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Dave Lee
2015-Oct-19, 06:03 PM
https://en.wikipedia.org/wiki/Globular_cluster
It is stated:
"A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers
Globular clusters, which are found in the halo of a galaxy, contain considerably more stars and are much older than the less dense galactic, or open clusters, which are found in the disk. Globular clusters are fairly common; there are about 150[2] to 158[3] currently known globular clusters in the Milky Way, with perhaps 10 to 20 more still undiscovered.[4] These globular clusters orbit the Galaxy at radii of 40 kiloparsecs (130,000 light-years) or more."
https://en.wikipedia.org/wiki/Star_cluster
"Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds or thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain fewer than a few hundred members, and are often very young.
Globular clusters, or GC, are roughly spherical groupings of from 10,000 to several million stars packed into regions of from 10 to 30 light years across. They commonly consist of very old Population II stars—just a few hundred million years younger than the universe itself—which are mostly yellow and red, with mass just less than two solar masses.
Until recently, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However, greatly improved distance measurements to globular clusters using the Hipparcos satellite and increasingly accurate measurements of the Hubble constantresolved the paradox, giving an age for the universe of about 13 billion years and an age for the oldest stars of a few hundred million years less.
No known globular clusters display active star formation, which is consistent with the view that globular clusters are typically the oldest objects in the Galaxy, and were among the first collections of stars to form.
Typical results for globular clusters are that they may be as old as 12.7billion years.[57] This is in contrast to open clusters which are only tens of millions of years old."

Let's summarize the basic information about a global cluster:
Size - 10 to 30 light years across.
Number of stars - Several millions.
Age of those stars – almost at the age of the Universe - about 13 billion years.
Age of Global cluster - may be as old as 12.7billion years
Gravity between the stars - Globular clusters are very tightly bound by gravity
Global clusters location - In the halo of a galaxy. (Not in the spiral disc)
Distance from the Milky way galactic center - 130,000 light-years or more.

Hence, my questions are as follow:
1- How could it be that the age of the stars in the global cluster are older than the age of the cluster itself?
2- How could it be that so many stars are located in so relatively small aria? (Please be aware that at that at the same distance from the Sun there are just few stars)
3- Why they don't collide with each other? (There is no supper massive mass at the center of the global cluster and those million stars are tightly bound by gravity)

antoniseb
2015-Oct-19, 07:38 PM
... Hence, my questions are as follow:
1- How could it be that the age of the stars in the global cluster are older than the age of the cluster itself?
2- How could it be that so many stars are located in so relatively small aria? (Please be aware that at that at the same distance from the Sun there are just few stars)
3- Why they don't collide with each other? (There is no supper massive mass at the center of the global cluster and those million stars are tightly bound by gravity)

1. They aren't. The clusters and the stars in them are the same age give or take a few million years.
2. They formed from a dense cloud of gas, but as to number of stars, please ignore Omega Centauri, which is probably a remnant of a galactic core, and not a globular cluster in the usual sense.
3. They do collide. Look up "Blue Stragglers" in wiki.

Hornblower
2015-Oct-19, 07:39 PM
Hence, my questions are as follow:
1- How could it be that the age of the stars in the global cluster are older than the age of the cluster itself?
2- How could it be that so many stars are located in so relatively small aria? (Please be aware that at that at the same distance from the Sun there are just few stars)
3- Why they don't collide with each other? (There is no supper massive mass at the center of the global cluster and those million stars are tightly bound by gravity)
1. Age of cluster, a few hundred million years less than universe as a whole. Age of oldest stars, a few hundred million years less than the universe as a whole. Some uncertainty and perhaps some sloppy writing leaves plenty of wiggle room for the stars not predating formation of the cluster.

2. Cluster probably formed from massive primordial clump of gas under conditions no longer occurring.

3. There almost surely have been some collisions, as indicated by "blue straggler" stars. After some initial fireworks, two colliding stars the size of the Sun or a bit smaller will coalesce into a single more massive one similar to Sirius or Vega. Any stars of such mass initially have long since evolved into red giants, some of which have further evolved into white dwarfs and are unobservable at that distance.

Even with this close packing, with the separation about 1/10 that in our stellar neighborhood, the spacing is still stupendously large compared to the diameters of the stars. Thus collisions are few and far between.

ETA: I see antoniseb posted simultaneously, and is pretty much in agreement.

JCoyote
2015-Oct-20, 05:25 AM
1- How could it be that the age of the stars in the global cluster are older than the age of the cluster itself?

The same way Paul McCartney can be older than the Beatles; a group can't be be called a group until the constituents exist.

I have my own query here. I once had a semi-hard sci fi concept (excepting for the usual conceit of faster than light travel but Alcubierre based with an adaption for drama) where long term refugees from earth were arriving in a globular cluster to kick off a series start; they were going to suddenly encounter a lot more things a lot more often.

But is a globular cluster a reasonable place to hunt for a habitable planet?

antoniseb
2015-Oct-20, 11:04 AM
... is a globular cluster a reasonable place to hunt for a habitable planet?
Probably not life like ours. There should be planets, but they were also formed 12-13 billion years ago, before the major enrichment of heavier elements. For your science fiction story, you'd want creatures that don't need much Iron or Calcium (or any other heavier element), probably trapped in a Europa-like ocean environment.

George
2015-Oct-20, 03:15 PM
Probably not life like ours. There should be planets, but they were also formed 12-13 billion years ago, before the major enrichment of heavier elements. For your science fiction story, you'd want creatures that don't need much Iron or Calcium (or any other heavier element), probably trapped in a Europa-like ocean environment. While ToSeeking (defined as running over to Wiki or Google to learn me good), I see from here (http://icc.dur.ac.uk/~tt/Lectures/Galaxies/TeX/lec/node27.html), that about 1/4 of the clusters are labeled metal rich and, not surprisingly, are closer to the galactic plane. These average about 25% of the metals of the Solar system, though a small number of these are the same as the Solar system.

antoniseb
2015-Oct-20, 03:34 PM
While ToSeeking (defined as running over to Wiki or Google to learn me good), I see from here (http://icc.dur.ac.uk/~tt/Lectures/Galaxies/TeX/lec/node27.html), that about 1/4 of the clusters are labeled metal rich and, not surprisingly, are closer to the galactic plane. These average about 25% of the metals of the Solar system, though a small number of these are the same as the Solar system.
There are some current papers indicating that the metal rich globulars in the galactic plane are also the ones with multiple eras of star formation... that these are remnant cores of dwarf galaxies, and not globular clusters in the usual sense. I suppose origin doesn't matter for the purposes of JCoyote's story, the place would still have that same look. For Dave Lee's initial question though, I think that distinction is important.

Dave Lee
2015-Oct-20, 04:31 PM
Can you please elaborate about the meaning of: " Globular clusters are very tightly bound by gravity".
Normally, when we think about gravity, we also think about rotation. Without rotation, What is the meaning of gravity?
So does it mean that the stars in a Globular cluster rotate around each other or around something?
For example - if we just take two stars in an open space and set them close enough (to pull each other by gravity), they could collide. However, without any rotation, I would expect that this collision could break them both to pieces (if there are in a simmilar size). Do you agree?
Normally, two stars could merge to one in a new star forming aria as nebula or the core of spiral galaxy. In those systems the stars rotate around massive core. However, Globular cluster isn't a nebula and there is no supper massive B.H. in the core.
So how could it be that two stars merge to one in Globular cluster?

antoniseb
2015-Oct-20, 05:02 PM
There is still a lot of unfinished science about the way(s) that globular clusters form, but for the sake of following one example, imagine in the early universe, two clouds in the outskirts of a new large galaxy colliding and forming one large star-forming cloud. Imagine that the gasses and dust in this cloud are already within the limits of being gravitationally bound, so the stars that condense out of that cloud are also bound to each other. This is a kind of hand-waving explanation for how such a cluster might form. It doesn't have to be globular at the beginning, but after a few billion years of stars moving inside of it, it surely will end up looking like a globular cluster.

As to how could two stars merge in a globular cluster, the answer is either head-on, or with a slow inward spiral.

korjik
2015-Oct-20, 05:26 PM
Can you please elaborate about the meaning of: " Globular clusters are very tightly bound by gravity".
Normally, when we think about gravity, we also think about rotation. Without rotation, What is the meaning of gravity?
So does it mean that the stars in a Globular cluster rotate around each other or around something?
For example - if we just take two stars in an open space and set them close enough (to pull each other by gravity), they could collide. However, without any rotation, I would expect that this collision could break them both to pieces (if there are in a simmilar size). Do you agree?
Normally, two stars could merge to one in a new star forming aria as nebula or the core of spiral galaxy. In those systems the stars rotate around massive core. However, Globular clusters isn't a nebula and there is no supper massive B.H. in the core.
So how could it be that two stars merge to one in Globular cluster?

Rotation has nothing to do with gravity. It is caused by the conservation of angular momentum. It is the angular momentum that causes orbits. For a group of separate bodies, each body attracts every other body. This averages out to each body orbiting the center of mass of the group.

Stars arent rigid bodies. They are large bodies of gas, and so they will tend to coalesce into one instead of shattering.

Stars collide when there are alot of stars in a very small area. Like in a globular cluster. Even the core of a large galaxy dosent approach the star density of a globular cluster.

Dave Lee
2015-Oct-20, 06:34 PM
Rotation has nothing to do with gravity. It is caused by the conservation of angular momentum. It is the angular momentum that causes orbits. For a group of separate bodies, each body attracts every other body. This averages out to each body orbiting the center of mass of the group.

So, do you mean that each star in a globular cluster orbits the center of mass of the whole group?

antoniseb
2015-Oct-20, 07:21 PM
So, do you mean that each star in a globular cluster orbits the center of mass of the whole group?
Essentially that is the way it works, but the "orbit" isn't strictly elliptical. The gravitational forces guiding the path of the star are more complex than that, but you could treat it as an orbit around the center of mass as your first approximation.

Jeff Root
2015-Oct-20, 09:03 PM
What you are calling "rotation" is probably better called
"revolution". The Sun rotates; the Earth rotates, the Moon
rotates; the Moon revolves around the Earth; the Earth and
Moon together revolve around the Sun, along with the other
planets; the Sun and planets together revolve around the
center of the Galaxy, along with the other stars.

Stars in a globular cluster sort of revolve around the center
of the cluster. On average, that is what they do, but their
orbits tend to be only somewhat elliptical, and vary greatly
in shape and orientation, and those orbits vary over time
as the stars pass one another.

Some stars -- mostly the more massive stars -- stay near
the center of the cluster, while some -- which tend to be
lighter -- stay far out near the edges. Most stars take long,
looping almost-elliptical paths from near the center to near
the periphery. The process in which more massive stars
end up near the center is called "virialization". This also
causes some stars to be thrown out of the cluster, which
results in the cluster becoming more tightly packed due to
the reduced angular momentum of the remaining stars.

The total angular momentum of the individual stars is still
very high, of course, but the average angular momentum
of all the stars considered together as a unit may be close
to zero, which is why the cluster is spherical in shape.

-- Jeff, in Minneapolis

antoniseb
2015-Oct-21, 01:14 AM
Concerning how they formed, this pape (http://arxiv.org/abs/1510.05671)r looks interesting.

JCoyote
2015-Oct-21, 01:41 AM
Actually origin WOULD matter for the purposes of a tv series and the pressures to present something as plausible as possible when constrained by the almighty budget. As many complaints as I see in science forums about rubber forehead aliens and every planet looking like British Columbia, I don't usually see anyone offering up the financing to show something else in an outer space-and-aliens oriented show.

I like the idea of a globular cluster because a mostly sustainable refugee ship, after traveling for at least a hundred years of encountering star systems every month or two... could suddenly be approaching new systems almost every week! ;) Also, with other spacefaring species active in a cluster, the search, negotiatons, and unprecedented level of involvement in regional politics become plot points.

So more heavy metals and terrestrial planets are desired. And remember, most science fictions tv shows don't have so many human looking aliens because the writers lack imagination... they have them because they don't have the cash to have an alternative.

Dave Lee
2015-Oct-21, 02:30 AM
Concerning how they formed, this pape (http://arxiv.org/abs/1510.05671)r looks interesting.

Thanks
http://arxiv.org/pdf/1510.05671v1.pdf
It is stated: "However, how dense structures,such as GCs, form in a cosmological context remains an interesting puzzle."
Hence, could it be that the current simulation and modeling are only hypothetical ideas about how the GC had been formed?

Dave Lee
2015-Oct-21, 09:51 AM
Essentially that is the way it works, but the "orbit" isn't strictly elliptical. The gravitational forces guiding the path of the star are more complex than that, but you could treat it as an orbit around the center of mass as your first approximation.
With regards to the center of mass;
Can we assume that each star has its own unique virtual center of mass?
In this case, each star should revolve around its virtual center of mass without the need to orbit around the center of the GC.
Therefore, the star revolves in a strictly elliptical orbit with regards to its own virtual center of mass. However, for the observer, it might be seen as it isn't strictly elliptical with regards to the center of the GC.
Do you agree?

Hornblower
2015-Oct-21, 11:18 AM
With regards to the center of mass;
Can we assume that each star has its own unique virtual center of mass?
In this case, each star should revolve around its virtual center of mass without the need to orbit around the center of the GC.
Therefore, the star revolves in a strictly elliptical orbit with regards to its own virtual center of mass. However, for the observer, it might be seen as it isn't strictly elliptical with regards to the center of the GC.
Do you agree?
My bold. No.

Dave Lee
2015-Oct-21, 11:30 AM
My bold. No.

Why?

antoniseb
2015-Oct-21, 11:40 AM
Why?
Each star follows its path at any instant accelerated by the sum of the gravitational influences of every other (moving) object in the GC. It changes continuously. Further, as the star goes toward the center of the GC, it is not following a Keplerian orbit anymore because some of the attractive masses are behind it and slowing it down. The pure Kepler-style elliptical orbit is only applicable in the case of the two body problem, and the GC has many more than two bodies to consider.

Hornblower
2015-Oct-21, 01:25 PM
Why?

Let me elaborate a bit. As Antoniseb pointed out, the actual orbits are not simple ellipses. They are bent out of shape because the effective central mass for gravitational acceleration purposes varies as the star moves in and out. My educated guess is that we could use a precessing reference ellipse as a rough first approximation, with all of these having a focus at the same place, that is, the center of mass of the cluster.

01101001
2015-Oct-21, 01:26 PM
Therefore, the star revolves in a strictly elliptical orbit with regards to its own virtual center of mass. However, for the observer, it might be seen as it isn't strictly elliptical with regards to the center of the GC.
Do you agree?

Are you randomly traveling down the same path you took in Random velocities of Solar neighborhood (http://cosmoquest.org/forum/showthread.php?158608-Random-velocities-of-Solar-Neighborhood&p=2316978#post2316978)?

Your questions, and resistance to answers, seem somehow familiar.

Or am I reading your assumptions the wrong way?

BigDon
2015-Oct-21, 02:31 PM
As a small but on-topic side track...

Who else besides myself assumed the people in the Firefly series had colonized an globular cluster?

Fiery Phoenix
2015-Oct-21, 02:58 PM
The total angular momentum of the individual stars is still
very high, of course, but the average angular momentum
of all the stars considered together as a unit may be close
to zero, which is why the cluster is spherical in shape.

-- Jeff, in Minneapolis
Mind elaborating on this one, Jeff? How does the average angular momentum give a cluster its spherical shape? Some kind of balance?

Hornblower
2015-Oct-21, 03:06 PM
Mind elaborating on this one, Jeff? How does the average angular momentum give a cluster its spherical shape? Some kind of balance?

I would say the spherical shape and the net zero angular momentum are concurrent consequences of a spherically symmetrical pattern of stellar motions.

Jeff Root
2015-Oct-21, 07:39 PM
Yeah, zero net angular momentum is closely connected to
the spherical shape but doesn't quite require it.

But when the stars are all in randomly-oriented orbits with
random energies, the angular momenta will very nearly
cancel out to a net angular momentum of zero.

-- Jeff, in Minneapolis

Hornblower
2015-Oct-22, 03:09 AM
Yeah, zero net angular momentum is closely connected to
the spherical shape but doesn't quite require it.

But when the stars are all in randomly-oriented orbits with
random energies, the angular momenta will very nearly
cancel out to a net angular momentum of zero.

-- Jeff, in Minneapolis

Now that I have thought about it some more, yes indeed, I can envision an oblate, prolate or even triaxial ellipsoidal cluster that nevertheless has zero net angular momentum. This would be the result of having a suitable distribution of counterrevolving orbits. I will leave it to the celestial mechanics experts to ponder how it would be formed and whether or not it would evolve toward being spherical.

Dave Lee
2015-Oct-22, 03:55 AM
I'm quite confused. So, let me ask again:
With regards to the Center of mass v.s the center of the GC.
One answer was:

Essentially that is the way it works, but the "orbit" isn't strictly elliptical. The gravitational forces guiding the path of the star are more complex than that, but you could treat it as an orbit around the center of mass as your first approximation.
Other answer was:


Stars in a globular cluster sort of revolve around the center of the cluster.
-- Jeff, in Minneapolis
So please, is it the center of mass or the center of the GC? Is it the same?

korjik
2015-Oct-22, 05:18 AM
I'm quite confused. So, let me ask again:
With regards to the Center of mass v.s the center of the GC.
One answer was:

Other answer was:

So please, is it the center of mass or the center of the GC? Is it the same?

Any orbit is only truly elliptical in a two body system. In that case the orbits are around the center of mass.

When you have a multi body system, the average orbit is around the center of mass, but with all the different bodies interacting there can be alot of perterbation of the elliptical orbit. If two stars have a close approach, there will be a large change in both orbits.

The end result is that on the average the stars orbit the center of mass, and that on the average the center of mass is at the geometric center of the cluster, but both move around a bit due to interactions between individual members of the cluster.

Dave Lee
2015-Oct-22, 05:33 AM
The end result is that on the average the stars orbit the center of mass...

Thanks
So the correct answer is: The Center of mass.
Is there any objections?

Jeff Root
2015-Oct-22, 05:49 AM
The answer to your question is easy and obvious to me,
and probably to most of the readers here. You appear to
be having a somewhat unusually difficult time with it, so
I will try to be as clear and complete as I can. But I am
neither a physicist nor a professional teacher, so don't
assume that my answer is the best you can get.

You know that planets orbit the Sun in elliptical orbits.
Newton figured out why that is. The Sun's gravity pulls
on them in such a way that as they fall toward the Sun
they speed up, and as they rise away from the Sun, they
slow down again. So they can keep going around and
around on these elliptical orbits.

But if there are multiple planets orbiting the Sun, it isn't
just the Sun pulling on the planets-- the planets also pull
on each other. So that the orbits are very close to being
perfect ellipses, but aren't quite. The orbits are slightly
distorted in ways that get very, very complicated very,
very quickly.

In a globular cluster, each star is pulled by the vector sum
of the gravity of all the other stars in the cluster, not just the
gravity of one central star that is bigger and more massive
by far than anything else in the system. That vector sum is
different at every different location in the cluster, and varies
over time at any given location as the stars move around.

So a star inside a globular cluster is constantly pulled in all
different directions, and usually doesn't follow a path that is
all *that* much like an elliptical orbit. It's just *sorta* like
an elliptical orbit.

On the other hand, a star which is outside the cluster, or at
the outermost fringes of the cluster, will see the cluster as
essentially a point source of gravity, just like the Sun is for
planets in the Solar System. The cluster's center of gravity
is the point toward which the vector points from locations
outside the cluster or at the outermost fringes.

If a star at the outermost fringes of a globular cluster is
moving too slowly to be in a circular orbit at that distance
from the center, it will begin to fall toward the center, and
speed up as it falls. After it passes the center it will slow
again as it rises back toward the periphery. So the star
will follow a trajectory through the cluster which only
somewhat resembles an elliptical orbit.

Whether the center of mass of the cluster is the same as
the geometric center of the cluster depends only on how
mass is distributed in the cluster. So if the stars were
distributed in a perfectly symmetrical pattern, the center
of mass would be the same as the geometric center.

-- Jeff, in Minneapolis

Dave Lee
2015-Oct-22, 05:57 AM
Each star follows its path at any instant accelerated by the sum of the gravitational influences of every other (moving) object in the GC. It changes continuously. Further, as the star goes toward the center of the GC, it is not following a Keplerian orbit anymore because some of the attractive masses are behind it and slowing it down. The pure Kepler-style elliptical orbit is only applicable in the case of the two body problem, and the GC has many more than two bodies to consider.


If I understand that message correctly, each star should have a unique center of mass based on its location.
As it is stated:
"as the star goes toward the center of the GC, it is not following a Keplerian orbit anymore because some of the attractive masses are behind it and slowing it down"
Therefore, a star closer to the center of GC will fill different gravity from a one at further location. Therefore, each one should have different center of mass. It might be very minor, but it is still different. However, this center of mass is not constant. It might be considered as an adaptive center of mass, as the star change its location and as all other stars in the GC change their locations too.
Do you agree?

antoniseb
2015-Oct-22, 11:00 AM
.However, this center of mass is not constant. it change as the star change its location and actually as the whole other stars in the GC also change their locations.
Do you agree?
So why even talk about a center of mass if it is always moving, and changing its apparent mass? You don't have simple Kepler-style elliptical orbits that sweep out equal area in equal time, so there is no mathematical advantage to trying to model a center of mass in this situation. It is a many-body situation and you'll want to use a computer to model the motions of the individual stars over time.

korjik
2015-Oct-22, 12:17 PM
If I understand that message correctly, each star should have a unique center of mass based on its location.
As it is stated:
"as the star goes toward the center of the GC, it is not following a Keplerian orbit anymore because some of the attractive masses are behind it and slowing it down"
Therefore, a star closer to the center of GC will fill different gravity from a one at further location. Therefore, each one should have different center of mass. It might be very minor, but it is still different. However, this center of mass is not constant. It might be considered as an adaptive center of mass, as the star change its location and as all other stars in the GC change their locations too.
Do you agree?

The center of mass is the same for all the stars. It is the force on the star that changes

antoniseb
2015-Oct-22, 12:31 PM
The center of mass is the same for all the stars. It is the force on the star that changes
You bring up a good point about terminology. What Dave Lee is asking about appears to be the apparent center of mass about which two bodies orbit in a two-body system. In the globular cluster both the gravitational acceleration of any individual star changes in both magnitude and direction significantly differently from a two-body system's. In an extreme but common example, when one of the stars passes close to another of the stars, the acceleration can briefly be in the opposite direction of the center of gravity.

George
2015-Oct-22, 01:45 PM
It was interesting to learn, if true, that only a two-body system can be directly addressed by equations to yield "exact" results. Three bodies or more require iterations, apparently.

Perhaps understanding the change in position of the c.g. could use a boost. Here (http://www.brighthub.com/science/space/articles/117934.aspx) is a nice illustration of how the barycenter (center of gravity) of the Solar system is constantly on the move. [Click on the small yellow Sun (*cough*) image labeled "Solar System" located a page down in the article.]

Hornblower
2015-Oct-22, 04:16 PM
Let me point out that in an inertial frame of reference, the barycenter is either stationary or moving in a straight line. It is the Sun that is moving in that loopy path, not the barycenter.

George
2015-Oct-22, 11:40 PM
Let me point out that in an inertial frame of reference, the barycenter is either stationary or moving in a straight line. It is the Sun that is moving in that loopy path, not the barycenter.
That's an interesting and good point. I will guess that the path would simply be flipped from the one linked, 180 deg. phase angle, but I haven't given it much thought.

Reality Check
2015-Oct-23, 01:49 AM
So the correct answer is: The Center of mass.
Is there any objections?
A body always moves around the center of mass under gravity so no objection.
This may be of interest: Astroquizzical: Why doesn’t a globular cluster collapse? (https://medium.com/starts-with-a-bang/astroquizzical-why-doesn-t-a-globular-cluster-collapse-422c4b985ec2#.lz7guxjd7)

pzkpfw
2015-Oct-23, 03:20 AM
Thanks
So the correct answer is: The Center of mass.
Is there any objections?


A body always moves around the center of mass under gravity so no objection.
This may be of interest: Astroquizzical: Why doesn’t a globular cluster collapse? (https://medium.com/starts-with-a-bang/astroquizzical-why-doesn-t-a-globular-cluster-collapse-422c4b985ec2#.lz7guxjd7)

He was replying to korjik who wrote (my bold):


... The end result is that on the average the stars orbit the center of mass, and that on the average the center of mass is at the geometric center of the cluster, but both move around a bit due to interactions between individual members of the cluster.

That seemed like it was taking something with "wiggle" and making it "concrete", potentially to draw a desired conclusion ...

Dave Lee
2015-Oct-23, 05:56 AM
A body always moves around the center of mass under gravity so no objection.
This may be of interest: Astroquizzical: Why doesn’t a globular cluster collapse? (https://medium.com/starts-with-a-bang/astroquizzical-why-doesn-t-a-globular-cluster-collapse-422c4b985ec2#.lz7guxjd7)

Thanks
It is great article as it gives some more highlight about the GC.
However, I would like to focus on the GC location vs its shape.
We know that a typical GC could be located at the distance of 130,000 LY from the Milky way core.
In this article it is stated that If the cluster is located near the galaxy, it will look as a long stream of stars, barely detectable in the night sky. At least four such globulars exist in the interior of our own Milky Way.
My questions are as follow:
1.What is the distance of those long stream of stars GC to the core of the Milky way?
2.What is the shape of a GC if it is too far away from the core of the galaxy (Lets say 200,000 or 500,000 Ly away)?
3.Up to what distance from the core we still can find GC?
4.Is there any change in the shape of those far end GC?

chornedsnorkack
2015-Oct-23, 03:14 PM
In this article it is stated that If the cluster is located near the galaxy, it will look as a long stream of stars, barely detectable in the night sky.

No. It is alleged to leave behind an elongated stream of stars. The core still remains.

Dave Lee
2015-Oct-24, 05:11 AM
Some of the Globular Clusters are located directly on the Milky Way Galaxy disc.
For example - Omega Centauri
https://en.wikipedia.org/wiki/Omega_Centauri

"Omega Centauri (ω Cen), or NGC 5139, is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. Located at a distance of 15,800 light-years (4,850 pc), it is the largest globular cluster in the Milky Way galaxy at a diameter of roughly 150 light-years.[10] It is estimated to contain approximately 10 million stars and a total mass equivalent to 4 million solar masses.[11]"
So why the science doesn't distinguish between the different types of globular clusters based on their locations (in/out the disc) and shape?

korjik
2015-Oct-24, 05:50 AM
Because they are Globular, hence the name

chornedsnorkack
2015-Oct-24, 06:43 AM
The orbits of globular clusters in Milky Way, and of stars inside each globular cluster, are usefully compared to comets.
There is little point in classifying comets by location inside or outside zodiac, because they have to cross zodiac twice each orbit. Just because they are in zodiac when we look at them does not mean they stay there.
But their orbits, if known (and in Solar System, unlike Milky Way, they easily become known) are relevant. The distinction between comets like Hale-Bopp (exactly on polar orbit, inclination 89,4 degrees) and Halley (near zodiac - but orbiting opposite the planets, inclination 162,3 degrees) is a useful one.
Omega Centauri is suspected to have last passed Milky Way disc about 25 million years ago - with consequences for the disc, but not for the cluster:
http://www.researchgate.net/publication/24014986_On_the_possible_generation_of_the_young_m assive_open_clusters_Stephenson2_and_BDSB122_by_Om ega_Centauri

Dave Lee
2015-Oct-24, 06:49 AM
Because they are Globular, hence the name

O.K.
So why a long line of stars is also called Globular cluster?
How can we distinguish between a GC on spiral disc to a GC outside the disc?

Dave Lee
2015-Oct-24, 07:11 AM
Omega Centauri is suspected to have last passed Milky Way disc about 25 million years ago - with consequences for the disc, but not for the cluster:
http://www.researchgate.net/publication/24014986_On_the_possible_generation_of_the_young_m assive_open_clusters_Stephenson2_and_BDSB122_by_Om ega_Centauri

However, do we assume that it should come back to the disc?
So it should moves in a sin. Wave- up and down, with related to the galactic disc.
Similar to the Sun path.
Is it correct?

Shaula
2015-Oct-24, 07:41 AM
O.K. So why a long line of stars is also called Globular cluster?
The answer is in the article. It would help if you quoted the piece in full:

The other thing that can cause change in a globular cluster is if its orbit takes it too close to the massive galaxy it sits near. Each globular cluster is, as a whole, orbiting a fairly massive galaxy, particularly in comparison to the cluster. This orbit takes a very long time to complete, and it seems that many globular clusters can successfully orbit the galaxy without getting too near — but if the cluster does get too near, then the cluster will experience an intense tidal force because of the gravitational pull of the galaxy. This tidal force can shear off the outer layers of the cluster (if it’s a mass-segregated cluster, this means it looses the smallest stars). These outer layers get pulled away into a long stream of stars, barely detectable in the night sky even with a powerful telescope, and leaving an even denser nucleus of stars behind; at least four such globulars exist in the interior of our own Milky Way.
So the linear feature you refer to is not called a globular cluster - it is made up of what was stripped away from the globular cluster due to an interaction with the galaxy. The denser core remains behind as the globular cluster. It the globular cluster is entirely torn apart it is no longer a globular cluster.

Dave Lee
2015-Oct-24, 11:02 AM
The answer is in the article. It would help if you quoted the piece in full:
So the linear feature you refer to is not called a globular cluster - it is made up of what was stripped away from the globular cluster due to an interaction with the galaxy. The denser core remains behind as the globular cluster. It the globular cluster is entirely torn apart it is no longer a globular cluster.

Do you mean that:

These outer layers get pulled away into a long stream of stars, barely detectable in the night sky even with a powerful telescope, and leaving an even denser nucleus of stars behind .
is no longer globular cluster?

If so, why it is stated:

at least four such globulars exist in the interior of our own Milky Way.

However, based on your knowlage, if it is not globular, what is the correct name for this cluster?

Shaula
2015-Oct-24, 11:26 AM
Do you mean that: is no longer globular cluster?
If so, why it is stated:
What is left behind is a smaller, denser globular cluster. The stages are outlined in that article:
1) Globular cluster gets close enough to the main galaxy that the tidal forces affect it
2) The outer layers of the globular cluster are stripped away leaving behind a smaller, denser globular cluster and a streak of stars now not part of the globular cluster
3) The globular cluster carries on in its orbit, the stripped away stars follow whatever new orbit the interaction with the galaxy results in - usually forming tails ahead of and behind the globular cluster in its orbit until further interactions disperse them into the general halo.


However, based on your knowlage, if it is not globular, what is the correct name for this cluster?
Which stars are you talking about? The stripped off ones? They are less a cluster than a stellar association. When they are fairly distinct they are sometimes called tidal tails.
See: https://en.wikipedia.org/wiki/Globular_cluster#Tidal_encounters

Dave Lee
2015-Oct-24, 03:15 PM
With regards to those four globulars:
Do you know which clusters are those four globulars? Can you please specify their names?
How far are they located from the galactic center?

Hornblower
2015-Oct-24, 03:50 PM
With regards to those four globulars:
Do you know which clusters are those four globulars? Can you please specify their names?
How far are they located from the galactic center?

The author did not choose to identify them in the article, so we have virtually nothing to guide us in an online search. This is the sort of question for which someone with access to a university astronomy department library could find answers with the help of the faculty and a good reference librarian. Trying to find it with a Google search is like trying to find a needle in a haystack.

StupendousMan
2015-Oct-24, 05:21 PM
Go to ADS:

http://adsabs.harvard.edu/abstract_service.html

Type into the "Abstract Words" box the terms

globular cluster tidal tail milky way

and set the "and" radio button. Then click on "Send Query".

You'll receive a list of many references, in order by relevance (first) and chronological order (second). Read the titles; if promising, read the abstracts; if still promising, read the papers.

Hornblower
2015-Oct-24, 05:43 PM
Go to ADS:

http://adsabs.harvard.edu/abstract_service.html

Type into the "Abstract Words" box the terms

globular cluster tidal tail milky way

and set the "and" radio button. Then click on "Send Query".

You'll receive a list of many references, in order by relevance (first) and chronological order (second). Read the titles; if promising, read the abstracts; if still promising, read the papers.

Thanks for the advice on searching.

Jeff Root
2015-Oct-24, 05:57 PM
I'm saving that advice. Hope I can find it when I need it.

-- Jeff, in Minneapolis

Dave Lee
2015-Oct-24, 08:23 PM
Go to ADS:

http://adsabs.harvard.edu/abstract_service.html

Type into the "Abstract Words" box the terms

globular cluster tidal tail milky way

and set the "and" radio button. Then click on "Send Query".

You'll receive a list of many references, in order by relevance (first) and chronological order (second). Read the titles; if promising, read the abstracts; if still promising, read the papers.

Thanks

Based on your advice I have found one globular cluster with tidal tail in the Milky Way – the Palomar_5:
https://en.wikipedia.org/wiki/Palomar_5
"Palomar 5 is a globular cluster discovered by Walter Baade in 1950.
- in fact there are many stars leaving this cluster in the form of a stellar stream. The stream has a mass of 5000 solar masses and is 30,000 light years long.[6] The cluster is currently 60.6 kly (18.6 kpc) from the Galactic Center. It shows a noticeable amount of flattening, with an aspect ratio of0.62 ± 0.23 between its semimajor axis and semiminor axis.[7]

http://www.obspm.fr/spip.php?page=imprimer&id_article=1855&lang=fr
At a distance of 23.2 kpc from the Sun and of 16.6 kpc above the Galactic plane, Palomar 5 is a halo globular cluster characterized by very low total mass, a large core radius, and low central concentration.

Hence, the Palomar_5, is a globular cluster with tidal tail, which located at the Halo above the galactic plane, at a distance of 60.6 Kly from the galactic center.
However, one of the biggest globular clusters - Omega Centauri, is located at the galactic plane with a distance of less than 30 Kly from the galactic center (based on my verification).
Therefore, the Omega Centauri is much bigger than the Palomar 5, much closer to the galactic center and has no tidal tail. Based on this info, we could easily believe that the biggest GC must be formed at the galactic plane.
Hence, those evidences could contradict the basic idea that the GC is moving inwards to the galaxy as in the following statement:

"This orbit takes a very long time to complete, and it seems that many globular clusters can successfully orbit the galaxy without getting too near — but if the cluster does get too near, then the cluster will experience an intense tidal force because of the gravitational pull of the galaxy. This tidal force can shear off the outer layers of the cluster (if it’s a mass-segregated cluster, this means it looses the smallest stars). These outer layers get pulled away into a long stream of stars, barely detectable in the night sky even with a powerful telescope, and leaving an even denser nucleus of stars behind"
Actually, as Palomar_5 is much smaller globular cluster with tidal tail we could easily believe that it might be a product of a bigger GC which had been drifted from the galactic plane to that location at the Halo.

So, what is the correct answer for that contradiction?

Shaula
2015-Oct-24, 08:40 PM
So, what is the correct answer for that contradiction?
The individual histories of each GC

antoniseb
2015-Oct-24, 08:42 PM
... Therefore, the Omega Centauri is much bigger than the Palomar 5, much closer to the galactic center and has no tidal tail. Based on this info, we could easily believe that the biggest GC must be formed at the galactic plane. ...
Careful how you draw your conclusions. Omega Centauri is thought to the the core of a dwarf galaxy that had previously been stripped of its outer gasses and stars by this tidal process. It is not in the size/mass range of other globular clusters, and should NOT be considered representative of them.
Thanks p for noting the typo.

slang
2015-Oct-24, 10:34 PM
So, what is the correct answer for that contradiction?

Reread post #2 in this thread...

Hornblower
2015-Oct-25, 02:26 AM
My educated guess is that Omega Centauri left a massive tidal tail during a close encounter with the galaxy's core, and that the tail has long since dispersed beyond easy visibility.

Dave Lee
2015-Oct-25, 03:15 AM
Thanks.
I assume that I didn't set my question correctly. I didn't want to ask about my conclusions, but about their conclusions.
So:


The individual histories of each GC

Fully agree.
However, in the article it is stated that in order to set the tail for a GC, it must move inwards to the galaxy. That should create a tidal force which can shear off the outer layers of the cluster and set the tail.
My question is – on which kind of evidences this conclusion is based on?
Why the Tail can't be formed without a tidal force, and without the need to move closer to the galactic center?



My educated guess is that Omega Centauri left a massive tidal tail during a close encounter with the galaxy's core, and that the tail has long since dispersed beyond easy visibility.

So, how could it be that a small GC can keep its tail, while Omega Centauri has no tail. Please remember that it is very close to us. Therefore, if it had a tail, we could see it much more easily than any other further away GC.




Omega Centauri is thought to the the core of a dwarf galaxy that had previously been stripped of its outer gasses and stars by this tidal process. It is not in the size/mass range of other globular clusters, and should NOT be considered representative of them.
Well, based on the article which I have found, Omega Centauri has currently all the features to set it as one of the biggest GC.
So why should we consider it as the core of a dwarf galaxy? It actually complicates the situation. It is quite difficult to understand how Omega Centauri had been formed as a GC. Now we will need to find how it had been formed as a dwarf galaxy and move inwards to its current location in the galactic disc. Do we have an answer for that? Why we can't find its tail, although it should have a massive tail and it is so close to us?

Shaula
2015-Oct-25, 05:32 AM
However, in the article it is stated that in order to set the tail for a GC, it must move inwards to the galaxy. That should create a tidal force which can shear off the outer layers of the cluster and set the tail.
My question is – on which kind of evidences this conclusion is based on?
Why the Tail can't be formed without a tidal force, and without the need to move closer to the galactic center?
The usual evidence, I suspect. Modelling, simulation and some observations. If you are interested in the topic at this level of detail I'd suggest you perform a detailed literature review.

In order to remove the stars from a cluster to form tails some interaction is required. The most common one, that fits the parameters, is a tidal one with the galaxy. There may be other ones (say two clusters interacting).


Why we can't find its tail, although it should have a massive tail and it is so close to us?
What simulation evidence is your statement that it should have a tail based on?

chornedsnorkack
2015-Oct-25, 06:45 AM
However, do we assume that it should come back to the disc?
So it should moves in a sin. Wave- up and down, with related to the galactic disc.
Similar to the Sun path.
Is it correct?
Yes, unless it were completely unbound to Milky Way and on its only passage.
If Milky Way were spherically symmetric, the reasoning would be obvious. Globular cluster like any other body would orbit in a plane, that has to go through the centre of mass distribution of Milky Way. It therefore has to cross any imaginary plane that also goes through Milky Way centre, like the disc plane, twice in orbit.
If it crosses the disc at least once, it has to do so once each orbit. The other crossing of disc plane might be crossing of the imaginary extension of the disc plane, outside the edge of the disc. Or it might also be in disc.
Since Milky Way is actually somewhat oblate, with the disc possessing nonzero mass, the orbiting objects pass disc plane more than twice per orbit.

Noclevername
2015-Oct-25, 11:16 AM
As a small but on-topic side track...

Who else besides myself assumed the people in the Firefly series had colonized an globular cluster?

According to the RPG based on the series, it's a multiple star system. They apparently have some means of heating outer planets and moons without building giant solar collectors.

antoniseb
2015-Oct-25, 11:40 AM
... Well, based on the article which I have found, Omega Centauri has currently all the features to set it as one of the biggest GC.
So why should we consider it as the core of a dwarf galaxy? It actually complicates the situation. It is quite difficult to understand how Omega Centauri had been formed as a GC. Now we will need to find how it had been formed as a dwarf galaxy and move inwards to its current location in the galactic disc. Do we have an answer for that? Why we can't find its tail, although it should have a massive tail and it is so close to us?
Based on one article you found you've decided not to listen to what we are telling you? As to the question of how Omega Centauri formed as a dwarf galaxy, models show that galaxies the size of the Milky Way should have nearly a hundred dwarf galaxy satellites. We've found dozens so far, and some are very small and dim. They orbit the Milky Way, just as Omega Centauri does. The ones that pass through the spiral arms in their orbits get stripped of gasses and outer stars. Several of the outer satellite dwarfs do have tidal tails, but OCent has passed through often enough that the easy pickings have long ago been picked.

chornedsnorkack
2015-Oct-25, 12:16 PM
We've found dozens so far, and some are very small and dim. They orbit the Milky Way, just as Omega Centauri does. The ones that pass through the spiral arms in their orbits get stripped of gasses and outer stars.
What strips the outskirts of globular clusters/galaxies? Passage through spiral arms, passage through disc between arms - or passage through Milky Way core?

Hornblower
2015-Oct-25, 01:26 PM
What strips the outskirts of globular clusters/galaxies? Passage through spiral arms, passage through disc between arms - or passage through Milky Way core?
Passage through the disk means encountering gas in the disk, so that any gas in the cluster will be impeded in its orbital motion while the stars just keep going. The relatively weak gravitational gradients in and near the disk will scatter weakly bound outer stars from the cluster. These effects will be somewhat stronger in the spiral arms, where the galactic density is somewhat greater than in other parts of the disk. A close encounter with the massive core of the large galaxy will do more violence to the more tightly bound inner parts of the cluster.

chornedsnorkack
2015-Oct-25, 01:48 PM
Passage through the disk means encountering gas in the disk, so that any gas in the cluster will be impeded in its orbital motion while the stars just keep going.

Or else the gas in the disc/arms is seized by the cluster?

Hornblower
2015-Oct-25, 02:11 PM
Or else the gas in the disc/arms is seized by the cluster?
No, I would expect the cluster gas and a corresponding sample of disk gas to be smeared out while trailing the cluster stars, because of conservation of momentum. The cluster's stars would continue in their orbit and leave the gas behind.

Jeff Root
2015-Oct-25, 02:46 PM
A couple of ideas I've picked up and think are true, and are
relevant if they are true:

The density of massive, bright stars in a globular cluster or
galactic core is so great that the combined stellar wind should
blow away all the gas and dust.

The density of stars between spiral arms is virtually identical
to the density of stars in the arms.

-- Jeff, in Minneapolis

Hornblower
2015-Oct-25, 04:48 PM
It has been asked why Palomar 5 has a detectable tidal tail while Omega Centauri does not. My hypothesis is that Palomar 5 was recently deflected into harm's way by a close encounter with another cluster or perhaps a passing dwarf galaxy near the apoapsis of its orbit, after billions of years of uneventful orbiting well away from troublesome gravitational gradients. This tail is a transient feature that should disperse and fade into the general stellar population of the galaxy over the coming eons. If the reputed dwarf-galaxy progenitor of Omega Centauri got into harm's way several billion years ago, there has been plenty of time for its tail to disperse that way. The dense core that remains is so tightly bound that proportionately very little stripping has occurred since then. If I understand the linked article correctly, Palomar 5 is far less dense and thus more susceptible to tidal disruption.

cjameshuff
2015-Oct-25, 06:00 PM
It was interesting to learn, if true, that only a two-body system can be directly addressed by equations to yield "exact" results. Three bodies or more require iterations, apparently.

Perhaps understanding the change in position of the c.g. could use a boost. Here (http://www.brighthub.com/science/space/articles/117934.aspx) is a nice illustration of how the barycenter (center of gravity) of the Solar system is constantly on the move. [Click on the small yellow Sun (*cough*) image labeled "Solar System" located a page down in the article.]

There is no general solution for systems of 3 or more bodies. There are solutions for certain specific configurations, but in general, you have to fall back on numerical integration to compute the motions. Kepler's laws are limited to two-body systems, and can only ever apply in very approximate ways to things like galactic orbits.

As for the tidal tails...I'm not sure what the problem is there. The only contradictions seem to be due to various unfounded assumptions, and things that I really don't understand why Dave Lee sees as problems...apparently finding it remarkable that a dwarf galaxy colliding with the Milky Way can at some point be within the disk, for example. And then there's a really wild leap to a conclusion that Omega Centauri formed in the disk and somehow drifted away from it?

Dave Lee, can you clarify what exactly it is you're trying to ask here?

Hornblower
2015-Oct-26, 01:04 AM
By the way, Omega Centauri is some 5,000 lightyears north of the disk plane, which puts it in the sparse region where the thick disk fades into the halo.

Jeff Root
2015-Oct-26, 02:46 AM
I know of only one relatively detailed illustration showing the
Milky Way face-on since they realized it is a barred spiral, and
that illustration has appeared in lots of places. Does it include
Omega Centauri? I've never seen it labeled as such.

-- Jeff, in Minneapolis

Hornblower
2015-Oct-26, 12:53 PM
Here is a nice one as viewed from the north side, with galactic longitude and distance from the Sun in the plane shown in polar coordinates.
http://i0.wp.com/www.universetoday.com/wp-content/uploads/2014/10/Milky_Way_Annotated732X520.jpg
By plotting the cluster's position on my Norton's Star Atlas galactic chart I find it about longitude 308 degrees and latitude 16 degrees. With its published distance from us as 15,800 lightyears, I find it to be about 4,350 lightyears above a point in the plane that is about 15,200 lightyears from the Sun, at the aforementioned longitude. Plotting it on this image should be straightforward.

antoniseb
2015-Oct-26, 01:17 PM
Here is a nice one as viewed from the north side, with galactic longitude and distance from the Sun in the plane shown in polar coordinates.
http://i0.wp.com/www.universetoday.com/wp-content/uploads/2014/10/Milky_Way_Annotated732X520.jpg
By plotting the cluster's position on my Norton's Star Atlas galactic chart I find it about longitude 308 degrees and latitude 16 degrees. With its published distance from us as 15,800 lightyears, I find it to be about 4,350 lightyears above a point in the plane that is about 15,200 lightyears from the Sun, at the aforementioned longitude. Plotting it on this image should be straightforward.
On the scale of that image, Omega Centauri would be pretty small. I'm not sure it would stand out.

chornedsnorkack
2015-Oct-26, 07:10 PM
On the scale of that image, Omega Centauri would be pretty small. I'm not sure it would stand out.

From which distance along Milky Way axis would Milky Way look like this?

antoniseb
2015-Oct-26, 07:37 PM
From which distance along Milky Way axis would Milky Way look like this?
I'm not sure I know what you mean. This is an artists rendering which probably implies a projection as seen from a very long way off (much more than a million light years), but then it would need to be magnified and light amplified to see it like that. We are seeing something about 100,000 light years across. Omega Centauri is about 150 light years across, so if the image were 600 pixels wide, OC would take up one pixel.

chornedsnorkack
2015-Oct-27, 05:00 AM
What is the ratio of surface luminosity of Milky Way disc between arms to the surface luminosity of arms?

StupendousMan
2015-Oct-27, 02:57 PM
What is the ratio of surface luminosity of Milky Way disc between arms to the surface luminosity of arms?

NGC 6674 has been mentioned as a "twin" of our Milky Way. There are plenty of images of it in different places. Why not grab one and measure it yourself?

Hornblower
2015-Oct-27, 03:08 PM
NGC 6674 has been mentioned as a "twin" of our Milky Way. There are plenty of images of it in different places. Why not grab one and measure it yourself?

The images may or may not accurately display the actual relative surface brightness levels.

antoniseb
2015-Oct-27, 03:54 PM
What is the ratio of surface luminosity of Milky Way disc between arms to the surface luminosity of arms?
Is this question somehow connected to Dave Lee's questions about Globular Clusters?

George
2015-Oct-27, 04:11 PM
There is no general solution for systems of 3 or more bodies. There are solutions for certain specific configurations, but in general, you have to fall back on numerical integration to compute the motions. Kepler's laws are limited to two-body systems, and can only ever apply in very approximate ways to things like galactic orbits. Is "numerical integration" synonymous with "iterative" for orbital solutions?

chornedsnorkack
2015-Oct-27, 05:00 PM
Trying to figure out how the brightness of Omega Centauri compares to surface brightness of Milky Way disc seen along axis.
Omega Centauri apparent brightness +3,9, distance 4,8 kpc, so absolute brightness -9,6 by my reckoning. As the name shows, catalogued as star.
47 Tucanae apparent brightness +4,9, distance 5,1 kpc, so absolute brightness -8,6. Also catalogued as star.
On the Milky Way map, I see a number of dots in interarm spaces - all of them blue. What are they? Individual young massive stars? Open clusters? Globular clusters?

47 Tucanae is much less bright than Small Magellanic Cloud - but much more concentrated. Sometimes easier to see.

How big a piece of Milky Way disc is as bright as Omega Centauri?
Is Omega Centauri 4000 lightyears from a spot on an arm, or from interarm gap?
Looking along Milky Way axis, could Omega Centauri be spotted as a point source of light against dimmer background, even if it is not resolved to an extended object?

antoniseb
2015-Oct-27, 06:00 PM
Trying to figure out how the brightness of Omega Centauri compares to surface brightness of Milky Way disc seen along axis.... could Omega Centauri be spotted as a point source of light against dimmer background, even if it is not resolved to an extended object?
So the answer is yes... by something like Hubble. Rather than look at this manufactured image of what the Milky Way probably looks like, why not look for images of nearby galaxies that have globular clusters around them. Without a lot of magnification, they look (disappointingly) like stars. Without some expert pointing them out you'd never have your eye drawn to them. Even a giant pseudo GC (former dwarf-galaxy core) like Omega Centauri) looks small and star-like in such images. See for example this item (http://www.researchgate.net/publication/1787122_Hubble_Space_Telescope_Observations_of_Sta r_Clusters_in_M101) about M101, and the candidates to be globular clusters around it.

ngc3314
2015-Oct-27, 06:17 PM
ngc 6674 has been mentioned as a "twin" of our milky way


NGC 6744?

http://saraobservatory.org/wp-content/uploads/2013/10/ngc6744.jpg

chornedsnorkack
2015-Oct-27, 06:30 PM
Without a lot of magnification, they look (disappointingly) like stars. Without some expert pointing them out you'd never have your eye drawn to them. Even a giant pseudo GC (former dwarf-galaxy core) like Omega Centauri) looks small and star-like in such images.
How do they compare with stars?
Eta Carinae, at absolute magnitude -8,6, is as bright as 47 Tucanae.
How many stars brighter than -8 exist in Milky Way? How many globular clusters meeting that limit?

antoniseb
2015-Oct-27, 07:23 PM
How do they compare with stars?
Eta Carinae, at absolute magnitude -8,6, is as bright as 47 Tucanae.
How many stars brighter than -8 exist in Milky Way? How many globular clusters meeting that limit?
Most globular clusters are about 10,000 stars, and are collectively 10,000 times as bright as the Sun (so abs. mag. -4). There are a good number of individual stars brighter than that. If you are asking because you want to know if globulars are easily discriminated from stars, the answer is that it takes magnification, long collection times, and spectroscopy, but yes you can tell the difference.

George
2015-Oct-27, 07:38 PM
Most globular clusters are about 10,000 stars, and are collectively 10,000 times as bright as the Sun (so abs. mag. -4). Did you mean to say -5 or so?

antoniseb
2015-Oct-27, 07:52 PM
Did you mean to say -5 or so?
No, that's not what I meant, but I was wrong. Thanks for catching it.

chornedsnorkack
2015-Oct-27, 08:08 PM
Most globular clusters are about 10,000 stars, and are collectively 10,000 times as bright as the Sun (so abs. mag. -4). There are a good number of individual stars brighter than that. If you are asking because you want to know if globulars are easily discriminated from stars, the answer is that it takes magnification, long collection times, and spectroscopy, but yes you can tell the difference.

In young galaxies, like Milky Way, besides globular clusters and individual stars, the point sources of light also include open clusters, right?
Even if a young open cluster is much less massive than a globular cluster, the globular cluster is relatively dim because most of its stars are low mass dwarfs. The brightest stars in globular clusters are normally red giants, small fraction of all stars, and even individually less bright than red, yellow or white supergiants.
A young open cluster might have much smaller actual mass, and even smaller star count, but containing several massive young stars, like main sequence white and blue dwarfs, and white and red supergiants would add up to a lot of light.

By contrast, in an elliptical galaxy, neither young massive stars nor open clusters thereof exist. Therefore everything in an elliptical galaxy that looks like a star isn´t - it is a globular cluster. Correct?

antoniseb
2015-Oct-27, 08:48 PM
... everything in an elliptical galaxy that looks like a star isn´t - it is a globular cluster. Correct?
Again, I'm not sure where you're going with this. We were talking about the Milky Way, and no suddenly not the Milky Way.
So here's the difference. Most globular clusters are 12-13 billion years old, and so are all of the stars in them. Mostly dwarfs as you say with the occasional temporary evolved giant.
Elliptical galaxies might have ended star-forming much more recently, and have a higher number of evolved giants, and otherwise fairly bright stars... but yes, nothing like Eta Carina, and yes, no bright young open clusters.
If you are saying that in spiral galaxies it might be difficult to tell a globular from a bright star, but in an elliptical it is easier. Yes, I think you're right... but it still takes some spectroscopy and magnification to be certain.

cjameshuff
2015-Oct-27, 10:28 PM
Is "numerical integration" synonymous with "iterative" for orbital solutions?

No. Iteration is an implementation detail, numerical integration isn't necessarily iterative. It might be that all reasonable ways of numerically computing orbital trajectories are iterative, but I wouldn't guarantee it, and iteration doesn't mean numerical integration.

antoniseb
2015-Oct-28, 01:11 AM
Not especially important to questions so far, but because there is curiosity about these things: http://arxiv.org/abs/1510.07834

George
2015-Oct-30, 01:25 PM
No. Iteration is an implementation detail, numerical integration isn't necessarily iterative. It might be that all reasonable ways of numerically computing orbital trajectories are iterative, but I wouldn't guarantee it, and iteration doesn't mean numerical integration. Thanks. I ask because I recall hearing that iteration is the way it is done, but I don't recall who said it.

chornedsnorkack
2015-Oct-31, 07:44 AM
How big are the interarm distances of Milky Way?