The Milky Method broke one in all its arms

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

One of the ways 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 position and movement of more than a billion stars. Gaia has already revealed details in the structure of the Milky Way, such as a wave pattern between some stars.

The Eagle, Omega, Triffid, and Lagoon Nebulae, captured by NASA’s Spitzer Infrared Space Telescope. Photo credit: NASA / JPL-Caltech

Another method is to look at certain objects in our galaxy, such as star formation nebulae. Star-forming nebulae tend to be in the spiral arms of a galaxy, where there is the most gas and dust. The Spitzer Infrared Space Telescope measured the distances to young stars in many nebulae, which helped us confirm that the Milky Way has four main spiral alarms.

A new study combines data from Gaia and Spitzer and compares the position of some nebulae with the total spiral distribution of the stars.[^1] The study focused on a main spiral alarm within the galaxy known as the Sagittarius Arm. It is the spiral arm straight inward from 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 a spiral arm is wrapped. The larger the pitch angle, the more open the spiral arms are. With the Sagittarius arm, the pitch angle is about 12 degrees. But the angles of inclination of some nebulae are very different.

Astronomers have found a break in our galaxy’s archery arm. Photo credit: NASA / JPL-Caltech

The team looked at 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 in the same general region and were used in the 1950s to confirm the existence of the archer arm. This new study determined the location of these nebulae and other stars and found the region to be tilted at 60 degrees.

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

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

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