|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
NET Angular Momentum of Globular cluster of stars
Is the NET angular momentum of a globular cluster of stars, zero?
e.g. Omega Centauri or M13 for example. Is the NET angular momentum of an elliptical galaxy and or the central bulge of a spiral, zero or close to zero..........oblate spheroids might have a net angular momentum if the oblate geometry is due to rotation in a single direction. But if it's due to net rotation 50 / 50 in two opposite directions, again, there would be no net angular momentum, right? rt [[Mod. note -- I think the answers are Yes (or very close to zero), Yes, and Yes (respectively). -- jt]] |
#3
|
|||
|
|||
NET Angular Momentum of Globular cluster of stars
In article , Martin Brown
writes: Is the NET angular momentum of an elliptical galaxy and or the central bulge of a spiral, zero or close to zero..........oblate spheroids might have a net angular momentum if the oblate geometry is due to rotation in a single direction. [[Mod. note -- I think the answers are Yes (or very close to zero), Yes, and Yes (respectively). -- jt]] Indeed. A reasonable treatment of the evolution of gravitationally bound clusters of stars is online at Caltech minus a few diagrams: http://www.astro.caltech.edu/~george...y20-Lec15x.pdf I'm no expert in this field, but I think that it might be a bit more complicated. Look up "fast rotator" and "slow rotator" in the context of elliptical galaxies. This might be a place to start: http://www-astro.physics.ox.ac.uk/~b...rly-types.html |
#4
|
|||
|
|||
NET Angular Momentum of Globular cluster of stars
OK, so far great, close to zero net angular momentum in a globular or
elliptical. Please keep in mind I'm attempting to march down a Gedanken path, and am not proposing anything that is likely. Now, if I have an elliptical with zero angular momentum, this is not to say there is no momentum. Clearly there is a lot of momentum and each star contributes an angular momentum vector to the total sum, which is zero. If I establish a coordinate system X,Y,Z, then I can sum the angular momentum about each axis with a SMBH at coordinate 0,0,0 at the origin as a reference position. As this has been decades since I've crunched these equations, perhaps my terms need updating so please feel free to change my variables. I'll use L for angular momentum. I can sum angular momentum about each of the principle axis'. For each axis, I get some number of objects moving around the axis with one orientation, and another number of objects moving in the reverse sense. So, I can sum them as total angular momentum L L = aL_x + (-bL_x) + cL_y + (-dL_y) + eL_z + (-fL_z) = 0 So, a=b, c=d, and e=f. And for a typical elliptical or globular, without oblateness, we can probably go further and say that a=b=c=d=e=f. In other words, objects are moving about the origin in all possible orientations, they are thermalized, and there is no net angular momentum. But this does not mean there is no angular momentum. The stars possess a lot of momentum, it's just all randomized. [Moderator's note: This is not necessarily the case. It is also possible---probable?---that each individual star has zero angular momentum with respect to the SBH at the center, i.e. executes a back-and-forth motion, passing through the centre of the galaxy. Stars are few and far between, so this is possible without an appreciable number of collisions. -P.H.] If this seems about correct, then I'll move forward to the next step in the Gedanken path, working with a galaxy that starts off with only L = aL_x and with all of the rest of the components initially zero. Have I made a mistake so far? rt |
#5
|
|||
|
|||
NET Angular Momentum of Globular cluster of stars
On Thursday, March 16, 2017 at 12:46:59 PM UTC-4, wrote:
Is the NET angular momentum of a globular cluster of stars, zero? e.g. Omega Centauri or M13 for example. .... [[Mod. note -- I think the answers are Yes (or very close to zero), Yes, and Yes (respectively). -- jt]] I'm not sure that's the case. Searching for "globular cluster rotational flattening" gives a lot of papers that discuss the rotational dynamics of globulars. I would say the consensus is, there is significant angular momentum in some globulars. |
#6
|
|||
|
|||
NET Angular Momentum of Globular cluster of stars
In article ,
writes: Is the NET angular momentum of a globular cluster of stars, zero? I think that's the case for most clusters, but I vaguely remember that there are a tiny number that show rotation. I might be mistaken on either part of this. Is the NET angular momentum of an elliptical galaxy and or the central bulge of a spiral, zero or close to zero For elliptical galaxies, a quick web search turned up https://academic-oup-com/mnras/artic...6.2011.18496.x The authors claim 86% of early type galaxies are "fast rotators." I haven't studied the paper to find out what that means or what sample they defined. I expect there are many more works on this subject; as I say, it was a very quick search. There are also lots of theory papers simulating major mergers, from which elliptical galaxies are supposed to form. The simulation results must include a final angular momentum. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#7
|
|||
|
|||
NET Angular Momentum of Globular cluster of stars
On Tuesday, March 21, 2017 at 1:18:55 PM UTC-7, Steve Willner wrote:
In article , writes: Is the NET angular momentum of a globular cluster of stars, zero? OK, it can be difficult to communicate in words rather than pictures and sketches.............trying again: I'm just trying to get us on board for a qualitative exploration. Sure, with ellipticals, as with globulars, there is a range of oblatness to the spherical geometry. A flattened structure may (and may not) have net angular momentum, it depends on whether there are stars rotating in a preferred (or in counter rotational e.g. the spiral ngc4138 is counter rotating, so it's possible for stars in ellipticals to also have this feature https://en.wikipedia.org/wiki/NGC_4138) directions about the flattened plane's normal axis. ie, we have E0 through E5 ellipticals. BUT, COMPARED TO.... a simple spiral disk, with no central bulge, with all stars moving around a center in the SAME direction..........like Saturn's rings. ........... an elliptical and or a globular have ESSENTIALLY, ZERO angular momentum . In other words, can we agree that if I have the same number of stars, and the same orbital radii, ellipticity etc. for every star, with the sole exception that the globular has stars that appear as spherical morphology, where as a flat spiral has purely single angular momentum axis............. The globular will be nearly zero compared to the flat spiral. Agreed? If so then I can move on. rt |
#8
|
|||
|
|||
NET Angular Momentum of Globular cluster of stars
On Thu, 16 Mar 2017 12:46:58 EDT, wrote:
Is the NET angular momentum of a globular cluster of stars, zero? ... Is the NET angular momentum of an elliptical galaxy and or the central bulge of a spiral, zero or close to zero I think the current status of thinking is long on theory and short on observations. I don't think there's been a single comprehensive survey of the actual stellar movements in any globular cluster. I'm not aware of any done for an elliptical galaxy either. Not long ago there was an idea of "tri-axial" rotation for these which implied that individual star orbits could be decomposed into 3 axial components which would somehow serve to keep the stars from plunging into the core. That doesn't seem to be de rigueur anymore. Also there was speculation some decades ago that globular cluster stars simply fell through the cluster cores in a sort of mass yo-yo action. Also not popular of late, I don't think. I had a notion which I publicized on these forums some years ago, that there's a gravitational scalar, as yet unmodelled, which allowed stars to freely float in such environments. I plead guilty for hand-waving on this, but it seems an elegant concept. Ugly facts begone! I mention all this because it looks to me that other answers on this thread are muddying up the distinction between theory and observation. If there've been cogent observations made recently on actual stellar orbits in globular clusters or ellipticals, I'd be glad to be directed to them. |
#9
|
|||
|
|||
NET Angular Momentum of Globular cluster of stars
On 23/03/2017 22:23, Eric Flesch wrote:
On Thu, 16 Mar 2017 12:46:58 EDT, wrote: Is the NET angular momentum of a globular cluster of stars, zero? ... Is the NET angular momentum of an elliptical galaxy and or the central bulge of a spiral, zero or close to zero I think the current status of thinking is long on theory and short on observations. I don't think there's been a single comprehensive survey of the actual stellar movements in any globular cluster. I'm not aware of any done for an elliptical galaxy either. I'm no expert in this apart from having known a few of the early practitioners of computational stellar dynamics but I think there are now some precision observations of stars in local globular clusters like Omega Centuri which show that things are never simple. http://www.aanda.org/articles/aa/pdf.../aa3061-05.pdf and https://www.astro.umd.edu/~richard/A...ergy_equip.pdf There are plenty of velocity dispersion and luminosity measurements along lines of sight through various of the brighter Messier globular clusters dating from way back. eg M15 http://articles.adsabs.harvard.edu//...00251.000.html Not long ago there was an idea of "tri-axial" rotation for these which implied that individual star orbits could be decomposed into 3 axial components which would somehow serve to keep the stars from plunging into the core. That doesn't seem to be de rigueur anymore. Also there was speculation some decades ago that globular cluster stars simply fell through the cluster cores in a sort of mass yo-yo action. Also not popular of late, I don't think. I'm sure I have seen somewhere a claim that stars out on the fringes of clusters tend to be in very elongated orbits rather than circular ones. Certainly the eye tends to pick out spiky rays in for example M13 around the edges of the core. I had a notion which I publicized on these forums some years ago, that there's a gravitational scalar, as yet unmodelled, which allowed stars to freely float in such environments. I plead guilty for hand-waving on this, but it seems an elegant concept. Ugly facts begone! At a handwaving level a single test particle in an evolved spherically symmetric globular cluster moves in a gravitational potential that far out is roughly 1/r^2 but closer in becomes weaker and in the dense core becomes almost linear. This implies that a test particle will no longer follow an ellipse around the centre of mass but will instead trace out a somewhat more circular petal shape staying further away from the core. Self consistent models in phase space seem to be one interesting way of approaching it using maximum entropy or some other computational heuristic to create the most non-committal distribution of matter in consistent with a given mass M, energy E and angular moment L. Going through the core must increase the possibility of a three body close encounter from time to time resulting in ejection of stars and the condensation of the rest. I mention all this because it looks to me that other answers on this thread are muddying up the distinction between theory and observation. If there've been cogent observations made recently on actual stellar orbits in globular clusters or ellipticals, I'd be glad to be directed to them. On the simulation early evolution of star clusters this was published fairly recently, but I don't know enough to comment on its veracity. http://rsta.royalsocietypublishing.o...t/368/1913/829 -- Regards, Martin Brown |
#10
|
|||
|
|||
NET Angular Momentum of Globular cluster of stars
On 24 Mar 2017, Martin Brown wrote:
http://www.aanda.org/articles/aa/pdf.../aa3061-05.pdf https://www.astro.umd.edu/~richard/A...ergy_equip.pdf ... At a handwaving level a single test particle in an evolved spherically symmetric globular cluster moves in a gravitational potential that far out is roughly 1/r^2 but closer in becomes weaker ... Thanks for the references -- the measurements of proper motion are a great step forward, but the authors go on to calculate orbitals (or fractions thereof) using certain standard assumptions. Stars shooting out of the globular cluster are assumed to have highly eccentric elliptical orbits, etc. The notion of orbits is a given. I'm realizing nowadays that my old notion of "gravitational scalar" really is just the same as today's notion of "dark matter". If dark matter resides throughout an elliptical galaxy or globular cluster (or the Galactic halo) then it could raise the ambient background gravitational level to where a resident star no longer feels the gravitational effect from its neighbours or from the system centre. Instead, the star would follow thre contours (potentials) of the dark matter structure. So stars would just mingle throughout without following orbits as such. Thus my concern about the current narrative which puts all star motions in a framework of orbits. cheers. |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
New M14: Globular Cluster with BRC250 + variable stars | Danilo Pivato | CCD Imaging | 1 | July 19th 11 08:45 AM |
New M14: Globular Cluster with BRC250 + variable stars | Danilo Pivato | Amateur Astronomy | 3 | July 17th 11 11:49 PM |
Angular Momentum | G=EMC^2 Glazier[_1_] | Misc | 6 | July 26th 08 11:52 PM |
HUBBLE SEES FAINTEST STARS IN A GLOBULAR CLUSTER (STScI-PR06-37) | INBOX ASTRONOMY: NEWS ALERT | Amateur Astronomy | 3 | August 19th 06 10:56 PM |
HUBBLE SEES FAINTEST STARS IN A GLOBULAR CLUSTER (STScI-PR06-37) | INBOX ASTRONOMY: NEWS ALERT | Astronomy Misc | 1 | August 18th 06 04:31 AM |