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Astronomers uncover a massive 'cavity' in the Milky Way

Across the beautiful Milky Way, two clouds of gas appear side by side. These enormous provinces of star-forming gas, known as "molecular clusters," stretch across the sky, appearing to form a bridge between the Taurus and Perseus constellations, where new suns will be able to grow and thrive for billions of years to come.

It's a celestial love story about star-crossed lovers — and, according to new research; it's also a massive optical illusion that's been created.

Using data from the European Space Agency's Gaia space observatory, new 3D maps of the region reveal that these scudding clouds are actually hundreds of light-years apart, separated by an enormous, empty orb where neither gas nor dust nor stars can find a place to take root.

According to a study published on Sept. 22 in the Astrophysical Journal Letters, this newly discovered chasm spans approximately 500 light-years in width, or roughly 115 times the distance between Earth and the nearest alien sun, Proxima Centauri. Even though hundreds of young stars have already formed around the bubble's periphery, the large, spherical void within the bubble points to one obvious cause, according to the authors: a catastrophic supernova explosion.

"We believe that either a single supernova exploded at the center of this bubble, pushing gas outward to form what we now call the 'Perseus-Taurus Supershell,' or a series of supernovae that occurred over millions of years created it over time," said lead study author Shmuel Bialy, a postdoctoral researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Astronomers have known about the molecular clouds in the constellations Taurus and Perseus for decades, but all previous research has been based on two-dimensional observations only. Following up on their findings with data from Gaia, the study's authors developed a new technique for mapping dust in distant corners of the galaxy in three dimensions. (The authors go into greater detail about their methods in a second study, which was published on September 22 in The Astrophysical Journal.)

When the researchers mapped the seemingly connected clouds of gas, they discovered that there was no physical connection between them, but that they instead resided on opposite sides of an invisible, empty space. They discovered that the long filament of gas that appeared to connect them was actually just a "coincidental projection" that was created on the closer Taurus side of the bubble and only appeared to connect with the farther Perseus side, according to the research team's findings.

In light of the molecular clouds' relative positions in space and the ages of the stars contained within them, the researchers hypothesized that both clouds formed as a result of the same supernova explosion approximately ten to twenty million years ago. Stars like our sun run out of fuel, shed their outer layers of hot gas, and then collapse under their own gravity, resulting in explosions like this. This sudden collapse generates a powerful shockwave, which pushes all of the leftover gas and dust far away from the ramshackle remains of the ex-shattered star's body.

According to the researchers, in this instance, two large blobs of gas appear to have congregated on opposite sides of the shockwave, where each one began to condense and eventually form new stars.

In Bialy's words, "this demonstrates that when a star dies, its supernova causes a chain of events that may eventually result in the birth of new stars."

After all, there is a happy ending to this tale of star-crossed clusters. However, the researchers believe that the new mapping technique itself is the more positive takeaway from the study. According to the authors, this study marks the first time that molecular clouds have been imaged in three dimensions, and it opens the door to many potential discoveries about the way gas rearranges itself to form stars throughout the galaxy.

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