selden wrote:Don't forget that the density of stars in the disk is almost uniform. Spiral arms stand out because they contain bright, young stars, not because there are many more stars in them.
Selden,
this subject is much more complex than popular Web pages tend to present it. The often quoted 50 year old qualitative explanation of the spiral arm formation is based on the so-called
Density Wave theory,
http://en.wikipedia.org/wiki/Density_wave_theoryIt states that the spiral arm pattern we see is actually a
(density) wave pattern, that rotates independently of the almost uniformly distributed stars and inter-stellar matter in the galactic disk. Typically like so:
Spiral_galaxy_arms_diagram.jpg
(Astro)physicists like myself refer usually to the much more recent
quantitative results of N-body simulation of galaxy formation by means of
supercomputers. The simulated dynamics among the huge number N of considered particles comprises self-gravity, hydrodynamics, radiative cooling, star formation, and supernova feedback, to
follow the chemical enrichment histories of both the stars and gas.In straight contradiction to your above statement and the Density Wave theory, let me quote the very recent results presented also on this year's international conference of the Royal Astronomical Society (NAM21). These simulations allow to separate the density of disk stars and gas:
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MULLARD SPACE SCIENCE LABORATORY ASTROPHYSICS
Click!m33sim.jpg
Caption: Stellar (left) and gas (right) density maps of a simulated galaxy similar in size to M33.
(from
http://www.mssl.ucl.ac.uk/theory/images.html)
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Here is the corresponding press release at the Royal Astronomical Society conference NAM21 that was widely reprinted:
http://www.ras.org.uk/news-and-press/21 ... alaxy-armsThe essence lies in this simulation result together with the quoted caption from Robert Grand's NAM21 talk:
Image Caption: Snapshots of face-on view of a simulated disc galaxy. A Brighter colour indicates
higher density. The Image shows two examples of star particles: the red star are travelling at the leading side of the arm, and the blue star are at the trailing side. It can be seen that the blue and red stars interchange their radial distances, with rapid migration within 40 million years. The dotted lines trace circles with radii of 4, 5 and 6 000 parsecs (1 parsec = 31 trillion kilometres), to guide the eye.
clickgrand.jpg
(Image credit: Robert Grand/UCL/MSSL, Copyright© 2011 Royal Astronomical Society.)
It is evident that the stars actually rotate with the arms!
Fridger
. A major experimental breakthrough by means of precise 3D mapping of the galactic spiral arm structure and chemical composition will come from the Gaia mission (launch 2013)
