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What does the Milky Way galaxy look like from the outside? Currently, we can’t really know because we’re inside it. Even if we were to send out a probe to take a photo from the outside, with our current technology, it would take a period many times longer than the history of human civilization to get to a place where it could get a good shot. However, that’s not stopping astronomers from trying to answer that question by attempting to account of as many stars and gas clouds as they can observe, assigning their positions and motions in three dimensions, and analyzing their spectra to determine what elements are present, which provide clues to their ages. With this information, not only will it be possible to observe the constructed model of the Milky Way from the outside, it will be possible to learn about the history of our galaxy as well. Recent speed-based observations have been systematically collecting data on stars in the Milky Way, to the tune of nearly 2 billion stars. That’s still just a small fraction of the estimated 100 billion stars in the galaxy, but it’s a reasonable start.
For a long time, astronomers and astrophysicists has assumed that the Milky Way was in what they called “static equilibrium,” where beyond the rotation of the galaxy, and star-birthing and supernovae and the like, were all happening within the galaxy itself. Recent observations have shown that this point of view is completely wrong, with the Milky Way interacting with dwarf galaxies and globular clusters orbiting it, mostly stealing their newborn stars.
The parts of a spiral galaxy consist of the central bulge, the disk (where the arms are located), and the halo, which is a roughly spherical region centered on the bulge, but which extends to perhaps ¾ of the way to the disk’s perimeter. The halo is quite faint, as far as light is concerned, so it has not been studied much until recently. It turns out that the halo contains streams of new stars, born in either a nearby dwarf galaxy or a globular cluster, that are fated to become incorporated into the Milky Way. The fact that the stars within a particular stream are traveling together and have the same spectra signatures suggests that they were all created in the same environment. As of 2021, 60 star streams in the halo had been identified.
There is another set of stars in the halo that are rotating in the direction opposite to the principle direction of rotation of the galaxy, and so that needed to be explained. It was determined that these were stars that were born within the Milky Way, but were “splashed” out after the early Milky Way collided with a second galaxy between 8 and 10 billion years ago. Eventually, the colliding galaxy was incorporated into the Milky Way, but these “splash” stars serve as evidence that this collision took place.
Then there are the gas clouds, which are where stars are formed. Clouds are much harder to observe, or determine their shapes or locations. Nonetheless, it was determined that the clouds form long, threadlike structures throughout the galaxy’s spiral arms. (At his point, we don’t even know how many arms our galaxy has.)
A picture of the Milky Way as a complex and dynamic body interacting with the collections of stars in its gravitational hold is rapidly evolving as astronomers continue to learn more about what’s out there.
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