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Discovered a Giant, Empty Cavity Lurking in Space

It may not appear so to us, but the space between the stars isn’t completely empty. Tenuous and not-so-tenuous clouds of gas and dust drift within the darkness.

A region of space some 700 light-years away could be a fascinating exception. There, among the constellations of Perseus and Taurus, astronomers have found an outsized, spherical void over 500 light-years in diameter. Around its perimeter are the Perseus and Taurus molecular clouds – dense clouds of cold gas and dust where stars form.

It’s called the Per-Tau Shell, and it seems to be the product of at least one giant supernova explosion many years ago. It’s likely that this phenomenon compressed and triggered star formation within the two molecular clouds.

“Hundreds of stars are forming or exist already at the surface of this giant bubble,” said theoretical astrophysicist Shmuel Bialy of the Harvard-Smithsonian Center for Astrophysics (CfA).

“We have two theories – either one supernova went off at the core of this bubble and pushed gas outward forming what we now call the ‘Perseus-Taurus Supershell’, or a series of supernovae occurring over many years created it over time.”

Mapping things in space is a tricky prospect. In two dimensions, it’s pretty straightforward, but the dimension – depth – takes a bit more work. we have a number of the way of doing so, but there are gaps in our knowledge, and lots of uncertainties remain.

To explore the Perseus and Taurus molecular clouds, the researchers used data from Gaia, the ECU Space Agency’s satellite observatory, which has been working since 2013 to map the Milky Way galaxy in three dimensions with the foremost detail and highest precision achievable. It’s one of the foremost powerful tools we’ve for helping us understand the architecture – and therefore the history – of our home galaxy.

These data were analyzed using visualization software called glue, which allows scientists to create interactive 3D visualizations. From this, astronomers were ready to construct 3D maps of the gas in these and other molecular clouds, appearing during a separate paper.

Side-on view of the Per-Tau Shell. (Bialy et al., ApJL, 2021)

“We’ve been ready to see these clouds for many years, but we never knew their true shape, depth, or thickness. We also were unsure how distant the clouds were,” said astronomer Catherine Zucker, also of Harvard-Smithsonian CfA.

“Now we know where they roll in the hay only one percent uncertainty, allowing us to discern this void between them.”

According to Bialy’s team’s analysis, the almost spherical void is probably going the result of a powerful supernova explosion, sending a shockwave out in all directions into interstellar space. As this shockwave expands, it pushes into and compresses the material within the interstellar medium, sweeping it up to make a spherical shell.

This also reveals how molecular clouds are often triggered into star formation, the scientists said.

“There are many different theories for how gas rearranges itself to form stars,” Zucker said.

“Astronomers have tested these theoretical ideas using simulations in the past, but this is often the first time we will use real – not simulated – 3D views to match theory to observation, and evaluate which theories work best.”

Star formation is assumed to occur when a denser region in a molecular cloud collapses, spinning, under its own gravity. When the shockwave from a supernova expands into the space around it, it can sweep up the gas within the interstellar space to form molecular clouds with dense regions that then start forming stars.

This is what the team thinks happened with the Per-Tau Shell. Between 6 and 22 million years ago, their reconstruction suggests, multiple supernova events carved out a cavity within the interstellar medium. This created both the shell and therefore the Perseus and Taurus molecular clouds. Currently, the bubble does not seem to be expanding – it serves, now, as a monument to the cosmic circle of life.

“This demonstrates that when a star dies, its supernova generates a chain of events which will ultimately lead to the birth of new stars,” Bialy said.

An interactive 3D model of the Per-Tau shell are often explored on the Harvard website. The team’s paper has been published within the Astrophysical Journal Letters.

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