Astronomers recently discovered a previously unrecognized spur of young stars and star-forming gas clouds sticking out of the Milky Way’s Sagittarius spiral arm –one of the most striking arms in our galaxy, noted for its young stars and beautiful nebula that connects to the major Orion arm that harbors our solar system. “Spiral arms in galaxies like the Milky Way typically form from long-lived spiral density waves that periodically cause a bunching up of stars and clouds in a regular symmetric pattern, like a pinwheel,” wrote astronomer Debra Elmegreen, president of the International Astronomical Union, in an email to The Daily Galaxy.
“Gravitational instabilities in the clouds lead to new star formation,” Elmegreen continued, “and since most of the gas mass is in the spiral pattern, that’s where we see new stars preferentially forming. As these stars and gas move through the wave crest, other stars and gas move in. Spurs are common in spiral galaxies and can form when the material moves into the interarm region and experiences a local shear that causes an instability or twist.”
First Dramatically Different Structure Identified
Astronomers have a rough idea of the size and shape of the Milky Way’s arms, but much remains unknown: They can’t see the full structure of our home galaxy because Earth is inside it. “In a sense, the galaxy hardest for us to see is our own,” wrote science-fiction icon, Isaac Asimov. “For one thing, we are imprisoned within it, while the others can be viewed as a whole from outside. Furthermore, we are far out from the center, and to make matters worse, we lie in a spiral arm clogged with dust. In other words, we are on a low roof on the outskirts of the city on a foggy day.”
Stretching some 3,000 light-years, the newly discovered spur is the first major structure identified with an orientation so dramatically different than the arm’s, reports the Jet Propulsion Laboratory (JPL).
Astronomer Thomas Bania with the Boston University Institute for Astrophysical Research told The Daily Galaxy: “How spiral arms form in galaxies like our Milky Way is still poorly understood. A myriad of processes contribute, including density waves – compression waves propagating across the galaxian disks, gravitational interactions with other, nearby large galaxies, and star formation itself which can stimulate the gravitational collapse of nearby clouds of gas and dust into new stellar generations.”
Focus on the Sagittarius Arm
To learn more, the authors of the new study focused on a nearby portion of one of the galaxy’s arms, called the Sagittarius Arm. Using NASA’s Spitzer Space Telescope prior to its retirement in January 2020, they sought out newborn stars, nestled in the gas and dust clouds (called nebulae) where they form. Spitzer detects infrared light that can penetrate those clouds, while visible light (the kind human eyes can see) is blocked.
Young stars and nebulae are thought to align closely with the shape of the arms they reside in. To get a 3D view of the arm segment, the scientists used the latest data release from the ESA (European Space Agency) Gaia mission to measure the precise distances to the stars. The combined data revealed that the long, thin structure associated with the Sagittarius Arm is made of young stars moving at nearly the same velocity and in the same direction through space.
Increasing Pitch Angle
“A key property of spiral arms is how tightly they wind around a galaxy,” said Michael Kuhn, an astrophysicist at Caltech and lead author of the new paper. This characteristic is measured by the arm’s pitch angle. A circle has a pitch angle of 0 degrees, and as the spiral becomes more open, the pitch angle increases. “Most models of the Milky Way suggest that the Sagittarius Arm forms a spiral that has a pitch angle of about 12 degrees, but the structure we examined really stands out at an angle of nearly 60 degrees.”
“This structure,” Kuhn told The Daily Galaxy, “has many similarities to mass condensations that appear in simulations of spiral galaxies. In these simulations, mass condensations can form due to gravitational instabilities within spiral arms, and these are sometimes stretched to extend from the arm to the inter-arm regions by the shear produced by galactic rotation. So, it’s possible that the structure we observe could either be a mass condensation within the Sagittarius arm or part of a spur extending out of an arm.”
Similar structures – sometimes called spurs or feathers – are commonly found jutting off the arms of other spiral galaxies. For decades scientists have wondered whether our Milky Way’s spiral arms are also dotted with these structures or if they are relatively smooth.
Sagittarius Spur Harbors Four Nebula
The newly discovered feature contains four nebulae known for their breathtaking beauty: the Eagle Nebula (which contains the Pillars of Creation), the Omega Nebula, the Trifid Nebula, and the Lagoon Nebula. In the 1950s, a team of astronomers made rough distance measurements to some of the stars in these nebulae and were able to infer the existence of the Sagittarius Arm. Their work provided some of the first evidence of our galaxy’s spiral structure.
Shown here are the Eagle, Omega, Triffid, and Lagoon Nebulae, imaged by NASA’s infrared Spitzer Space Telescope. These nebulae are part of a structure within the Milky Way’s Sagittarius Arm that is poking out from the arm at a dramatic angle. (NASA/JPL-Caltech)
“Distances are among the most difficult things to measure in astronomy,” said co-author Alberto Krone-Martins, an astrophysicist and lecturer in informatics at the University of California, Irvine and a member of the Gaia Data Processing and Analysis Consortium (DPAC). “It is only the recent, direct distance measurements from Gaia that make the geometry of this new structure so apparent.”
In the new study, researchers also relied on a catalog of more than a hundred thousand newborn stars discovered by Spitzer in a survey of the galaxy called the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE).
“When we put the Gaia and Spitzer data together and finally see this detailed, three-dimensionalmap, we can see that there’s quite a bit of complexity in this region that just hasn’t been apparent before,” said Kuhn.
Uncertainties About Large-scale Structure of the Milky Way
The stars in the newly discovered structure likely formed around the same time, in the same general area, and were uniquely influenced by the forces acting within the galaxy, including gravity and shear due to the galaxy’s rotation.
“Ultimately, this is a reminder that there are many uncertainties about the large-scale structure of the Milky Way, and we need to look at the details if we want to understand that bigger picture,” said one the paper’s co-authors, Robert Benjamin, an astrophysicist at the University of Wisconsin-Whitewater and a principal investigator on the GLIMPSE survey. “This structure is a small piece of the Milky Way, but it could tell us something significant about the Galaxy as a whole.”