Beautiful spiral - with a question

https://www.legacysurvey.org//viewer/?ra=343.2438&dec=2.6279&layer=hsc-dr2&zoom=14

Could someone please explain why the cores of spiral galaxies are usually yellow/white and only the spiral arms are blue?

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That has to do with the actual density of stars near the core of the galaxy. The amount of stars increases dramatically as you get closer to the center black hole. Those are also the oldest stars in the galaxy as well.

Yes, the different colors are more about the different populations of stars, as you say; old stars have that yellowy color, while young stars forming in the disk are doing their hot, blue, live-fast-and-die-young tricks. In spirals, the core is called the “bulge” and the spiral arms/outer part is called the “disk”, eg as described

http://hosting.astro.cornell.edu/academics/courses/astro201/galaxies/types.htm

cheers,
–dustin

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in the center of galaxies, the density of stars is so high that any gas there is heated up enough so it can’t form new stars. All the stars there are old and red.
Stars can only form from cold gas because they need to very dense in their cores, and heat makes things be less dense.