Talk:Spiral galaxy

Merger from Spiral nebula

Article merged: See old talk-page here. Cosmo0 18:54, 21 September 2007 (UTC)

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How is it that something that seems so simple can be so complicated? All of our lives are in this galaxies hands and the is nothing that we can do but sit and wait for somthing to happen all over again.--207.12.157.2 15:38, 6 September 2005 (UTC)Sara Huey207.12.157.2 15:38, 6 September 2005 (UTC)

It's mesmerizing!

i try not to think about it too much cos it can get pretty confusing pretty quick!

Explanation for Spiral Structure

I'm having some trouble with the proposed explanation for spiral structure in the article, "The first acceptable theory was devised by C. C. Lin and Frank Shu in 1964. They suggested that the spiral arms were manifestations of spiral density waves. They assumed that the stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated, i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center." The diagram illustrates a sequence of concentric ellipses with differing orientations, all centered at the galactic center, and postulates these elipses as the orbits of stars in the galaxy: that the stars orbit in elipses *centered* at the galactic center. But shouldn't eliptical orbits aroung the galactic center have the galactic center at one focus, rather than at the center? Or is the fact that there is a distribution of mass over the whole galaxy, and not just at the center the cause? In any case, I found this point confusing, and I think it merits an explanation. Althai 22:14, 3 October 2006 (UTC)

The picture is a somewhat oversimplistic explanation for spiral arms in galaxies. For one thing, if you simply imagine stars orbiting on fixed elliptical orbits like the one shown then it implies a stationary spiral pattern. In fact, the spiral pattern propagates around the galaxy. The ellipticities are also highly exaggerated in the figure - in reality the density contrast in spiral arms is very low and the orbits nearly circular. You're right about the fact that elliptical orbits should be focussed on the centre of mass and that you have to consider how mass is distributed throughout the galaxy. The last point is key, because the stars in the disk orbit in their own combined potential of their own self-gravity. This is completely different to, say, a planet orbiting a star. Self-gravitating systems are prone to resonant oscillations and it is this that leads to the formation of spiral arms. The whole theory of orbits is very complicated - I've studied it and I still don't understand it. Cosmo0 09:05, 21 September 2007 (UTC)

Interference patterns caused by gravitational waves from two black holes orbiting each other? If the spiral density wave diagram is accurate, then two things must be true. Firstly, the center of mass would have to be elliptical and therefore binary.Secondly, it would have to be rotating,(or it wouldn't be binary for long.) This would have the effect of creating an S-shaped stream of point sources of intense gravity travelling outward (presumably at the speed of light), and tapering off in all directions.It would look much like the stream of water from a sprinkler with two rotating jets. Any mass that comes in contact with the stream, would either give energy to or receive energy from the binary system causing its rotation to speed up and slow down. Each time the "wave" passes the whole system would become more and more balanced. An interesting note is that if enough mass is swept into the arms to overpower the binary system's gravity, it could conceivably start to come apart, creating perhaps a barred spiral, and eventually two galaxies that fly apart. amateur 1 December 2006

The sphere of influence of a black hole (and even a black hole binary) is very small compared to an entire galaxy, so it doesn't explain the spiral structure in galaxies. Which is not to say that some of what you say isn't true in some circumstances: orbiting binaries can cause spiral-like structures, e.g. the tidal arms thrown off by 2 merging galaxies. Cosmo0 09:09, 21 September 2007 (UTC)

New Explanation for Spiral Structure

In a recent paper published in Proceedings of The Royal Society A, Charles Francis and Erik Anderson presented a model of spiral arms, matching observations and showing how the mutual gravity between stars causes orbital rosettes to align so as to form logarithmic spirals. Density wave theory is shown to be based on elementary mathematical mistakes and incorrect physics, and makes observationally incorrect predictions about stellar motion (stars do move along the arms, and orbits are not nearly circular). I have editted this article accordingly RQG (talk) 07:25, 25 August 2009 (UTC)

I'm gravely concerned that a user whose name RQG matches rqgravity.net, the site of the author of a theory that is not widely accepted (despite having made it into a peer review journal). The material should not be stated as fact until this hypothesis is widely accepted, and ideally shouldn't be in the article at all. 87.194.227.22 (talk) 23:28, 28 August 2009 (UTC)

I concede there is a certain awkwardness. However, the material given is not really a theory of physics, but rather a straightforward application of Newtonian gravity, which does have widespread acceptance. In so far as it is possible to tell, the mechanism describe is largely accepted by those who study it. A theory of science is scientific not because it is well known, but because it adheres to observation and because it is shown to adhere to fundamental scientific laws by correct mathematical argument. It should also be noted that the previous material on density wave theory had also not achieved universal acceptance, and moreover contained a number errors of empirical fact. Worse, if we define a crank theory to be one which is based on elementary mathematical and/or physical errors, density wave theory is a crank theory. However, because it has been prominant, density wave theory remains in the article and still has its own page. RQG (talk) 08:11, 29 August 2009 (UTC)

I'd be surprised if it were widely accepted when the animation of the model in question clearly doesn't resemble the actual kinematics of spiral galaxies. I've had bad experiences of edit wars in wikipedia and I'm not going to invest time on this but perhaps a more regular wikipedia editor would care to contact a recognised expert for an opinion. Arguing that this material should be in because other material is in wikipedia is also a poor argument. Either better justify this, or argue for the deletion of the material that offends you. 87.194.227.22 (talk) 08:47, 29 August 2009 (UTC)

In fact the animation is based on a flat rotation curve. The idea that it is not is an illusion resulting from the fact that if velocity is independent of radius, then angular velocity is inversely proportional to radius. Note too that the graph in the referenced article is a simplification and and does not show the observed velocity dispersion. The match between the model and actual data is shown in some depth in the refereed paper in Proc Roy Soc A. An independent view (in addition to that of two reviewers) has been given by Astronomer Rainer Klement, of the Max Planck Institute for Astronomy in Heidelberg, Germany, who has said the paper "comes up with an elegant way of explaining the velocity distribution we observe in the solar neighborhood." RQG (talk) 07:09, 30 August 2009 (UTC)

I'm aware of the potential for a misleading illusion but it's as plain as day that the outer stars are moving more slowly than the inner ones. There's no clear discussion of the rotation curve in the article, no match to anything other than local stellar data and radio data for the Milky Way, with no attempt made to fit to other galaxies. Note also that I never referred to this theory as crank and it may well turn out that it can be made to work but this is an important article which has had this hypothesis dropped into it in a manner that suggests it as fact by an author of the paper, presumably. 87.194.227.22 (talk) 08:09, 30 August 2009 (UTC)

Merge spiral nebula

Agree. Merge into a historical section and make spiral nebula redirect here. --IanOsgood 16:09, 4 June 2007 (UTC)

Merge - The term spiral nebula is merely a footnote in the history of astronomy. Spiral galaxy could use some observational history.--mikeu 22:41, 4 June 2007 (UTC)

Agree with both of the above comments - an alternative, historical name doesn't deserve it's own article. I'm thinking of doing some work on the spiral galaxy article anyway, so I'll do it then, if no-one else does it first and no-one objects. Cosmo0 22:56, 20 September 2007 (UTC)

Done. Cosmo0 19:06, 21 September 2007 (UTC)