M13 Globular Cluster (APOD 2009 June 17)

[smitty]

It appears to be pretty well accepted that there are black holes at the center of many spiral galaxies, but does anybody know (or has there been educated speculation about) what is at the center of a typical globular cluster? Is it a black hole? Other? And while I'm asking, are there any known reasons to suspect that planets would be more likely or less likely to form around stars in a globular cluster, as opposed to around stars in in a spiral galaxy?

[bystander]

http://apod.nasa.gov/apod/ap090617.html

Globular clusters are in galaxies. M13 happens to be in the same spiral galaxy that spawned Earth, the Milky Way. Elliptical, lenticular, irregular, and spiral galaxies can all contain globular and open clusters, as well as planets and black holes. Black holes are thought to exist at the center of M15 and the Mayall II cluster in Andromeda, but this doesn't seem to be the norm.

[Chris Peterson]

It appears to be pretty well accepted that there are black holes at the center of many spiral galaxies, but does anybody know (or has there been educated speculation about) what is at the center of a typical globular cluster? Is it a black hole? Other? And while I'm asking, are there any known reasons to suspect that planets would be more likely or less likely to form around stars in a globular cluster, as opposed to around stars in in a spiral galaxy?

With a few possible (unconfirmed) exceptions, globular clusters don't contain massive black holes.

Planetary systems are probably less likely in globular clusters simply because the star density is high enough that gravitational perturbations and tidal effects will destabilize the orbits of any planets that form. Since the stars in globular clusters are very old, it is likely that most planetary systems that formed with the stars have long since been disrupted- especially in the denser central regions.

It's also likely that conditions for planet formation were poor to begin with. These are low-metallicity stars, meaning that they probably formed from little more than hydrogen and helium gas clouds, not the dusty gas clouds that we see later stars forming from. I don't think the dynamics of planet formation are very well understood in a gas-only environment.

[Doum]

I'll add to that that many nova and may be a few supernova might have keep the 150 light year radius of M13 sterile.

[neufer]

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<<Explanation: In 1714, Edmond Halley noted that M13 "shows itself to the naked eye when the sky is serene and the Moon absent." Of course, M13 is now modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. A system of stars numbering in the hundreds of thousands, it is one of the brightest globular star clusters in the northern sky. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter, but approaching the cluster core over 100 stars would be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. This stunning view of the cluster combines recent telescopic images of the cluster's dense core with digitized photographic plates recorded between 1987 and 1991 using the Samuel Oschin Telescope, a wide-field survey instrument at Palomar Observatory. The resulting composite highlights both inner and outer reaches of the giant star cluster. Among the distant background galaxies also visible, NGC 6207 is above and to the left of M13 [and K2 orange giant star HD 150998 to the left of M13]>>

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<<Spiral Galaxy NGC 6207 in Hercules is very near globular cluster M-13, and it is a favorite of visual observers using 8 inch or larger scopes. There is a bright Milky Way star superimposed near the center. This galaxy has complex knotty spiral arms, faint outer arms, and a bright central lens without a definite nucleus.


Supernova 2004 A: Type: II
Magnitude 15.7 at discovery
The supernova is magnitude 17.64 in the Kopernik image, taken on June 24, 2004.
The expansion velocity is about 12,000 km/s.>>

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HD 149026 has a transiting hot Jupiter planet and is one of the most prominent and studied.

<<HD 149026 is a yellow subgiant star approximately 257 light-years away in the constellation of Hercules. The star is thought to be much more massive, larger, and brighter than the Sun. In 2005 they discovered an unusual extrasolar planet orbiting the star. The planet, designated HD 149026 b, was detected transiting the star allowing its diameter to be measured. It was found to be smaller than other known transiting planets, meaning the planet is unusually dense for a closely-orbiting giant planet. The temperature of the giant planet is calculated to be 3,700°F (2,040° C), generating so much infrared heat that it glows. >>

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<<Hercules was one of the 48 constellations listed by the 1st century astronomer Ptolemy, and it remains one of the 88 modern constellations today. It is the fifth largest of the modern constellations. The constellation was not always identified with Heracles/Hercules. In earlier times, for example in the Rudolphine Tables, an alternative Greek name was Engonasin, meaning "on his knees" or "the Kneeler".

Mu Herculis is 27.4 light years from Earth. The solar apex, i.e., the point on the sky which marks the direction that the Sun is moving in its orbit around the center of the Milky Way, is located within Hercules, close to Vega in neighboring Lyra.>>

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Arecibo M13 (1974) radio signal has already passed Mu Herculis.
Coordinates: 17h 46m 27.5s, +27° 43′ 14″

Arecibo M13 (1974) radio signal has gone 1/8th of the way to hot Jupiter planet of HD 149026
Coordinates: 16h 30m 29.619s, +38° 20′ 50.31″

Arecibo M13 (1974) radio signal has gone 1/52nd of the way to K2 giant star HD 150998
Coordinates: 16h 43m 04.22s, +36° 30′ 40″

Arecibo M13 (1974) radio signal has gone 1/750th of the way to M13.
Coordinates: 16h 41m 41.44s, +36° 27′ 36.9″

Arecibo M13 (1974) radio signal has gone less than a millionth of the way to NGC 6207.
Coordinates: 16h 43m 04.3ss, +36° 49' 59"

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"In the deepest sense the search for extraterrestrial intelligence is a search for ourselves."

<<The communication of quite complex information is not very difficult, even for civilizations with extremely different biologies and social conventions. For example, arithmetical statements can be transmitted, some true and some false, and in such a way it becomes possible to transmit the ideas of true and false concepts which might otherwise seem extremely difficult to communicate.

But by far the most promising method is to send pictures. The message might consist of an array of zeros and ones transmitted as long and short beeps, or tones on two adjacent frequencies, or tones at different amplitudes, or even signals with different radio polarizations. Properly arranged in rows and columns, the zeros and ones form a visual pattern - a picture similar to those an imaginative typist can create by using the letters of the alphabet as a medium. Just such a message was transmitted to space by the Arecibo Observatory, which Cornell University runs for the National Science Foundation, in November 1974 at a ceremony marking the resurfacing of the Arecibo dish the largest radio/radar telescope on Earth. The signal was sent to a collection of stars called M13, a globular cluster comprising about a million separate suns, because it was overhead at the time of the ceremony. Since M13 is 24,000 light years away, the message will take 24,000 years to arrive there. If anyone is listening, it will be 48,000 years before we receive a reply. The Arecibo message was clearly not intended as a serious attempt at interstellar communication, but rather as an indication of the remarkable advances in terrestrial radio technology.

The decoded message forms a kind of pictogram that says something like this: "Here is how we count from one to ten. Here are five atoms that we think are interesting or important: hydrogen, carbon, nitrogen, oxygen and phosphorus. Here are some ways to put these atoms together that we think interesting or important - the molecules thymine, adenine, guanine and cytosine, and a chain composed of alternating sugars and phosphates. These molecular building blocks are put together to form a long molecule of DNA comprising about four billion links in the chain. The molecule is a double helix. In some way this molecule is important for the clumsy looking creature at the center of the message. That creature is 14 radio wavelengths or 5 feet 9.5 inches tall. There are about four billion of these creatures on the third plant from our star. There are nine planets altogether, four big ones toward the outside and one little one at the extremity. This message is brought to you courtesy of a radio telescope 2,430 wavelengths or 1,004 feet in diameter. Yours truly." Especially with many similar pictorial messages, each consistent with and corroborating the others, it is very likely that almost unambiguous interstellar radio communication could be achieved even between two civilizations which have never met. Of course our immediate objective is not to send such messages, because we are very young and backward; we wish to listen.>>

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"Very old & backward" Art Neuendorffer

[zbvhs]

So how do globular clusters work? Are the stars orbiting a common barycenter at the gravitational center of the cluster? If that works for globular clusters, why would black holes be needed at the centers of normal galaxies?

[neufer]

So how do globular clusters work? Are the stars orbiting a common barycenter at the gravitational center of the cluster? If that works for globular clusters, why would black holes be needed at the centers of normal galaxies?

It works and black holes are NOT needed at the centers of normal galaxies.

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.         King Lear > Act II, scene IV
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KING LEAR: O, reason not the need: our basest beggars
.   Are in the poorest thing superfluous:
.   Allow not nature more than nature needs,
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[neufer]

I'll add to that that many nova and may be a few supernova might have keep the 150 light year radius of M13 sterile.

Assuming that most life in the universe is bacterial-like and that it develops beneath the surface of earth like planets I don't think this is a problem.

Gold's "Deep-hot biosphere" model

<<In the 1970s, Thomas Gold proposed the theory that life first developed not on the surface of the Earth, but several kilometers below the surface. The discovery in the late 1990s of nanobes (filamental structures that are smaller than bacteria, but that may contain DNA) in deep rocks might be seen as lending support to Gold's theory. It is now reasonably well established that microbial life is plentiful at shallow depths in the Earth, up to 5 kilometres (3.1 mi) below the surface, in the form of extremophile archaea, rather than the better-known eubacteria (which live in more accessible conditions). It is claimed that discovery of microbial life below the surface of another body in our solar system would lend significant credence to this theory. Thomas Gold also asserted that a trickle of food from a deep, unreachable, source is needed for survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct. Gold's theory is that flow of food is due to out-gassing of primordial methane from the Earth's mantle; more conventional explanations of the food supply of deep microbes (away from sedimentary carbon compounds) is that the organisms subsist on hydrogen released by an interaction between water and (reduced) iron compounds in rocks.>>

[smitty]

Wow. I'd like to thank everybody who responded to my question for providing such helpful, informative replies, and for expanding the scope of the discussion in such interesting ways! Cheers to all!
Smitty

[Chris Peterson]

So how do globular clusters work? Are the stars orbiting a common barycenter at the gravitational center of the cluster?

Essentially, yes. It's going to be a highly perturbed environment, though, so I wouldn't expect anything like stable, elliptical orbits.

If that works for globular clusters, why would black holes be needed at the centers of normal galaxies?

What do you mean by "needed"? The supermassive black holes found in the center of most galaxies are somehow related to the galaxy formation process- in a way not understood yet (sort of a chicken and egg problem). Dynamically, they are not important for the evolved galaxy. They represent only a tiny fraction of the galactic mass. Presumably you could remove the central black hole from a galaxy and that galaxy would continue on substantially unaltered in terms of its structure and rotation.

[neufer]

So how do globular clusters work? Are the stars orbiting a common barycenter at the gravitational center of the cluster?

Essentially, yes. It's going to be a highly perturbed environment, though, so I wouldn't expect anything like stable, elliptical orbits.

Actually, one might well expect quasi-stable, quasi-elliptical orbits with the center (not the foci) of the ellipses being at the center of the cluster.

[Chris Peterson]

Actually, one might well expect quasi-stable, quasi-elliptical orbits with the center (not the foci) of the ellipses being at the center of the cluster.

Why? With millions of bodies involved, that isn't obvious to me. Or maybe I should ask, how do you quantify "quasi"?

Certainly the orbits are locally elliptical, but I don't know what a complete orbit looks like, or several complete orbits. And I'm not suggesting the system is entirely chaotic, or it would not survive at all. Clearly, the system is well bounded. But I wonder just how deterministic it is.

[neufer]

Actually, one might well expect quasi-stable, quasi-elliptical orbits with the center (not the foci) of the ellipses being at the center of the cluster.

Why? With millions of bodies involved, that isn't obvious to me. Or maybe I should ask, how do you quantify "quasi"?

Certainly the orbits are locally elliptical, but I don't know what a complete orbit looks like, or several complete orbits. And I'm not suggesting the system is entirely chaotic, or it would not survive at all. Clearly, the system is well bounded. But I wonder just how deterministic it is.

I'm picturing a galactic cluster ball of roughly constant density emulating the harmonic oscillator gravitational potential well interior to our own earth.

I'm also assuming that with an average star separation of ~ 45,000 AU that the mean free path for binary collisions will be > 10,000 light years.

[Chris Peterson]

I'm picturing a galactic cluster ball of roughly constant density emulating the harmonic oscillator gravitational potential well interior to our own earth.

Well, a globular doesn't have anything close to a uniform density (which is what I assume you mean by "constant density"). The Wikipedia article on the matter states:

The results of N-body simulations have shown that the stars can follow unusual paths through the cluster, often forming loops and often falling more directly toward the core than would a single star orbiting a central mass. In addition, due to interactions with other stars that result in an increase in velocity, some of the stars gain sufficient energy to escape the cluster. Over long periods of time this will result in a dissipation of the cluster, a process termed evaporation. The typical time scale for the evaporation of a globular cluster is 1e10 years.

The reference for this looks credible, but is a book so I can't check much online.

I'm also assuming that with an average star separation of ~ 20,000 AU that the mean free path for binary collisions will be > 1,000 light years.

I would expect collisions in a globular cluster to be rare to the point of not happening. I actually did the math for that some time back. Is that what you are referring to, or are "binary collisions" something else?

[neufer]

I'm picturing a galactic cluster ball of roughly constant density
emulating the harmonic oscillator gravitational potential well interior to our own earth.

Well, a globular doesn't have anything close to a uniform density (which is what I assume you mean by "constant density").

I'm thinking along the lines of a 3D Gaussian distribution for which a uniform density sphere is a reasonable approximation.
(The 3D Gaussian distribution would come about from random "collisions/interactions" over billions of years.)

The Wikipedia article on the matter states:

The results of N-body simulations have shown that the stars can follow unusual paths through the cluster, often forming loops and often falling more directly toward the core than would a single star orbiting a central mass. In addition, due to interactions with other stars that result in an increase in velocity, some of the stars gain sufficient energy to escape the cluster. Over long periods of time this will result in a dissipation of the cluster, a process termed evaporation. The typical time scale for the evaporation of a globular cluster is 1e10 years.

The reference for this looks credible, but is a book so I can't check much online.

I'm simply referring to a dozen or so orbits over ~10 million years during which any individual star would have a quasi-stable quasi-elliptical orbit with the ellipse center being the center of the cluster. What happens over ~10 billion years is a whole different matter.

I'm also assuming that with an average star separation of ~ 45,000 AU that the mean free path for binary collisions will be > 10,000 light years.

I would expect collisions in a globular cluster to be rare to the point of not happening. I actually did the math for that some time back. Is that what you are referring to, or are "binary collisions" something else?

I'm definitely NOT referring to actual rare physical collisions which sometimes result in blue stragglers.

I'm referring to Rutherford scattering where two stars approach close enough that the gravitational forces will noticeably deflect their original trajectories. I would think that this would require stars to pass each other within a few AU of each other.

[Chris Peterson]

I'm thinking along the lines of a 3D Gaussian distribution for which a uniform density sphere is a reasonable approximation.

Well, the radial density profile is determined by the self gravity of the cluster, and varies over about 1000:1 from the core to the edges. I don't see anything Gaussian here, or anything that can be reasonably approximated by a uniform density sphere. Perhaps I'm just not understanding your approach.

I'm simply referring to a dozen or so orbits over ~10 million years during which any individual star would have a quasi-stable quasi-elliptical orbit with the ellipse center being the center of the cluster. What happens over ~10 billion years is a whole different matter.

The reference addressed two things, individual odd orbits and long term evaporation. The first is really at issue here. If your "quasi" means just a few orbits, then you may be right. But I don't have any sense one way or the other if that's really the case.

I'm referring to Rutherford scattering where two stars approach close enough that the gravitational forces will noticeably deflect their original trajectories. I would think that this would require stars to pass each other within a few AU of each other.

I don't think so. The mean distance between stars near the core of a globular is just a few thousand AU, and for stars that are quite near their escape velocity already (as many will be) you only need the tiniest transfer of angular momentum to significantly shift the orbit. I've seen calculations showing that our own sun's galactic orbit has probably been altered many times in the past by interactions with stars light years away.

[apodman]

The reason for the low abundance of unusual blue straggler stars in M13 remains unknown.

Thanks for the oxymoron. Is this from the same people who brought us jumbo shrimp? Does a low abundance simply mean a low number, or does it mean an large number (an abundance) that is not as great as a medium or high abundance? Are we being told that there are many or not many blue straggler stars? And compared with what? Compared with theory? Compared with other globular clusters? I would give a small fortune to know.

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M3 has a specific frequency of blue stragglers 3 times larger than that of M13.

Okay, that partly answers my question.

[neufer]

I'm thinking along the lines of a 3D Gaussian distribution for which a uniform density sphere is a reasonable approximation.

Well, the radial density profile is determined by the self gravity of the cluster, and varies over about 1000:1 from the core to the edges. I don't see anything Gaussian here, or anything that can be reasonably approximated by a uniform density sphere. Perhaps I'm just not understanding your approach.

Where do you get the 1000:1 from

If the relative gravitational (potential?) was that weak at the edges it would be evaporating stars like crazy!

And even if it were 1000:1 (which it isn't) then the velocities of the stars at the core would be 31 times faster
and hence they would stick around for only 1/30th the time ... resulting in low densities not high.

I'm simply referring to a dozen or so orbits over ~10 million years during which any individual star would have a quasi-stable quasi-elliptical orbit with the ellipse center being the center of the cluster. What happens over ~10 billion years is a whole different matter.

The reference addressed two things, individual odd orbits and long term evaporation. The first is really at issue here. If your "quasi" means just a few orbits, then you may be right. But I don't have any sense one way or the other if that's really the case.

1) What makes you think that the planets in our own solar system are elliptical or stable in a time frame of ~10 billion years?

2) Where would a galactic cluster get the energy to totally "evaporate"
Any "evaporating" stars would "evaporatively cool" the residual cluster and, hence, cause it to shrink.

I'm referring to Rutherford scattering where two stars approach close enough that the gravitational forces will noticeably deflect their original trajectories. I would think that this would require stars to pass each other within a few AU of each other.

I don't think so. The mean distance between stars near the core of a globular is just a few thousand AU, and for stars that are quite near their escape velocity already (as many will be) you only need the tiniest transfer of angular momentum to significantly shift the orbit. I've seen calculations showing that our own sun's galactic orbit has probably been altered many times in the past by interactions with stars light years away.

Even if the mean distance between stars near the core of a globular is just a few thousand AU (and it isn't)
the core velocities would be so fast that stars would need to pass much closer than 1 AU to significantly affect the orbit.

If there is no central black hole then THERE IS NO CORE
There is just a uniform homogeneous gas of stars.

Forget your old ideas about a galactic cluster "core" and think in terms
of a galactic gaseous star condensate with a Boltzmann velocity distribution.

[Boner]

Okay. In layman terms. Does anyone know how far apart the stars are in cluster? And do they orbit a a central object? Is the cluster expanding, compacting or static? I would assume that they don't move in opposition to each other? I started reading the replies but most of them went over my head.

[Chris Peterson]

Well, the radial density profile is determined by the self gravity of the cluster, and varies over about 1000:1 from the core to the edges. I don't see anything Gaussian here, or anything that can be reasonably approximated by a uniform density sphere. Perhaps I'm just not understanding your approach.

Where do you get the 1000:1 from
The star density at the outside of a globular is less than 0.1 stars per cubic parsec; in the core the star density can be 1000 stars per cubic parsec or greater. So that's actually a density variation of 10,000 to 1.

If the relative gravitational (potential?) was that weak at the edges it would be evaporating stars like crazy!

Why? The stars further out have a lower escape velocity, of course, but that doesn't mean they'll just float away. It's like the Oort Cloud- the escape velocity out there is next to nothing, but there's no sign the cloud is evaporating at any significant rate.

And even if it were 1000:1 (which it isn't) then the velocities of the stars at the core would be 31 times faster
and hence they would stick around for only 1/30th the time ... resulting in low densities not high.

I don't understand the argument. Don't more central stars, in the denser part of the cluster, see a lower effective mass? Any star essentially sees only the cluster mass interior to its orbit. And escape velocity is higher near the center; while interior orbits are going to be less stable because of the closer groupings, that doesn't mean that the stars will be ejected from the cluster.

1) What makes you think that the planets in our own solar system are elliptical or stable in a time frame of ~10 billion years?

I didn't say that was the case, but I do believe that is on the order of the stability that most models show.

2) Where would a galactic cluster get the energy to totally "evaporate" :?:
Any "evaporating" stars would "evaporatively cool" the residual cluster and, hence, cause it to shrink.

Would it? The mass would drop, of course. So the escape velocity would drop as well. How effectively it would evaporate depends on how tightly bound the object is to begin with.

Even if the mean distance between stars near the core of a globular is just a few thousand AU (and it isn't)
the core velocities would be so fast that stars would need to pass much closer than 1 AU to significantly affect the orbit.

The HST images of M15 indicate a core density of over 100,000 stars per cubic parsec (this is one of the candidate clusters to contain a black hole, however). This equates to a typical distance of 4000 AU, which is certainly close enough to perturb a stellar orbit.

If there is no central black hole then THERE IS NO CORE :!:
There is just a uniform homogeneous gas of stars.

You may choose your own semantics, but the literature refers to the central part of globular clusters as "the core". And I still don't understand your assertion that a cluster is homogeneous. Even ignoring all the literature describing the surface and spatial density curves as a function of radius, you only need to look at one (as in the APOD) to see that there is a very strong density gradient.

[Chris Peterson]

Okay. In layman terms. Does anyone know how far apart the stars are in cluster? And do they orbit a a central object? Is the cluster expanding, compacting or static? I would assume that they don't move in opposition to each other? I started reading the replies but most of them went over my head.

A few light years apart at the outside, a few thousand AU apart in the core. There is no central object (except for the possibility that a very few globulars might have central black holes- but even in that case, the black hole isn't an object most stars are orbiting around, any more than the black hole in the center of a galaxy is). The stars are all in independent orbits, nominally around the cluster's center of mass, but certainly perturbed by other stars in their area. Clusters must change over billions of years due to loss of material, but for most purposes can reasonably be considered static in terms of size and mass.

[ALCHEMIST1]

If y'all look closely at the full res image, it looks like the stars in the globular, particularly the dim ones at the periphery are substantially out of focus. Or am I imagining it?

[bystander]

If y'all look closely at the full res image, it looks like the stars in the globular, particularly the dim ones at the periphery are substantially out of focus. Or am I imagining it?

Nah, stay off the Hg and Pb for a while, you'll be ok.

[Chris Peterson]

If y'all look closely at the full res image, it looks like the stars in the globular, particularly the dim ones at the periphery are substantially out of focus. Or am I imagining it?

The star profiles don't look to be out of focus. What I see are dark cores in many of the core stars, which gives a sense of them being defocused. That's probably a processing artifact. Globular clusters are notoriously difficult to process, as the central stars run together into odd shapes.

[neufer]

Well, the radial density profile is determined by the self gravity of the cluster, and varies over about 1000:1 from the core to the edges. I don't see anything Gaussian here, or anything that can be reasonably approximated by a uniform density sphere. Perhaps I'm just not understanding your approach.

Where do you get the 1000:1 from

The star density at the outside of a globular is less than 0.1 stars per cubic parsec; in the core the star density can be 1000 stars per cubic parsec or greater. So that's actually a density variation of 10,000 to 1.

OK, I apologize, I misread you. And once again we are mostly arguing over semantics (I think).

But where do you get the "1000 stars per cubic parsec or greater" from

<<Explanation: ...approaching the M13 cluster core over 100 stars would be contained in a cube just 3 light-years [~ 0.9 parsecs on a side.]>>

It is the stars in this uniform density M13 cluster core that concern me most and that I was referring to originally.

These cluster core stars will be in quasi-stable quasi-elliptical orbits with the center of the ellipses being at the center of M13.

Clearly many of the outlier stars will be in quasi-stable quasi-elliptical orbits with the center of M13 being at one of the foci.