The Milky Way has broken one of its arms

The Milky Way galaxy is our home, and yet in some ways it is the least understood galaxy. One of the biggest challenges for astronomers is understanding its large-scale structure. Because we’re in the middle of it all, mapping our galaxy is a bit like trying to map the size and shape of a woodland park while standing in the middle.

One of the ways that astronomers can map our galaxy is by measuring the position and distance of thousands and thousands of stars. This is one of the main objectives of the Gaia mission, which studies the location and movement of more than a billion stars. Gaia has previously revealed details in the structure of the Milky Way, such as a pattern of waves among some stars.

Another method is to look at specific objects in our galaxy, such as star-forming nebulae. Star-forming nebulae tend to be located in the spiral arms of a galaxy, where there is the most gas and dust. The Spitzer infrared space telescope has measured distances to young stars in many nebulae, allowing us to confirm that the Milky Way has four main spiral arms.

A new study combines data from Gaia and Spitzer, comparing the location of some nebulae with the global spiral distribution of stars.[^1] The study focused on a main spiral arm in the galaxy known as the Sagittarius arm. It is the spiral arm just inside Orion’s solar arm. The team hoped to measure an aspect of the spiral arm known as the pitch angle. It tells you how tightly coiled a spiral arm is. The greater the tilt angle, the more open the helical arms. In the case of the Sagittarius arm, the pitch angle is around 12 degrees. But the pitch angle of some nebulae is very different.

The team examined four prominent nebulae in our night sky: the Eagle Nebula (which contains the Pillars of Creation), the Omega Nebula, the Trifid Nebula, and the Lagoon Nebula. These four nebulae are found in the same general region and were used in the 1950s to confirm the existence of the Sagittarius arm. This new study pinpointed the location of these nebulae and other stars and found that the region has a pitch angle of 60 degrees.

This doesn’t mean our original Sagittarius arm measurement is wrong, but it does indicate a type of structure known as galactic spurs. Some spiral galaxies have very smooth spiral arms, where the regions of gas, dust, and star formation all lie along the same curve. Other spiral galaxies have more broken spiral arms, with small, feathery branches called spurs. We don’t know for sure what type of galaxy the Milky Way is, but this new study indicates that it is the latter.

Reference: Kuhn, MA, et al. “A high step angle structure in the Sagittarius arm.” Astronomy & Astrophysics 651 (2021): L10.