The Perseus Molecular Cloud. Image: JPL
Astronomers have discovered a giant spherical void in space that extends for nearly 500 light years and reveals “unprecedented” insights about the births and deaths of stars, a pair of new studies report.The huge cavity is located between two well-studied molecular clouds of gas and dust, called Taurus and Perseus, which are located about 400 and 1,000 light years from Earth, respectively. Given their relative proximity to our solar system, these clouds have been known to scientists for decades as regions where stars are born and nourished, but the discovery of the void between them has resolved a longstanding mystery about their formation that has widespread implications for astrophysics as a whole.
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Observational advances within the past few years have enabled astronomers “to analyze the 3D spatial structure and thicknesses of famous nearby star-forming regions for the first time,” a breakthrough that provides “unprecedented insight into the origins and fates of molecular clouds in the interstellar medium,” according to a study published on Wednesday in The Astrophysical Journal.The cavity between the Taurus and Perseus clouds, which has been named the Per-Tau shell, was spotted thanks to these recent mapping efforts. The discovery “offers the first 3D observational view of a phenomenon long-hypothesized theoretically,” referring to the idea that deaths of stars, called supernovae, trigger the formation of molecular clouds that birth new stars, reports a related study in The Astrophysical Journal Letters that was also published on Wednesday. “This theory that star formation is triggered by the death of previous stars has been around for a very long time,” said Catherine Zucker, a postdoctoral researcher at the Center for Astrophysics | Harvard & Smithsonian who led The Astrophysical Journal study and co-authored the The Astrophysical Journal Letters study, in a call. “It's been seen in simulations where people try to recreate conditions we see in the interstellar medium in the galaxy,” she added. “But we have never actually seen it in observation because we didn't have the 3D spatial view that you can get in simulations until now.”
This observational limitation has been lifted thanks to the European Space Agency’s Gaia spacecraft, which has been building the largest and most detailed map of the Milky Way since its launch in 2013. As Gaia has released new batches of data, Zucker and her colleagues around the world have fine-tuned maps of interstellar dust, exposing the distances and structures of nearby molecular clouds for the first time. In 2020, for instance, scientists at the Max Planck Institute for Astrophysics published a map that estimated cloud distances with just 1 percent uncertainty, an enormous improvement over previous constraints. “That map was amazing because we can essentially map out where clouds are at 1 percent distance uncertainty, whereas pre-Gaia, it was 30 percent uncertainty,” Zucker said. “This field is rapidly changing on the monthly timescale because of Gaia and because of these new techniques that are being applied to the Gaia data to create these new pretty pictures of our Milky Way.”These maps proved to be especially useful for entangling the mysterious origins of the Perseus and Taurus clouds, which previously had only been observed in 2D. For years, scientists have wondered if these clouds formed independently or if they are somehow related, but it has been difficult to resolve these questions without a 3D image, especially since Taurus is located in front of Perseus in our line-of-sight from Earth.
The spectacular accuracy of the dust maps enabled Zucker and her colleagues to spot the huge Per-Tau shell that separates the clouds, which strongly implies that both molecular structures were created by a supernova, or many supernovae, about 10 million years ago. This explosive event hollowed out the central void by ejecting stellar material into the surrounding medium. Over time, it cooled into the Perseus and Taurus clouds, which have now started the cycle over again as sites of active star formation. “We were able to essentially place these two clouds in their larger environment,” Zucker said. “That’s how we discovered the shell, and that these star-forming regions are probably formed due to the death of a previous star.”Alyssa Goodman, a Harvard astronomer and a co-author on both studies, also created 3D renderings of these maps in augmented reality so that anyone can interact with the new findings.With the pair of groundbreaking studies published, scientists are already planning to chase a new set of questions. For instance, Zucker and her colleagues hope that they will be able to use the next Gaia release, due in 2022, to track down the star cluster that contained the now-dead progenitor of the Per-Tau shell. Since the spacecraft tracks the positions and movements of stars across space, the team will search for a stellar gaggle that looks like it might have been at the right place and time to create the bubble that now lies between the Taurus and Perseus clouds.Zucker also looks forward to mapping out the shadowy interiors of other nearby molecular clouds and interstellar structures that could contain more insights about the life cycle of stars. “We think, and we're actually exploring, other examples of star formation at the edge of these shells,” she said. “There are other famous regions of the galaxy where we can look at that, but what Per-Tau gives us is an archetype, or an example, of what the 3D morphology and motion of the shells might look like. It gives us a good framework that we can then look for similar regions nearby, and we can use that as a way to transfer knowledge to other regions.” “This is definitely happening across the galaxy and there's probably many more examples in our neighborhood that we can find based on this one example,” Zucker concluded.