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Spiral galaxies: winding up or winding down?

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#1 Ira

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Posted 04 August 2013 - 05:50 AM

Do spiral galaxies wind up or wind down? Do they all wind the same way?

/Ira

#2 Pess

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Posted 04 August 2013 - 07:14 AM

Do spiral galaxies wind up or wind down? Do they all wind the same way?

/Ira


Not sure what you mean by 'wind up' or 'wind down'. In any event nobody knows for sure what they do.

The latest, greatest, tastest theory is that the stars spin with the spiral arms (as opposed to most remaining stationary as the arms pass through them).

The stars at the leading edge of the spiral gradually move inward while those at the trailing edge tend to move out further in the spiral.

The spiral itself is thought to break up and re-form due to gravitational shear forces over time.

Of course, this is all speculation subject to further investigation.

Pesse (eom) Mist

#3 Ira

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Posted 04 August 2013 - 09:19 AM

Does the spiral get tighter ("wind up") or looser ("wind down") over time? I assume the arms rotate in one direction or the other, so the spirals are either tightening or loosening. No?

/Ira

#4 GlennLeDrew

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Posted 04 August 2013 - 05:39 PM

The vast majority of galaxies have trailing arms; a very small percentage seem to have leading arms. The evidence comes from radial velocity measurements across the galaxy. I certainly don't know why spiral arms should be leading, but I suspect the condition is transitory, where throughout such a galaxy's life its arms usually trail. Again, that's my guess.

#5 Mister T

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Posted 04 August 2013 - 05:49 PM

Do spiral galaxies wind up or wind down? Do they all wind the same way?

/Ira


Not sure what you mean by 'wind up' or 'wind down'. In any event nobody knows for sure what they do.

The latest, greatest, tastest theory is that the stars spin with the spiral arms (as opposed to most remaining stationary as the arms pass through them).

The stars at the leading edge of the spiral gradually move inward while those at the trailing edge tend to move out further in the spiral.

The spiral itself is thought to break up and re-form due to gravitational shear forces over time.

Of course, this is all speculation subject to further investigation.

Pesse (eom) Mist


We should know for sure in 5-10 BY :graduate:

#6 Ira

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Posted 04 August 2013 - 08:32 PM

The vast majority of galaxies have trailing arms; a very small percentage seem to have leading arms. The evidence comes from radial velocity measurements across the galaxy. I certainly don't know why spiral arms should be leading, but I suspect the condition is transitory, where throughout such a galaxy's life its arms usually trail. Again, that's my guess.


How would an entire galaxy change its direction of rotation? Seems like there's an aweful lot of kinetic energy to just begin going the other way.

Also, as we look back in time shouldn't we see galaxies with differing degrees of tightness as they wind (and unwind)? Do we see this or do all spirals have just about the same degree of windedness? Is there a name or mathematical expression for how tightly spirals are wound?

/Ira

#7 David E

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Posted 05 August 2013 - 04:36 AM

The vast majority of galaxies have trailing arms; a very small percentage seem to have leading arms. The evidence comes from radial velocity measurements across the galaxy. I certainly don't know why spiral arms should be leading, but I suspect the condition is transitory, where throughout such a galaxy's life its arms usually trail. Again, that's my guess.


How would an entire galaxy change its direction of rotation? Seems like there's an aweful lot of kinetic energy to just begin going the other way.

Also, as we look back in time shouldn't we see galaxies with differing degrees of tightness as they wind (and unwind)? Do we see this or do all spirals have just about the same degree of windedness? Is there a name or mathematical expression for how tightly spirals are wound?

/Ira


For the last paragraph, I would think that depends on how much dark matter is in that galaxy. Dark matter is holding our spiral arms together.

#8 Pess

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Posted 05 August 2013 - 06:35 AM

For the last paragraph, I would think that depends on how much dark matter is in that galaxy. Dark matter is holding our spiral arms together.


Keep in mind that computer modeling is really all we have right now..and with computers it is garbage in-garbage out. Dark matter and dark energy are relatively new concepts but are the dominant forces needed to be modeled.

That's why my first post was kinda tongue-in-0check about not knowing what we don't know. This whole field is in flux.

Pesse (So we need a flux capacitor) Mist

#9 Jarad

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Posted 05 August 2013 - 09:20 AM

How would an entire galaxy change its direction of rotation? Seems like there's an aweful lot of kinetic energy to just begin going the other way.



The galaxy doesn't change its direction of rotation. The arms are areas of denser stars - like density waves. They can be oriented either pointing forward or backward compared to the overall rotations. A galaxy could switch between the two not by changing overall rotation, but by having the density wave at the outer edges move faster or slower than the inner part.

Jarad

#10 GlennLeDrew

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Posted 05 August 2013 - 01:46 PM

As Jarad states, it's not that the *material* reverses direction (which is quite impossible), but that the *pattern* of the density wave alters. It must be borne in mind that the material and the wave pattern are not locked together; they pass through each other, and at different rates at different distances from the center of the system. Nearer to the center the material moves faster than the pattern. With increasing distance the speeds more and more closely match until the point of corotation. Beyond this the material moves more slowly than the pattern. Over the full range of galactocentric distance there will be resonances, where the speed ratios are tidy multiples or fractions, and which result in amplification of the density wave. These and other considerations could lead to periodic reversals of the pattern orientation.

#11 UND_astrophysics

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Posted 08 August 2013 - 12:20 AM

err guys .... The Spiral arms are not moving, As Glenn and Jared alluded, stars are not moving inward or outward. The arms are visible because of the blue giants forming. It is an illusion that arms are "moving".. the rotation combined with the formation sequence is what gives this effect.

#12 derangedhermit

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Posted 08 August 2013 - 02:12 AM

It's an optical illusion, works just like wagon wheels in old Westerns.

#13 GlennLeDrew

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Posted 08 August 2013 - 04:24 PM

According to theory, the wave pattern does indeed rotate, in a fixed configuration like a boomerang, if you will. At a certain distance from the center, called corotation, the disk material and the wave pattern speeds are the same. Inside corotation the material is faster, and outside corotation the material is slower. At various distances from the center, and depending on the number of spiral arms making up the wave pattern, there are resonances established by the ratios of orbital periods and the periods between the passage of material through successive wave troughs.

#14 llanitedave

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Posted 08 August 2013 - 05:31 PM

In the density wave model, at least, the wave does rotate, but it's not synchronized with the stars' orbits.

#15 UND_astrophysics

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Posted 08 August 2013 - 05:45 PM


But again, just to elaborate and just as important in addressing the original question , the spiral arm effect (what we can see) is due to the Blue giants which are on the order of 1,000 to 1,000,000 times higher luminosity and their short lives compared to other main sequence stars (sometimes only millions of years)Luminosity and lifespans are an inverse relationship of stars That is what is making the spiral arms visible. Simply more luminous stars being formed in the denser areas of GMC's as described by Glenn's (accurate and valid) previous post.

#16 GlennLeDrew

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Posted 08 August 2013 - 11:41 PM

Not just the short-lived, massive and bright stars delineate the spiral arms. The troughs in the spiral arm arm density wave cause a small over density among all stars. If we examine a near-IR image of a spiral galaxy, where the contribution of light comes more from the (generally much older) low mass K and M dwarfs, a diffuse spiral pattern is present as well, albeit of much lower 'contrast.' And of course we see the piling up of molecular clouds in these troughs, too.

#17 derangedhermit

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Posted 09 August 2013 - 01:12 AM

According to theory, the wave pattern does indeed rotate, in a fixed configuration like a boomerang, if you will. At a certain distance from the center, called corotation, the disk material and the wave pattern speeds are the same. Inside corotation the material is faster, and outside corotation the material is slower. At various distances from the center, and depending on the number of spiral arms making up the wave pattern, there are resonances established by the ratios of orbital periods and the periods between the passage of material through successive wave troughs.


Someone should do an animation, with a set of parameters to play with and some defaults for sample (realistic) motions. It is kind of hard to visualize the various components and factors at work and the resulting motions.

#18 Ira

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Posted 15 August 2013 - 12:56 AM

You guys are just confusing me, making me think that galaxies don't rotate at all. See here: http://youtu.be/d2oKs1qC_jY

/Ira

#19 GlennLeDrew

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Posted 15 August 2013 - 03:42 AM

Differential radial velocity across galaxy disks proves that galaxies rotate. But unlike a Keplerian potential, where the bulk of the system's mass resides in the center and orbital velocity decreases exponentially with distance, in a spiral galaxy's disk the orbital velocity is more or less uniform with distance (except very near the center, where velocity increases quickly, and essentially linearly with distance.)

But in spite of a basically constant velocity, there is a difference in orbital period with distance (due to larger orbit circumference), which results in a decrease in angular velocity with distance and hence shear.

This deals with the material. But superimposed upon this is a density wave which (at least most commonly) rotates in the same direction, in a fixed pattern as though 'solid.'

Simply picture a spiral pattern, as though printed on clear plastic, spinning about the center. All parts of the pattern move at constant angular velocity. This means that with increasing distance from the center, the linear velocity increases linearly. But stars and gas in the disk orbit at a constant linear velocity. And so nearer to the center the material moves soeeds abead of the density wave, at some particular distance out the material and wave pattern move at the same speed, and farther out the wave pattern speeds ahead of the material.

#20 StarWars

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Posted 16 August 2013 - 01:24 AM

For the last paragraph, I would think that depends on how much dark matter is in that galaxy. Dark matter is holding our spiral arms together.


Keep in mind that computer modeling is really all we have right now..and with computers it is garbage in-garbage out. Dark matter and dark energy are relatively new concepts but are the dominant forces needed to be modeled.

That's why my first post was kinda tongue-in-0check about not knowing what we don't know. This whole field is in flux.

Pesse (So we need a flux capacitor) Mist



Most likely.. :grin:

Spiral galaxies don't make much sense. The spiral arms seem to demonstrate an invisible resistance causing the curvature in the arms. Might be dark matter.... :question:

But it looks cool.... :grin:

http://isc.astro.cor..._collision_t...

#21 Ira

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Posted 11 September 2013 - 10:45 PM

Whether getting tighter or looser, they appear to have a 7% preference for spinning counter-clockwise. Hmmmm...

http://www.dailygala...nning-new-re...

/Ira






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