M51 - the Whirlpool Galaxy. Like most spiral galaxies, the spiral arms are really density waves. Drag forces produced by these density waves could drive the 'secular' evolution of galaxies. Credit: NASA-HST.
A traditional galaxy evolution model has it that you start with spiral galaxies ? which might grow in size through digesting smaller dwarf galaxies ? but otherwise retain their spiral form relatively undisturbed. It is only when these galaxies collide with another of similar size that you first get an irregular ?train-wreck? form, which eventually settles into a featureless elliptical form – full of stars following random orbital paths rather than moving in the same narrow orbital plane that we see in the flattened galactic disk of a spiral galaxy.
The concept of secular galaxy evolution challenges this notion ? where ?secular? means separate or isolated. Theories of secular evolution propose that galaxies naturally evolve along the Hubble sequence (from spiral to elliptical), without merging or collisions necessarily driving changes in their form. (...)
Read the rest of Astronomy Without A Telescope – Secular Evolution (499 words)
© Steve Nerlich for Universe Today, 2010. |
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Full story at http://www.universetoday.com/81814/astronomy-without-a-telescope-secular-evolution/
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