years and
rotates clockwise around the black hole. Around it are more
OB stars, some of which may inhabit a looser, counterclockwise-moving disk. Both the S stars and those in the OB disk
live fast, furious lives; now several million years old, they've
already burned through a fair fraction of their fuel.
Surrounding these big, bright orbs is a large cloud of red
giants. These older stars, roughly 100 million to a billion
years old, have passed through the main sequence of stellar
life and entered retirement. The red giants extend out about
5 light-years from the black hole.
The puzzle is, none of this looks anything like what
astronomers predicted they would see in the mid-20th century. "Let's be very precise: The expectation was no young
stars," Genzel says. The black hole's gravity should have
prevented starbirth; the only stellar neighbors ought to have
been a dense crowd of old, red stars. "That there are 200 O
stars in the central region and all of them more or less the
same age - that seemed outrageous, ja?"

Captive Audience
If they weren't born there, any stars near the black hole would
presumably have migrated to the spot over billions of years,
driven into the heart of the galaxy by gravitational interactions. But massive stars like those in the S cluster and OB disk

G A L ACTIC CEN TER: G REGG DINDER M A N / S&T, SOURCE T. A LE X A NDER / ANNUAL R E VIE W OF
ASTR ONOMY AND ASTR OPHYSICS 2017 A ND SMEDA R N AOZ; 3D VIZ: UCL A G A L ACTIC CEN TER
G ROUP / W.M. K ECK OBSERVATORY L ASER TE A M; SK E TCHES: LE A H TISCIONE / S&T

lack holes are not known for being particularly
chummy with stars. It's not just that they can rip
individual stars into pieces and eat them. The great
leviathans of the cosmos, supermassive black holes, have
such incredible gravitational ﬁelds that their wrenching tidal
forces shred clouds that would otherwise collapse and give
birth to new suns, sterilizing would-be stellar nurseries.
So astronomers were amazed to discover that roughly 200
young, massive stars huddle around the Milky Way's central
black hole, Sagittarius A*. The youngest are only a few million years old. They're not keeping their distance, either: One
comes within two-thousandths of a light-year of the black
hole, or four times Neptune's distance from the Sun.
These stars should not exist. For them to be born here,
their natal clouds would have had to be almost as dense as
a star in order to overcome the black hole's shearing effects,
says Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics, Germany). "It seems absolutely preposterous."
But if black holes are known for anything, it's for making
the preposterous real. Not only do the stars around Sgr A*
give us a glimpse into a fantastical world where stars and
black holes coexist, but one of these stars is also providing
astronomers with a long-awaited test of general relativity. As
the star swung through its closest approach to the black hole

Sky and Telescope - September 2018

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