Author Topic: Why barred spirals?  (Read 7737 times)

genel

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Why barred spirals?
« on: September 10, 2007, 01:47:18 am »
Having now seen many barred spirals, my question is what are the current theories about how they form?
I saw one explanation about some kind of resonance wave, but many I've seen seem far too symmetrical for that.
What can produce such a rigid looking structure, especially those which have strong bars which abruptly end and spin off long trails of spirals?
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pmf71

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Re: Why barred spirals?
« Reply #1 on: September 11, 2007, 10:13:02 pm »
I think the fact that some spirals retain a barred structure can be loosely compared to the lagrange points around earth, those are the points where the earth's gravity and the sun's have a certain balance (a satellite can stay there without using fuel to stay in that position). the individual gravity fields of stars and clusters probably work together and against the gravity of the center bulge (including the SMBH) to form a region of stability up to a certain distance from the center of the galaxy, with the result that the stars closer to the center orbiting slower than you expect, and the stars further out orbiting faster than you'd expect. Thus retaining the barred structure for a much longer period than normal.
« Last Edit: September 11, 2007, 10:14:49 pm by pmf71 »

gz

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Re: Why barred spirals?
« Reply #2 on: September 12, 2007, 01:00:18 am »
A Langrange point only exists when there are two massive objects--it is the point where their gravitational fields exactly cancel out.

pmf71

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Re: Why barred spirals?
« Reply #3 on: September 12, 2007, 07:12:12 pm »
A Langrange point only exists when there are two massive objects--it is the point where their gravitational fields exactly cancel out.

I also said LOOSELY compared to. And if earth is massive enough to have lagrange points between it and the sun, then stars and clusters most certainly are massive enough too.

Fusion_power

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Re: Why barred spirals?
« Reply #4 on: September 17, 2007, 04:00:12 am »
Lagrange points are a poor analogy.  Gravity is the place to start though.

The best theories today have to do with distributed mass in the galaxy with a central bar.  Basically, the mass is concentrated more toward the ends of the bar than the center.  This tends to lock the mass of the bar into a sustainable form.  The rest of the galactic mass is linked to the bar which makes these galaxies appear to have spirals attached to the bar ends.

DarJones
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gz

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Re: Why barred spirals?
« Reply #5 on: September 17, 2007, 04:33:38 pm »
That leaves the question, why is the mass more concentrated in those spots in the first place?

bamford

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Re: Why barred spirals?
« Reply #6 on: September 20, 2007, 09:48:20 am »
Actually pmf71's Lagrange point analogy isn't that bad.  One must try not to think of galaxies as solid objects.  Their appearance is due to the orbits of individual stars.  The motion of each of these stars is influenced by the gravity of all the other stars.  Calculating the resulting stellar orbits is quite technically involved.  However, it turns out that there are particular places in the galaxy which are more stable than others - where a star will tend to stay, or only move slowly, in the rotating frame of the galaxy.  At any given time there will therefore be more stars in these places, and so we will see features at these places, such as spiral arms, rings and bars.  These regions are sometimes called resonances, and do have close similarities to the Lagrange point resonances in a two body system, particularly the stable Lagrange points.  In some cases the stars accumulating at these special points further enhances their stability, which is roughly what I think Fusion_power was getting at.

genel

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Re: Why barred spirals?
« Reply #7 on: September 23, 2007, 03:15:35 am »
Yes, I agree galaxies are not solid - which is the essence of my question. Bars appear to rotate like rigid bodies, not independent masses orbiting the center. I could see the LaGrange point analogy if a large, say black hole or cluster formed in the outer part of the galaxy, but isn't the resonance a POINT, not a bar? Also, how could that generate such a symmetrical bar on the opposite side of the core? I can understand resonances, is there an analogy to ocean waves? Saturn's rings are a prime example of resonances. Something has to be creating the sweet spot for the stars to prefer. Some cataclysmic event? We see high energy jets all the time, but those are along the axis of rotation, not in the plane.

The other part of this is the abrupt end of the bars. A wave would not end so distinctly. Obviously, there is an evolution from barred spiral to rings. you can see some that are  just starting, others that are nearing the first complete revolution and full rings.

I'm not trying  to be argumentative, I'm just trying to understand and I can't picture the mechanism.

Thanks for your inputs - please keep trying
If you're not cold, it's not astronomy.


gz

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Re: Why barred spirals?
« Reply #8 on: September 23, 2007, 05:29:47 am »
We still haven't had a really good explanation for how the rotational symmetry of a sphere or disc could be broken to generate a bar. If gravitational interactions, as in Lagrange points, create resonances, wouldn't we expect those resonances to be symmetrical as well (i.e. a spherical shell in a spherical galaxy, or a circle in a disc galaxy)? Why is a bar cropping out out of the chaos?

Alice

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Re: Why barred spirals?
« Reply #9 on: September 23, 2007, 09:07:52 am »
Sounds like a neat PhD for you, seanparmeleeumn!  ;D

Update: For anyone confused, seanparmeleeumn was gz's old username.
« Last Edit: September 10, 2008, 10:38:39 pm by Alice »

zookeeperKevin

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Re: Why barred spirals?
« Reply #10 on: September 23, 2007, 10:03:07 am »
Sounds like a neat PhD for you, seanparmeleeumn!  ;D

Sean, if you're mathematically inclined, we could point you to the kinds of people who study bar formation. Getting funding for a PhD can be easy or difficult depending on your nationality....

gz

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Re: Why barred spirals?
« Reply #11 on: September 23, 2007, 10:37:20 am »
Hah. Well, a PhD would require a B.S., and astronomy is not my trade. I guess I (along with a few other folks here) am just looking for a simple explanation. Are you saying there really isn't one yet?

zookeeperKevin

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Re: Why barred spirals?
« Reply #12 on: September 23, 2007, 10:38:37 am »
I believe bar formation and its role in galaxy evolution is still a contentious topic.

genel

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Re: Why barred spirals?
« Reply #13 on: September 23, 2007, 04:21:23 pm »
I believe bar formation and its role in galaxy evolution is still a contentious topic.

based on this thread, that certainly seems to be the case.  ;)

I was just wondering -- I've been thinking in terms of existing stars being sheparded into the bars, but perhaps it's more of a disturbance sparking new star formation, favoring the bars. One possibility is a near right angle collision, but that seems TOO disruptive, as we can see from other mergers.

The two effects that really create problems are the symmetry, in the rotation plane, and the abrupt end of the bars.
If you're not cold, it's not astronomy.


genel

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Re: Why barred spirals?
« Reply #14 on: September 23, 2007, 05:38:01 pm »
I just reviewed the barred spirals thread and made a few observations. Most bars seem to consist of yellow, presumably old main sequence stars. In spirals with the raw materials, the spiral arms off the ends have a lot of young blue stars. In the older systems, there are some systems with spiral arms or rings. frequently there are larger concentrations at the ends of the bars. inside the ends, the bars do seem to have swept up material, compared to the density in the rest of the system.
If you're not cold, it's not astronomy.