This episode is sponsored by Coursera: one
hundred percent online learning from the world’s
best universities and companies.
[♪INTRO]
There is a supermassive black hole in the
middle of our Galaxy.
It’s as massive as about 4 million Suns,
and everything in the Milky Way orbits around
the region where it lives.
Studying it could even help us understand
ideas as enormous as gravity.
But for hundreds of years, no one was sure
this thing was out there.
Black holes don’t emit light, and our instruments
weren’t advanced enough to hone in on the
radio waves emitted by the matter that surrounds
them.
So for centuries, we were missing out on everything
this object had to teach us.
That is, until 1974 — when, for the first
time, a team of scientists was able to provide
convincing evidence that it exists.
Today, the black hole is called Sagittarius
A*, and finding it was one of the biggest
discoveries in astronomy.
Here’s how it happened.
This story began in the 1930s at, of all places,
a telephone company.
At the time, a physicist named Karl Jansky
was working for Bell Labs, and his job was
to find anything that might cause static on
transatlantic phone calls.
For the most part, his job was pretty chill.
Mainly, he just picked up static from thunderstorms.
But then there was this other source of static,
which he was able to trace to interstellar
space.
It was coming from a region at the center
of our Galaxy called Sagittarius A, which
gets its name from being in the constellation
Sagittarius.
Bell Labs didn’t care to follow up on this,
but astronomers did.
Over the next few decades, they began studying
this region and looking for whatever was emitting
those radio waves.
But unfortunately, even though they were getting
strong signals, they didn’t have telescopes
sensitive enough to pick out a single source
of this radiation.
It’s like their instruments were trying
to recreate a picture of the sky, but with
really big pixels.
So if they tried to capture the source of
the waves, they just got a blurry signal with
a bunch of other objects in it.
The good news is, radio astronomers didn’t
have to build one massive telescope dish to
solve this problem.
Instead, they used what’s called a radio
interferometer.
There are a few types of interferometers,
like the ones used to detect gravitational
waves.
But in this case, an interferometer is an
array of two or more dishes that work together
to collect signals from a single object.
As the Earth turns, the dishes in an interferometer
move in an arc.
And as the Earth completes a rotation, the
dishes trace out a circle.
That circle can act like a giant virtual telescope
dish.
It takes some really sophisticated computer
processing to sync up all the signals from
different dishes at different times, but the
results are worth it.
Because in this setup, the telescope’s resolution
depends on the distance between the dishes
instead of the size of a single dish.
And that means you can get some really sensitive
measurements.
Without a solution like this, the first teams
of astronomers to study Sagittarius A didn’t
have the resolution to pick out a compact
source.
But by the end of the 1960s, astronomers began
to narrow in on the prize.
One team of theorists published a paper suggesting
that this mysterious radio source had enough
energy to possibly be a black hole.
Then, in 1971, a pair of astronomers used
an interferometer in the U.K. to zoom in on
the region.
They were able to trace out some structure,
but they still didn’t have the resolution
to find the object itself.
Finally, in 1974, two astronomers focused
in on this thing at long last.
They used an interferometer in Green Bank,
West Virginia, along with a telescope in the
nearby town of Huntersville, to create a virtual
telescope with the resolution of a 35 kilometer
dish.
With this, they were able to pick out a single
bright source within the signal, which they
inferred was a black hole.
In the paper they published about their research,
they wrote that this black hole wasn’t just
at the center of our Galaxy, either; it likely
defined the center.
And this discovery was such a big deal that
one of the scientists, Bob Brown, felt inspired
to give the object its very nerdy name, Sagittarius
A*.
It might sound pretty normal, as far as space
names go, but Brown was actually inspired
by atomic physicists.
They use an asterisk to show that an atom
is in an excited state, so he added an asterisk
to Sagittarius A to denote the black hole…
because the discovery was “exciting.”
Which is really kind of cute.
And honestly, it was exciting, but this wasn’t
the end of all this.
Because at the time, scientists barely had
enough resolution to identify that A* was
a black hole.
They couldn’t actually figure out its mass
or radius, which would have proven that beyond
a doubt.
Since 1974, we’ve needed decades of follow-up
research to confirm the discovery and fill
out our picture of this massive object and
its immediate neighborhood.
Understanding our black hole has taken the
work of dozens of scientists around the world,
and in the early 2000s, all those researchers
allowed us to finally get our proof.
In 2002, a team based at UCLA discovered and
tracked the orbits of stars extremely close
to the black hole.
They used information from the stars’ orbits
to determine the mass and radius — and to
show for once and for all that this couldn’t
be anything but a black hole.
After all, it contains the mass of around
4 million suns in a space about as big as
Mercury’s orbit around the Sun.
And as far as we know, no other object could
be that dense.
So in the end, a story that began with static
and phone calls led to one of the biggest
discoveries in astronomy: a supermassive black
hole some 2,6000 light-years away.
But really, this story isn’t over yet.
Looking to the future, increasingly sensitive
telescopes will continue to tell us more about
Sagittarius A star and the area around it.
Observing this object and other black holes
will also help us test Einstein’s ideas
about gravity and the properties of spacetime.
And of course, there’s the fact that we
still haven’t taken a proper picture of
this thing.
In April 2019, scientists shared the first
picture of the edge of an another black hole
in the neighboring galaxy M87.
Now, they aim to do the same for Sagittarius
A*.
And when that happens, we’ll be able to
see the object that scientists have been studying
in the dark for more than 4 decades.
A lot of making big scientific discoveries
involves understanding data.
And if you want to learn more about data science,
check out Coursera.
Coursera offers 100% online learning, and
they’ve partnered with top universities
and industry leaders to offer you more than
3000 courses in everything from astronomy
to computer science.
Right now, they’re also offering the chance
to get a professional certificate in data
science from IBM.
It involves 9 online courses, and when you’re
done, you’ll have some new programming skills,
a certificate to recognize your work, and
something to put on your resume.
If you want to check it out, Coursera is offering
a free month of courses to the first 100 people
who sign up at the link in the description.
The offer will expire at the end of July,
but you’ll still be able to access the courses
long afterward.
[ ♪OUTRO ]
