Why does the Milky Way have spiral arms instead of being a smooth disk?
Quick Answer
Spiral arms are not fixed structures of stars; they are density waves moving through the galaxy. Gas compresses in those regions, triggering new star formation and making the arms stand out.
The Full Story
The Milky Way is a barred spiral galaxy, which means it has a central bar and spiral arms. The arms are best understood as regions of higher density that move through the disk rather than being permanent lines of the same stars. As gas enters these crowded regions, it gets compressed and forms bright young stars, clouds, and nebulae. Our galaxy’s structure was mapped in much greater detail by missions such as ESA’s Gaia, launched in 2013.
Key Facts
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