Credit: NASA/JPL-Caltech

These four nebulae (star-forming clouds of gas and dust) are 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. In a new study, astronomers have shown that these nebulae are part of a substructure within the arm that is angled differently from the rest of the arm.

A key property of spiral arms is how tightly they wind around a galaxy. 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 protruding structure has a pitch angle of nearly 60 degrees.

Similar structures – sometimes called spurs or feathers – are commonly found jutting out of 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.