For the first time ever, astronomers have pinpointed the location of a luminous light source on the opposite side of the Milky Way Galaxy, far beyond the galactic center. The source — a region of space where massive stars are being born — is located in a distant spiral arm, one of the large tentacles of gas that swirl around the middle of our galaxy. Knowing its location has allowed astronomers to trace the arm as it wraps around the center of the Milky Way, telling us more about the structure of the galaxy we live in.

It’s a significant discovery, since locating distant objects in our galaxy is an incredibly difficult process. The Milky Way is filled with interstellar dust that makes it nearly impossible to see any visible light coming from faraway sources. And our galaxy is incredibly big, stretching 100,000 light-years across. That means it takes a thousand centuries for light to cross from one end of the Milky Way to the other. Any radio waves coming from remote locations across the galaxy weaken considerably as they cross the vast distances on the way to Earth.

locating distant objects in our galaxy is an incredibly difficult process

That’s why astronomers use special measurement techniques to figure out where things are in our galaxy. To find this specific star-forming region, scientists leveraged the Earth’s orbit around the Sun, observing the source’s radio waves from different vantage points as the Earth travels through the Solar System. Such a technique can help astronomers accurately measure the distance of a far-off object — it’s been used to do so many times before — but a galactic object this far away has never been measured before. “This is certainly the first source we’ve ever measured a distance that far by a factor of two,” Mark Reid, a senior radio astronomer at Harvard and author of a study in Science detailing this discovery, tells The Verge. “So it’s twice as far away as the previous record holder.”

Reid and his team weren’t looking for this light source in particular; they found it as part of an ongoing mapping campaign of the Milky Way. Over the last five years, the team has been measuring the distances of star-forming regions all over the galaxy to learn more about the structure of our cosmic neighborhood. And they’ve located up to 200 sources so far. The team has been looking specifically for these regions — dense clouds of gas and dust that form new stars — because such places are known to pop up in the arms of spiral galaxies. It’s thought that gases within the arms bump up against each and other and become so compressed that they give birth to new stars.

“Imagine you’re trying to make a map of a city, but you’re not allowed to leave home.”

But so far, all of the regions that the team has mapped have been in the general vicinity of our Solar System. They hadn’t found any sources far beyond the galactic center — the supermassive black hole at the middle of the Milky Way. That’s because it’s fairly difficult to map the galaxy from Earth. “Imagine you’re trying to make a map of a city, but you’re not allowed to leave home,” says Richard Pogge, an astronomer at Ohio State University, who wasn’t involved in the study, tells The Verge. “You go and look at distant lights and try to map out where the population of the city is. But up until now we’ve only seen downtown and mapped out a few of the suburbs.”

For their mapping campaign, the astronomers have relied on a telescope known as the Very Long Baseline Array, run by the National Radio Astronomy Observatory. The array consists of 10 big radio telescopes located across parts of the Northern Hemisphere, from Hawaii to New England. The team has been using these telescopes to pick up emissions of water vapor and methanol from distant sources. Regions where stars form create a lot of these gases, which give off incredibly strong radio waves that we can observe from Earth.

But the key to really locating these regions is to observe their emissions at different times of the year — that way you see the sources from different points in space. It’s a technique known as trigonometric parallax. A great way to visualize it is by holding out your thumb in front of your face. If you alternate closing each of your eyes, your thumb will appear to change position. That’s because of the space between your two eyes; each one is viewing your thumb from a different distance.

The astronomers harness this idea but at a much bigger scale. They observe a source at one point of the year, and then wait six months before studying it again. By that time, the Earth has completed half of its trip around the Sun, allowing astronomers to view the source from two very distant vantage points. “It’s like your left eye is on one side of the Sun and your right eye is on the other side in the Earth’s orbit,” says Reid. As a result, the source appears to shift position ever so slightly in the sky, and astronomers can use this small change to accurately calculate how far away it is.

For this particular source, Reid and his team observed it in the spring and fall seasons between 2014 and 2015. The apparent change in distance was extremely tiny, shifting just 49 microarcseconds in the sky. “That’s about the size of a baseball put on the Moon and viewed from here,” says Reid. Then, they calculated the extreme distance of the source, placing the region more than 66,500 light-years away. Previously the farthest region measured with this technique was nearly 35,900 light-years away — also calculated by Reid’s team. “This is totally unexplored territory,” says Pogge. “When a technique makes a leap by a factor of 2, you say, ‘Whoa that’s big.’”

“This is totally unexplored territory.”

The astronomers were able to place the light source in what is known as the Scutum-Centaurus spiral arm. “Because of this source we’re confident we can trace this arm almost all the way around the Milky Way, almost one full revolution,” says Reid. That helps to flesh out the map we’re making of the galaxy’s structure.

But to fully understand its shape, the team has a lot more work to do. There’s a whole portion of the Milky Way that can only be mapped from the Southern Hemisphere of Earth, and the team’s campaign has only been done in the Northern Hemisphere so far. Reid says the astronomers are hoping to work with telescopes from the University of Tasmania in Australia in the coming years. That way, they’ll have an even more comprehensive map and maybe find even more faraway sources. “You do this long enough, and every so often you get lucky,” says Reid. “You get a really interesting one.”