Why do Galaxies look like that?

Why should a galaxy have bluish spiral arms dotted with red patches and dark lanes.  Why should it have a central region that is yellow and spherical rather than flat? Why are they flat to begin with?

M83: The Thousand-Ruby Galaxy Image Credit: Subaru Telescope (NAOJ), Hubble Space Telescope, European Southern Observatory – Processing & Copyright: Robert Gendler

Because Galaxies are so huge, and made from hundreds of Billions of stars that change over the course of their lives, a galaxy shows the entire life cycle of a star in its own structure.

Stars are born along spiral arms, where most of the thick dust and gas clouds are concentrated.  The dark dust lanes of spiral arms condense to form stars, whose light blasts away and ionizes remaining gas in the birth cloud, creating the reddish Hydrogen – alpha light you see in the above photo.

The most massive and hottest blue stars will only live a few hundred million years, exploding as supernovae and seeding more star formation in the spiral arms, but stars that live much longer go on adventures away from the spiral arms that gave them life. Those stars spend several billion years orbiting the galaxy, each orbit taking a few hundred million years.

Slowly but surely, dust clouds and interactions with the gravity of other stars will slow down the older stars, sending them closer to the galactic centre.  As stars age they cool and become redder on average, so the migrating central stars are much redder, giving the central regions of a galaxy a yellowish glow.  Because of those gravitational interactions, the trajectory of each star brings it above or below the galactic disk, giving the stars in the central galaxy a spherical distribution.

Galaxies look still, and over the course of a human lifetime they will not change their character.  But in astronomical time scales, a galaxy is a spinning, living, rotating, evolving nation of stars that are interacting, living their lives, and dying.  Maybe a bit like a city of earthlings.

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