Recent supernovae produced giant voids in the Orion Nebula

The Orion Nebula is a well-known feature in the night sky and can be seen in small backyard telescopes. Orion is a busy place. The region is known for active star formation and other phenomena. It is one of the most studied features in the sky, and astronomers have observed all kinds of activity there: planets forming in protoplanetary disks, stars starting their lives from fusion in collapsing molecular clouds, and the photoevaporation power of massive hot stars as they carve openings in clouds of interstellar gas.

But supernova explosions also leave their mark on the Orion Nebula. New research says supernova explosions in recent astronomical history are responsible for a mysterious feature first formally identified in the night sky in the late 1800s. It’s called Barnard’s Loop, and it’s a giant loop of hot gas about 300 light-years across.

In 1894, the American astronomer and astrophotographer EE Barnard published his sightings of Orion in popular astronomy. Remarkably, he was experimenting with the lens of a “cheap, oil-projecting lantern” – in his words – that he was using at the Lick Observatory. In his article he explained what he had found. “On my drawing, I’ve marked some of the haze, from one degree to two degrees east of Tau, with dots, because it’s so faint at this point I can’t be sure.” He added, “It’s brightest near 56 and 60 Orionis.” (Barnard also acknowledged that he wasn’t the first to see it.)

EE Barnard's drawing of the Orion Nebula from 1894. Barnard's Loop is clearly visible just below Orionis 60 and 56. At the time, brighter stars were marked with larger circles, and Betelgeuse and Rigel are clearly marked as the brightest stars in the constellation Orion.  Image Credit: EE Barnard, Popular Astronomy.
EE Barnard’s drawing of the Orion Nebula from 1894. Barnard’s Loop is clearly visible just below Orionis 60 and 56. At the time, brighter stars were marked with larger circles, and Betelgeuse and Rigel are clearly marked as the brightest stars in the constellation Orion. Image Credit: EE Barnard, Popular Astronomy.

In our more modern times, we just turn our browsers and beautiful images of Barnard’s Loop quickly appear on our screens.

This is a Hubble image of the emission nebula Barnard's Loop in Orion.  Image Credit: NASA, ESA, Hubble
This is a Hubble image of the emission nebula Barnard’s Loop in Orion. Image Credit: NASA, ESA, Hubble.

Astronomers’ understanding of Barnard’s Loop has increased, along with our powers of observation and our growing knowledge of everything in nature.

We know it’s an emission nebula made of ionized gases. The gases emit light at different wavelengths, although red alpha hydrogen is dominant. We know that the nearby stars in the Orion Nebula drive the ionization of the gases and their emissions. One thing astronomers aren’t sure about is what caused the loop of gases in the first place.

The most likely cause is a supernova explosion. A new paper backs that up, saying that sometime in the past four million years, at least one supernova carved out features in the region.

The paper is “A 3D Rendering of Orion: I. Barnard’s Loopand it is available online at the prepress site Authorea. Lead author is Michael Foley, a graduate student at the Center for Astrophysics | Harvard and Smithsonian. The team presented their results at a press conference hosted by the American Astronomical Society.

This image of the Orion Nebula shows the enigmatic Barnard's Loop feature, a structure made of hot ionized gas that was formally identified in 1898. Orion A, Orion B, and Orion Lam are giant clouds of gas where stars are born, although they are invisible to the naked eye. eye.  The white dots in the center of the image are a group of connected stars, all moving outward from a point in their center.  The blue star marker is the center of flux for Barnard's loop and the orange star is the center of expansion for the OBP-B1 stars.  The authors think that one or more stars in this group exploded as supernovae, creating Barnard's Loop, while also triggering active star formation.  Image credit: Michael Foley
This image of the Orion Nebula shows the enigmatic Barnard’s Loop feature, a structure made of hot ionized gas that was formally identified in 1898. Orion A, Orion B, and Orion Lam are giant clouds of gas where stars are born, although they are invisible to the naked eye. eye. The white dots in the center of the image are a group of connected stars, all moving outward from a point in their center. The blue star marker is the center of flux for Barnard’s loop and the orange star is the center of expansion for the OBP-B1 stars. The authors think that one or more stars in this group exploded as supernovae, creating Barnard’s Loop, while also triggering active star formation. The yellow dust ring is associated with the Orion Eridanus superb† Image credit: Michael Foley

The ESA’s Gaia spacecraft played a role in this investigation. Gaia is an ambitious project to create a 3D map of the Milky Way galaxy, and the project has just released its third dataset. This research team is known for creating interactive figures that help bring their research to life, and when they combined existing 2D data with Gaia’s 3D data, the team was able to map more of the busy Orion region in their interactive tool.

“Our first large-scale 3D look at Orion tells us so much,” says lead author Michael Foley† “Prior to this work, most of Orion’s studies were limited to two dimensions: top to bottom and left to right in the sky. By adding the third dimension – distance – we can map all kinds of interesting structures, such as huge cavities of gas and dust or star clusters with very interesting movements. Combining the information from the interstellar gas and the stars, we believe that some supernovae created cavities over the past few million years.”

“Orion has had a pretty exciting history,” he added.

The team's interactive tool allows users to rotate and manipulate the image to show the positions and velocities of various objects.  Image credit: Foley et al. 2022.
The team’s interactive tool allows users to rotate and manipulate the image to display the positions and velocities of various objects. Image credit: Foley et al. 2022.

the teams interactive visual tool is based on visualization software called Glue. It shows a star cluster moving outward from a point close to the center of Barnard’s Loop. They think one or more supernova explosions created the Loop and pushed the stars out. The position and velocity data show that this happened sometime in the past four million years.

Another clue lies in the formation of new stars at the edges of the Barnard’s Loop cavity. Supernovae generate powerful shock waves that compress gas and can cause star formation.

Their results do not stand alone. Other research, including previous research by the same team, shows that supernovae are responsible for carving out other bubbles in the interstellar medium and causing widespread star formation. This applies to the Per-Tau Gas Casing which is surrounded by the molecular clouds of Perseus and Taurus. And the more astronomers use advanced data like Gaia’s, the more they’ll find these gas bubbles with pronounced star formation at their edges, said study co-author Alyssa Goodman.

“It seems clear that we will see a ‘Swiss cheese’ image of the interstellar medium, with stars forming at the edge of the holes as we map more and more of the galaxy,” said Alyssa Goodman, a professor at Harvard, CfA astronomer and co-author of the study.

“We believe that shells and loops in Orion, the Per-Tau shell and the Local bubble are the first of many discoveries linking new star formation to old supernovae, Foley says. “Supernovae sweep gas and dust into dense clumps, creating the perfect birthplaces for new stars. The Orion region, rich in both star formation and supernovae, is the latest example.”

An artistic take on the hot local bubble. Image Credit: NASA

Thanks to the ESA’s Gaia mission and powerful computing resources in the hands of astronomers and data visualization experts, we are getting more advanced and detailed 3D images of the Universe. The Orion Nebula and its region is a great place to start because there is so much happening there: stars are born and some die in catastrophic explosions leading to more star births. But a 3D Orion is just the beginning for Foley, whose enthusiasm overflows.

“Thanks to the work of many incredible scientists, 3D data will transform our understanding of star formation in our galaxy,” Foley notes. “It could be a lot more explosive than we can even imagine!”

This new paper is the first in a series by this group of researchers. Next up will be supernovae present in the greater Orion region. Expect more Swiss cheese.

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