Spiral Galaxies

The spiral arms are actually regions of star formation characterized by bright blue giants and supergiants (O and B stars), and massive HII regions. The closer matter is to the center of a galaxy, the faster it revolves around the center of the galaxy. This would tend to soon wrap the spiral arms tighter than we see them, if the arms were constantly composed of the same matter.

Since star formation and HII regions correspond to regions of higher-than-average interstellar gas density, this suggests a way to explain what causes the spiral arms.

As explained by Lin and Shu in 1963, the spiral structure results from density waves in the interstellar gas, analogous to higher-density regions of cars in sections of highway undergoing construction (Shu, p. 275).

The galaxies seen here have random orientations to our line of sight - some are seen face-on (M 33), some edge-on (M 104), and most somewhere in between.
Click on a thumbnail image to see it full size.

M31, NGC 224

M33, NGC 598

M51, NGC 5194-5195

M65, NGC 3623

M74, NGC 628

M77, NGC 1068

M88, NGC 4501

M96, NGC 3368

M98, NGC 4192

M99, NGC 4254

M100, NGC 4321

M104, NGC 4594

M106, NGC 4258

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Last updated on 11-02-99.
Email:jbruton @northlink.com