"In 1974, an unusually compact source of radio waves at the exact center of the Milky Way, some 26,000 light-years away. Named Sagittarius A, after the constellation where it appears in the sky, it hints of
something strange: a trickle of X-rays from the same spot, tendrils of
agitated gas surrounding it and, most telling, a small group of stars
racing around madly for no obvious reason.
Tracking
the motions of those stars enabled astronomers to estimate the mass of
the unseen object directing the
action. From there they built a convincing case that Sagittarius A* was
in fact a black hole — the biggest one in the galaxy, with a mass 4.3
million times that of the sun and a diameter of about 25 million
kilometers.... when Sagittarius A* was acting up,
it was at least a million times as energetic as it is today. Put another
way, it briefly shone as brilliantly as a million suns.
"In
fact, other researchers may have already caught Sagittarius A* in the
act of such a feeding frenzy, albeit on a much smaller scale. Every day
or so, Chandra captures modest flare-ups during which the black hole
brightens up to a factor of 160 for a few hours.... those
events are burps caused when an asteroid or comet, at
least 10 kilometers across, passes within about 100 million kilometers
of the black hole, quickly getting shredded and consumed."
And
now a large gas cloud known as G2 is whipping past the black hole at 10
million kph. It is the first time scientists have ever seen such a
thing.... the
black hole’s potent gravity has warped G2 into a long, snaking blob,
with the leading part already coiled all the way around Sagittarius A*.
At its closest, G2 will pass about 25 billion kilometers from the event
horizon, far
enough that it is unlikely to get entirely sucked in. But the cloud is
so elongated that it will take a full year for all of it to clear the
black hole, and there is plenty of room for surprise.
G2
might make fireworks another way, because Sagittarius A* is probably
surrounded by smaller black holes. Much as hair collects around a
bathtub drain, the most massive objects in the Milky Way tend to spiral
downward toward the center. As a result, an estimated 20,000 black
holes, each about the size of a city and containing a few times the mass
of the sun, are thought to be circling Sagittarius A*. As G2 blows past
that dark swarm, the smaller black holes may scoop up bits of loose gas
and light up with bursts of X-rays that could be visible to Chandra or
NASA’s NuStar space telescope.