The different stellar components of the Milky Way are believed to have formed at different times with star formation still occuring in the thin disk , and evidence for all three of the currently recognised galaxy formation mechanisms primordial collapse, hierarchical clustering and secular evolution is present. Like most galaxies, the Milky Way does not exist in isolation.
Along with the Andromeda galaxy , the Large and Small Magellanic Clouds and approximately 30 dwarf galaxies , it forms part of the Local Group. The Milky Way is currently undergoing minor mergers with at least two of the Local Group dwarf galaxies — the Sagittarius and Canis Major dwarfs — and resides in the periphery of the great Virgo cluster with which it will also one day merge. The bright glow of the Milky Way stretching across the night sky is one of the greatest sights afforded us. There are several clues.
The first clue to the shape of the Milky Way comes from the bright band of stars that stretches across the sky and, as mentioned above, is how the Milky Way got its name.
This band of stars can be seen with the naked eye in places with dark night skies. That band comes from seeing the disk of stars that forms the Milky Way from inside the disk, and tells us that our galaxy is basically flat.
The concentration of stars in a band adds to the evidence that the Milky Way is a spiral galaxy. If we lived in an elliptical galaxy, we would see the stars of our galaxy spread out all around the sky, not in a single band. Another clue comes when astronomers map young, bright stars and clouds of ionized hydrogen in the Milky Way's disk. These clouds, called HII regions, are ionized by young, hot stars and are basically free protons and electrons.
These are both important marker of spiral arms in other spiral galaxies we see, so mapping them in our own galaxy can give a clue about the spiral nature of the Milky Way. There are bright enough that we can see them through the disk of our galaxy, except where the region at the center of our galaxy gets in the way.
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