If you ever find yourself on a tour of some of the most fantastic places in the galaxy,
make sure you stop by the center. It’s a pretty exciting place.
Mainly, that’s because of the supermassive black hole, more than 4.3 million times the
mass of our Sun, that’s sitting in the middle of the Milky Way.
Black holes are what’s left over when a star collapses, and they have an incredibly
strong gravitational pull.
Because of that super powerful force, the supermassive black hole’s neighborhood is
the hottest, densest, and most turbulent place in the galaxy, stretching nearly a thousand
light years across.
Over time, it’s drawn in about 80 percent of all of the dense, molecular hydrogen gas
in our galaxy -- the stuff that makes stars -- which is why it’s called the central
molecular zone.
But combine the gas clouds with the supermassive black hole’s gravitational pull, and you
get some weird effects.
So not only is the region full of dark clouds, it’s also scattered with super star clusters
and the corpses of dead stars.
Much of the gas in the molecular zone is clumped up, lying along a twisted ring of dusty gas
clouds around 326 light-years from the central black hole.
These gas clouds orbit the center of the Milky Way at almost 300,000 kilometers an hour,
following the same orbit like beads on a string.
But the ring is a little off-center, so at one point, the clouds end up within just 200
light-years of the black hole.
And that’s when things really start to get weird.
The gravitational pull compresses the clouds even more, and one of them is so dense that
it’s nicknamed the Brick.
At an average density of 10,000 atoms per cubic centimeter, the Brick would be considered
a high vacuum here on Earth. But out in space, it’s more than 10 times the density where
star formation would normally begin, and astronomers wanted to know why.
In 2012, a group of researchers got the first glimpse inside the Brick using a set of telescopes
called the Atacama Large Millimeter Array -- and in the process, they might’ve found
the answer to why it hasn’t started making stars yet.
They discovered that the gas is made of both filaments and dusty clumps, and it’s preparing
to form star clusters with hundreds of high luminosity, or super bright, high-mass stars
-- but the high energy environment moves the gas around so much that the lights haven’t
turned on quite yet.
But eventually, the Brick will reach just the right conditions where it can make stars,
and it’ll quickly form a lot of really big, bright ones.
Then, it’ll become a lot like another cloud in the molecular zone, called Sagittarius
B2.
Which is a huge cloud, almost 10 times as massive as the Brick, and the most active
site of star formation in the entire Galaxy.
It probably went through that whole black-hole-squeezing process at some point, and within the last
few tens of thousands of years, it’s burst to life with dozens of extreme, high-mass
stars.
And in the hottest spot in Sgr B2, there’s a young star cluster that produces more than
10 million times the luminosity of our Sun.
Those hot stars pour out so much heat and energy that all sorts of complex chemical
reactions can happen in the surrounding clouds.
So that’s where we tend to find complex organic molecules like formaldehyde, different
kinds of alcohols, and ethyl formate, a molecule that smells like rum and tastes like raspberries.
But Sgr B2 won’t stick around forever.
At some point, the fierce winds and radiation from the hot stars will blow the cloud apart,
leaving behind enormous super star clusters.
There are a few of these, formed within the last few million years, already in the molecular
zone.
Two of the most famous clusters are called Arches and Quintuplet, and they pack stars
in densities more than 100,000 times higher than in our part of the galaxy.
The Quintuplet cluster contains one of the most luminous stars known, the Pistol star.
And it probably couldn’t have formed anywhere else in the galaxy, because it would’ve
needed a lot of mass to pile up really quickly.
Overall, every few million years the central molecular zone turns so much gas into stars
that it uses up 100,000 times our Sun’s mass.
But any losses are quickly replaced by gas flowing in from farther out in the galaxy,
so the star formation has continued for millions, if not billions, of years.
With such a long history of forming heavy, bright stars, the molecular zone has another
group of residents that you won’t find anywhere else in the galaxy: lots and lots of dead
stars.
High mass stars burn through their fuel really quickly, and they’ll go supernova in as
little as 3.5 million years after they first form -- compare that to our Sun’s 10 billion
years!
After they explode, they leave behind stellar corpses in the form of small black holes and
neutron stars -- tiny, high-density cores.
Over the more than 10-billion-year lifespan of the galaxy, generations of stars have formed
and died in the molecular zone.
So, in addition to being the busiest stellar nursery in the galaxy, the most extreme place
in the galaxy is also its largest and most crowded graveyard.
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