297
See also: Radio astronomy 179 ■ Discovering black holes 254 ■
Studying distant stars 304–05I
n 1935, Karl Jansky found a
source of radio waves named
Sagittarius A (Sgr A) at the
center of the Milky Way. Hidden
from light telescopes by cosmic
dust, the radio waves emanated
from several sources. In 1974, radio
telescopes pinpointed the most
intense source, named Sagittarius
A* (Sgr A*). It was small and
produced intense X-rays, suggesting
that matter at the heart of the
galaxy was being ripped apart
by a gigantic black hole, emitting
X-rays in the process. However, this
remained hypothetical until Andrea
Ghez, an astronomer at UCLA,
used a method for observing stars
through the dust using infrared.In 1980, Hawaii’s Keck Observatory
began measuring the speed of stars
orbiting close to the galactic center.
This data made it possible to
calculate the mass of the invisible
object inside Sgr A*. Ghez’s team
found that the stars closest to Sgr A*
were orbiting at a quarter of the
speed of light. Such speed indicated
an immense gravitational presence:
a black hole 4 million times heavier
than the sun, which must have
swallowed up stars and other black
holes when the galaxy was young. ■THE TRIUMPH OF TECHNOLOGY
OUR GALAXY
HARBORS A MASSIVE
CENTRAL BLACK HOLE
HE HEART OF THE MILKY WAYT
IN CONTEXT
KEY ASTRONOMER
Andrea Ghez (1965 –)
BEFORE
1971 British astronomers
Martin Rees and Donald
Lynden-Bell propose that
the radio waves emanating
from Sagittarius A are
produced by a black hole.
AFTER
2004 A smaller black hole
is discovered in orbit around
S a g i t t a r i u s A*.
2013 The Chandra X-ray
Observatory sees a record-
breaking X-ray flare from
Sagittarius A*, perhaps caused
by an asteroid entering the
black hole.
2016 The LIGO experiment
makes the first detection of
gravitational waves, capturing
the moment when two black
holes merged into one.
An X-ray flare shoots from the black
hole at the heart of the Milky Way. The
discovery suggests all galaxies may
have black holes at their hearts.