So we've seen millions of galaxies, and all of them have a bright, yellow nucleus. What is the cause of the yellowness? Is it because of aging stars, redshift from the SMBH, or something else?


1 Answer 1


It's an observational fact that many disk galaxies have a color gradient with redder (or as you say 'yellow') stars in the central regions and bluer stars in the disk. There are two things that can cause this central redness.

  • Age, older stars being redder than younger stars
  • Metallicity, with red stars being more metal rich than blue stars

Whether it's age or metallicity or both causing these gradients is topic of current research. It's a popular theory that galaxies grow inside-out, which causes a red center.


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