Astronomers have used the Milky Way’s youngest objects to construct a new map of the galaxy’s spiral arms, and the results are far more confusing than expected.
While a galaxy’s spiral arms look stunning and awe-inspiring, they aren’t much denser with stars than the spaces between them. A typical spiral arm has only about 10% more stars than the average, so the spiral arms are not regions of increased stellar concentration.
Instead, the arms represent places of above-average star formation. The arms are density waves, the formation of which is triggered by gravitational interactions with the satellite galaxies, which travel through a galaxy at their own speed, like ripples in a pond. As the waves travel, they increase the density slightly in that region of space as they pass. When this happens, the gas clouds destabilize and collapse, leading to a new cycle of star formation.
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Those clouds go on to form all kinds of stars, from small ones red dwarfs to the blue giants. Those giant stars don’t last long; a typical star of that size lasts only a few million years. When the spiraling density wave finishes passing through it, all those stars will have disappeared, leaving behind only their smaller and fainter brothers.
So when we look at a galaxy from afar, bright giant stars outshine all others, giving rise to the appearance of spiral arms.
It is relatively easy for astronomers to map the spiral structures of many distant galaxies, especially if they appear in front of us. But creating a detailed map of the arms of our Milky Way is much more difficult.
Because we’re nestled in the same galactic disk as the arms, we have to contend with tens of thousands of light-years of gas and dust, which are very good at blocking many wavelengths of light, to make our observations. And so our maps of the Milky Way they are surprisingly vague and full of conjecture.
Recently, however, a team of astronomers developed a new map of the spiral structure of the Milky Way and reported their findings in a paper published in The Astrophysical Journal in April. To build their map, the astronomers focused on observations of three types of objects: massive star-forming regions, bright young stars and young open clusters. All three of these item types represent new rounds of star formation in their own way, so they should all prove to be relatively reliable tracers of spiral arms.
Massive star-forming regions are dense clouds of gas and dust that are actively forming giant stars. The interaction of radiation from stars with gas in the clouds generates naturally occurring “masers”, microwave lasers. Very long base interferometers, which are networks of telescopes scattered around the world, they have allowed researchers to measure the distances of dozens of these masers across the galaxy.
Because giant stars don’t live long, they don’t have much time to move away from their birthplaces. So where we see them today should be near a spiral arm location. The team used the positions of more than 23,000 young stars of the Gaia Catalog. By using only young stars with high-precision positions, the astronomers were able to ensure that they closely mapped the positions of the spiral arms.
Finally, the team used the locations of known young open clusters. Open clusters are star associations which have formed from the same gas cloud but have not yet had enough time to move away from each other. So these clusters, of which the team used nearly 1,000 samples, should be able to tell us where the spiral arms are.
A new map
Despite the volume of data, the locations of all these young objects don’t even come close to covering the entire Milky Way. Instead, the researchers had to take the most suitable positions of the spiral arms in our vicinity and extend them, reconstructing the full length of the arms from the edge of the galaxy to the core.
They found a total of seven spiral arms. Two of these, the coat of arms of Perseus and that of Norma, dominated the others; a distant observer would see these two arms stand out much more clearly. These arms begin at opposite ends of the Milky Way’s elongated core and wrap around each other, forming a symmetrical S-shaped pattern.
The other five arms – the Carena, Sagittarius, Centaur, External and Local arms – do not extend nearly as far as the two largest. Instead, they begin as bifurcations of main arms or as loose segments of their own, neither of which envelops the galaxy entirely.
From afar, our galaxy would be beautiful, if a little messy. The two main arms would elegantly wrap around each other, while the additional arms would fill in the gaps to create a grand show.
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