Starship Asterisk*

Omega Centauri: The Brightest Globular Star Cluster

Explanation: This huge ball of stars predates our Sun. Long before humankind evolved, before dinosaurs roamed, and even before our Earth existed, ancient globs of stars condensed and orbited a young Milky Way Galaxy. Of the 200 or so globular clusters that survive today, Omega Centauri is the largest, containing over ten million stars. Omega Centauri is also the brightest globular cluster, at apparent visual magnitude 3.9 it is visible to southern observers with the unaided eye. Cataloged as NGC 5139, Omega Centauri is about 18,000 light-years away and 150 light-years in diameter. Unlike many other globular clusters, the stars in Omega Centauri show several different ages and trace chemical abundances, indicating that the globular star cluster has a complex history over its 12 billion year age.

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Has anyone suggested possible origins of globular clusters, as compared to galaxies.

Do they spin, or otherwise rotate ?
And why don't they form into galaxies of some variety - say the common catherine wheel variety for example.

And would all galaxies have once been globular clusters ?

Answers on the back of a postage stamp ....

RocketRon wrote:Do they spin, or otherwise rotate ?

The stars of a globular cluster are said to be in chaotic orbits around the gravitational center. The motion has been likened to bees circling a beehive.

Quite indeed.

RocketRon wrote:Has anyone suggested possible origins of globular clusters, as compared to galaxies.

Do they spin, or otherwise rotate ?
And why don't they form into galaxies of some variety - say the common catherine wheel variety for example.

And would all galaxies have once been globular clusters ?

Answers on the back of a postage stamp ....

And, biggest question of all: why don't they have a concentration of the dark matter? They are the only thing that doesn't! And I like RocketRon's other questions as well!

@RocketRon, @heehaw, re your questions above : while comparing star clusters to galaxies,
please don't forget the scale is so different ! A few MILLION stars make a big cluster,
while galaxies hold dozens of BILLIONS. They really do not fight in the same league.

Is the proper motion of Omega Centauri the akin to digging up dinosaur bones? Apparently we are studying globular clusters to unravel our early galaxy. Perhaps other studies are under way to determine the origin of galactic species tracing our universal roots?

heehaw wrote:

RocketRon wrote:Has anyone suggested possible origins of globular clusters, as compared to galaxies.

And, biggest question of all: why don't they have a concentration of the dark matter? They are the only thing that doesn't! And I like RocketRon's other questions as well!

Ideas about globular clusters are developing richly right now. We've recently found that some globulars have stars with a wide range of ages (not all the same age as many models have suggested). We've recently found that some globulars are far too massive for their luminosity, and very likely do have dark matter halos. Basically, people are starting to think that what we call a "globular cluster" isn't a single thing, but an end product that could have developed in different ways. It may make no sense to talk generally about globular clusters, as different ones may be very different beasts.

Shouldn't there be a Black hole of some signicant size at the center of Omega Centauri?

snuggs28 wrote:Shouldn't there be a Black hole of some signicant size at the center of Omega Centauri?

There may be an intermediate mass black hole near the center of the cluster, but that's uncertain. A few globular clusters appear to have massive black holes in them, but most apparently do not. (And Omega Centauri may not even be a globular cluster!)

To spin today's topic in a different way - that the motion of the stars of a globular cluster differs from that of a spiral galaxy begs the question does the motion of the black hole at their centers offer a clue to their fundamental differences and disposition of energy over time.

If Omega Centauri has a black hole - its type will certainly be of interest some day.

Omega Centauri is one of the most spectacular objects in the sky for the visual astronomer.

Make an effort to view it, even if all you have are binoculars. It is worth the effort!

The distance from Earth to Omega Centauri is indicated to be 18,000 light years in today's APOD.
In the Omega Centauri link to May 1, 2008 the distance is estimated to be 15,000 light years.
With a diameter of 150 light years the difference in distance measurement seems too large.
Which is correct? 18,000 or 15,000 LY?

Rob v. wrote:The distance from Earth to Omega Centauri is indicated to be 18,000 light years in today's APOD.
In the Omega Centauri link to May 1, 2008 the distance is estimated to be 15,000 light years.
With a diameter of 150 light years the difference in distance measurement seems too large.
Which is correct? 18,000 or 15,000 LY?

What does the diameter measurement have to do with distance?

In any case, you need to define what you mean by "correct". Almost certainly, neither is perfectly accurate. Fairly recent studies suggest a distance somewhere between about 15 and 17 kly. But it is notoriously difficult to measure most astronomical distances. Realistically, a distance range of 15-18 kly for almost any astronomical object is considered pretty good.

Chris Peterson wrote:

Rob v. wrote:The distance from Earth to Omega Centauri is indicated to be 18,000 light years in today's APOD.
In the Omega Centauri link to May 1, 2008 the distance is estimated to be 15,000 light years.
With a diameter of 150 light years the difference in distance measurement seems too large.
Which is correct? 18,000 or 15,000 LY?

What does the diameter measurement have to do with distance?

In any case, you need to define what you mean by "correct". Almost certainly, neither is perfectly accurate. Fairly recent studies suggest a distance somewhere between about 15 and 17 kly. But it is notoriously difficult to measure most astronomical distances. Realistically, a distance range of 15-18 kly for almost any astronomical object is considered pretty good.

Hi Chris, Thanks for your reply and question. I did not think that the measurements to astronomical objects lacked so much precision.

As to the question about how diameter factors into my question: If the globular cluster measures 150 LY in diameter and the cluster is assumed to be somewhat spherical (which the study you quoted did not appear to find) then you could interpret the measurement to the globular cluster to refer to the approximate center of the sphere or to an approximate edge of the cluster's sphere.

If the distance to the center of globular cluster is let's say (per today's APOD description) 18,000 LY and it's diameter is 150LY (=radius of 75LY) then the nearest and furthest stars would range between 17,925 & 18,075 LY distance for a tolerance of =±0.416%. (Oops. Original post had incorrectly used diameter instead of radius in the calculation)

In that case I did not understand the 15,000 LY distance quoted on the APOD posting of 05/01/2008.

However if the range of the distance is between 15,000 and 18,000 LY (=16,500 nominal±1,500 LY) the distance tolerance is approximately ±9.4% and the distance to the nearest, center or furthest object in the globular cluster is irrelevant.

With Hubble measuring the distance to the farthest objects in the universe with an accuracy of 3% I wonder if the this distance measurement to Omega Centauri could be refined.

Rob v. wrote: With Hubble measuring the distance to the farthest objects in the universe with an accuracy of 3% I wonder if the this distance measurement to Omega Centauri could be refined.

Hubble may be measuring red-shift that accurately, but converting that to distance gets less precise.

See: https://en.wikipedia.org/wiki/Cosmic_distance_ladder

Visual_Astronomer wrote:Omega Centauri is one of the most spectacular objects in the sky for the visual astronomer. Make an effort to view it, even if all you have are binoculars. It is worth the effort!

I agree with you entirely, except not everyone is geographically located in a favorable position to do so. Omega Centauri has a declination of 47.5° south, so it's position is more favorable to observers in the southern hemisphere. At a latitude of 40 degrees north, which cuts across the middle of the United States, Omega culminates just several degrees above the southern horizon (including a little boost from atmospheric refraction). Therefore, it's difficult to see, let alone see well, because of considerable atmospheric absorption down that low (even on nights with nice clear skies overhead, it's often murky near the horizon). As a result, it's a challenge just to spot Omega from 40°N, although I've heard of observers seeing it from Long Island, NY, and the north shore of Lake Ontario.

I've seen Omega a number of times from East Point, NJ (the site of the APOD for January 8, 2012), on the north shore of the Delaware Bay (at 39.2°N latitude). When sky conditions permit, it's visible in binoculars, but typically, it's not really outstanding. On one lucky occasion (January 31, 2008), when transparency was unusually good over the bay, Omega was spectacular in my 16x70 binoculars, better than globular clusters M4 and M13 at the same time, which were at much higher altitudes. I've been trying to spot Omega for a while now from Coyle Field, an observing site closer to central NJ (at 39.8°N), but so far, I haven't had a definitive sighting yet.

Of course, the best thing is to be at a more southerly location, so if you're in the northern hemisphere and travel south in the winter, take your binoculars!

I find it intriguing that these globular clusters are like tiny galaxies with millions of stars chaotically revolving around a center of gravity. Is it not possible that they are "proto-galaxies" that were arrested in their development just like some proto-planets were? I find that idea fascinating.

Joe Stieber wrote:

Visual_Astronomer wrote:Omega Centauri is one of the most spectacular objects in the sky for the visual astronomer. Make an effort to view it, even if all you have are binoculars. It is worth the effort!

I agree with you entirely, except not everyone is geographically located in a favorable position to do so. Omega Centauri has a declination of 47.5° south, so it's position is more favorable to observers in the southern hemisphere.

I'm at 30° north in central Texas, so it is pretty low, but on a rare night of very steady air the view through my 20" looks a lot like this APOD picture!

http://earthsky.org/tonight/spica-guide ... a-centauri

Just in case anyone wants to look.

Rob v. wrote:With Hubble measuring the distance to the farthest objects in the universe with an accuracy of 3% I wonder if the this distance measurement to Omega Centauri could be refined.

The accuracy with which we're able to measure distance depends on the distance, and the available techniques for that distance. We can actually measure the distance to the edge of the observable universe with more accuracy than we can measure the distance to nearby galaxies, or even to many objects within our own galaxy.

Rob v. wrote: I did not think that the measurements to astronomical objects lacked so much precision.

Yes indeed, even the estimated distance to the great searchlight Deneb ranges from ~1400LY to 3,000 LY. So we don't know if the light left it after the end of the Roman Empire or just when they were putting the finishing touches to Stonehenge. Pretty big difference.

Visual_Astronomer wrote:

Rob v. wrote: With Hubble measuring the distance to the farthest objects in the universe with an accuracy of 3% I wonder if the this distance measurement to Omega Centauri could be refined.

Hubble may be measuring red-shift that accurately, but converting that to distance gets less precise.

See: https://en.wikipedia.org/wiki/Cosmic_distance_ladder

Hi Visual Astronomer,

Now that I had a chance to find some time away from work and commitments to revisit and go to look at your referred to link I have a little more of a grasp of how difficult it must be for scientists to perform the measurements. (Evaluating the proper technique to apply, obtaining or calibrating the instruments, finding adequate conditions and time to perform the measurements, collect, compile and analyze the data, etc.) Very cool. Thank you for the interesting information.

Rob v.

Chris Peterson wrote:

Rob v. wrote:With Hubble measuring the distance to the farthest objects in the universe with an accuracy of 3% I wonder if the this distance measurement to Omega Centauri could be refined.

The accuracy with which we're able to measure distance depends on the distance, and the available techniques for that distance. We can actually measure the distance to the edge of the observable universe with more accuracy than we can measure the distance to nearby galaxies, or even to many objects within our own galaxy.

Hi Chris,

That's really interesting to know. Going forward I will need to look a little more critically at the distances published in Astronomy related articles. It would be nice if quoted distances were accompanied in brackets by some form of abbreviation which would define either the measurement technique used, the estimated accuracy (±xxxx units) or statistical probability indicating the estimated measurement error. I appreciate your patience.

Regards, Rob v.

hamilton1 wrote:

Rob v. wrote: I did not think that the measurements to astronomical objects lacked so much precision.

Yes indeed, even the estimated distance to the great searchlight Deneb ranges from ~1400LY to 3,000 LY. So we don't know if the light left it after the end of the Roman Empire or just when they were putting the finishing touches to Stonehenge. Pretty big difference.

Hi Hamilton1,
That's for sure.... At least in human time-scale terms. When it comes to universe time-scale terms: Meh. Close enough.
Rob v.