New Feature Discovered in One of the Milky Way’s Spiral Arms
Shown here (from left) 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. Credit: NASA/JPL-Caltech
Scientists have spotted a previously unrecognized feature of our Milky Way galaxy: A contingent of young stars and star-forming gas clouds is sticking out of one of the Milky Way’s spiral arms like a splinter poking out from a plank of wood. The structure stretches some 3,000 light-years. What makes it special is its orientation which is dramatically different from the arm’s. The newly discovered feature offers insight into the large-scale structure of our galaxy.
Distances are among the most challenging things to measure in astronomy. Only recently, direct distance measurements from Gaia (Gaia is a mission to chart a three-dimensional map of our galaxy, in the process revealing the composition, formation, and evolution of the galaxy, ESA) made the geometry of this new structure apparent. It is hard to have an exact idea of the size and shape of the Milky Way´s arms because Earth is inside the Milky Way. It is like reading the label when you are inside the jar. Or, as NASA puts it, it’s akin to standing in the middle of Times Square and trying to draw a map of the island of Manhattan. Could you measure distances precisely enough to know if two buildings were on the same block or a few streets apart? And how could you hope to see all the way to the tip of the island with so many things in your way? (Astronomers Find a ‘Break’ in One of the Milky Way’s Spiral Arms, 2021)
Scientists studied the Sagittarius Arm, a portion of one of the galaxy´s arms, and sought out newborn stars nestled in the gas and dust clouds (called nebulae) where they form. They did this with data from Spitzer, a space telescope (NASA) that detects infrared light that can penetrate those clouds. 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 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. (Kuhn, et al., 2021)
A contingent of stars and star-forming clouds was found jutting out from the Milky Way’s Sagittarius Arm. The inset shows the size of the structure and distance from the Sun. Each orange star shape indicates star-forming regions that may contain anywhere from dozens to thousands of stars.
A contingent of stars and star-forming clouds was found jutting out from the Milky Way’s Sagittarius Arm. The inset shows the size of the structure and distance from the Sun. Each orange star shape indicates star-forming regions that may contain anywhere from dozens to thousands of stars. Image Credit: NASA/JPL-Caltech
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 with a pitch angle of about 12 degrees, but the structure that was discovered stands out at an angle of nearly 60 degrees. (Kuhn, et al., 2021)
Kuhn, et al., 2021
The newly discovered feature contains four nebulae known for their breathtaking beauty (see the main picture): the Eagle Nebula (which includes the Pillars of Creation), the Omega Nebula, the Triffid Nebula, and the Lagoon Nebula.
Astronomers don’t yet fully understand what causes spiral arms to form in galaxies like ours. Even though we can’t see the Milky Way’s full structure, the ability to measure the motion of individual stars is useful for understanding this phenomenon. There are still 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. This newly discovered structure is a small piece of the Milky Way, but it could tell us something significant about the galaxy as a whole. (Kuhn, et al., 2021)
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References
Astronomers Find a ‘Break’ in One of the Milky Way’s Spiral Arms. (2021, August 17). Retrieved from NASA Jet Propulsion Laboratory: https://www.jpl.nasa.gov/news/astronomers-find-a-break-in-one-of-the-milky-ways-spiral-arms?utm_source=iContact&utm_medium=email&utm_campaign=nasajpl&utm_content=gaia20210817-1
Kuhn, M., Benjamin, R., Zucker, C., Krone-Martins, A., de Souza, R., Castro-Ginard, A., . . . Hillenbrand, L. (2021, July 21). A high pitch angle structure in the Sagittarius Arm. Astronomy & Astrophysics, 651(L10). doi:10.1051/0004–6361/202141198
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