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.

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The offer will expire at the end of July, but you'll still be able to access the courses long afterward. [ ♪OUTRO ].