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starbux
sage
Reged: 02/08/06
Posts: 289
Loc: Silicon Valley, CA
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This may sound like a dumb question. But what makes me ask is this: globular clusters contain stars MUCH older than the Sun, it seems to me that other than blue stragglers the clusters would contain almost entirely orange and red dwarfs (a solar mass star would have already moved past the red giant stage to white dwarf status would it not?).
If my assumptions are correct (and DO correct me if I am mistaken), at the distance of several thousand light years that GCs lie, orange and red dwarfs would be too faint to be resolved individually in most amateur telescopes.
So when I see stars resolved in the brighter GCs in my 8" scope, exactly what kind of stars am I seeing? Are they still red giants and if so what position on the Main Sequence did they originate? I would presume the larger mass stars (except for blue stragglers) are long gone.
Thanks.
Malcolm
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Achernar
Postmaster
Reged: 02/25/06
Posts: 5024
Loc: Mobile, Alabama, USA
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The magnitude of the brightest stars versus the limiting magnitude your telescope can reach determines if a globular cluster resolves into stars or not. The apparent magnitude of a globular clusters brightest stars depends on three factors. One is the distance from Earth. The second is how much interstellar dust is between us and the globular cluster. Some globular clusters such as M-22 and M-62 are dimmed dramatically by the interstellar dust that reddens and absorbs their light as it makes it's way to us. The third is the true luminosity of the globular cluster's brightest stars, which will be red and yellow giants, not Main Sequence stars. Even in the closest globular clusters such as M-22, M-4 and NGC-6397, Main Sequence stars are very faint objects and would at best be 17th or 18th magnitude objects. In the most remote globular clusters such as NGC-2419 in Lynx, even red giants with a luminosity of hundreds of times that of the Sun are 16th magnitude and fainter objects. The reason why a good number of globulars are at least partially resolved in medium sized telescopes is the fact they have numerous red and yellow giant stars bright enough to be seen individually and not as an unresolved glow from hundreds of thousands of very faint stars.
Taras
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15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
Two curious cats
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3466
Loc: Cambridge, MA, USA
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Quote:
This may sound like a dumb question. But what makes me ask is this: globular clusters contain stars MUCH older than the Sun, it seems to me that other than blue stragglers the clusters would contain almost entirely orange and red dwarfs ...
If my assumptions are correct (and DO correct me if I am mistaken), at the distance of several thousand light years that GCs lie, orange and red dwarfs would be too faint to be resolved individually in most amateur telescopes.
Good thinking!
To some extent, you're right. Globular clusters are much harder to resolve than many open clusters, and the fact that globs are farther away is only part of the reason. Bright young stars like the ones that make up the Pleiades and many other open clusters are conspicuously absent from globular clusters.
However, you have the time scale a little wrong. Yes, globs are older than our Sun, but only by a factor of three or so. And our Sun is only halfway to its white-dwarf stage. So stars even slightly less luminous than our Sun are still going strong in globular clusters.
Moreover, shortly before a star settles down into old age, it goes through its red-giant phase, when it becomes much *more* luminous than when it was on the main sequence. Although red-giant stars are relatively rare, they're very prominent. So down to any given limiting magnitude, the fraction of visible stars that are red giants is usually surprisingly high. That's true for the naked-eye sky, and it's also true for globular clusters. Take a look at any colorful photo of a glob and note how many of the stars are reddish. No accident!
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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HfxObserver
scholastic sledgehammer
Reged: 11/12/04
Posts: 784
Loc: Regina, SK, Canada
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Hmmm, this leads to one of my conversations with another observer who deduced that what we are seeing is groups of stars tight together not individual stars....at least with 8-inch and under scopes.
Do we know that we are seeing individual stars in 8-inch scopes? OR are we seeing clusters?
-Chris
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Chris
7X50 Vixen,22X100 Antares
80mm William Optics Megrez II ED
Santel MK6
Borg 125SD f6 (Pentax/Oasis version)
Tak-Lapides
Pentax XW's 40,20,14,10,5mm XO, 3.5, 3.8XP, Speers 5-8, 30mm Widescan III
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8275
Loc: Beatrice, Nebraska
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Quote:
Hmmm, this leads to one of my conversations with another observer who deduced that what we are seeing is groups of stars tight together not individual stars....at least with 8-inch and under scopes.
Do we know that we are seeing individual stars in 8-inch scopes? OR are we seeing clusters?
-Chris
It is a little of both. Some of the very brightest giant stars in globulars are bright enough to be visible individually, while other star-like points may be just dense groups of several fainter stars that are just a little too close together to resolve in an 8 inch. If we put Arcturus out in M13 (26,000 light years away), it would have a magnitude of about 14.4, which is within range of an 8 inch on a good night. The brightest giant stars in most globulars have an absolute magnitude of around -2, making one of these in M13 have a visual magnitude of 12.5. However, a star identical to our sun if located in M13 would shine at a feeble magnitude 19.3. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3466
Loc: Cambridge, MA, USA
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Quote:
Hmmm, this leads to one of my conversations with another observer who deduced that what we are seeing is groups of stars tight together not individual stars....at least with 8-inch and under scopes.
Do we know that we are seeing individual stars in 8-inch scopes? OR are we seeing clusters?
The difference would be obvious in a photograph. And every time I've correlated the stars that I've sketched in a glob against a photograph, they've all turned out to be individual bright stars, not clusters.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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skyward_eyes
Carpal Tunnel
Reged: 12/12/06
Posts: 2101
Loc: Arizona
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Post deleted by skyward_eyes
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www.skywardeyes.webs.com
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3466
Loc: Cambridge, MA, USA
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Quote:
I really didnt start resolving individual stars in globulars until I got my 16".
I'm startled by that statement. Under modestly dark skies, I can see a number of individual stars in M4 and M22 using my 70-mm refractor at 60X. My 7-inch Dob at 120X shows dozens of stars in those clusters, and also shows many stars in M13, M5, M15, M2, M92, M10, and M12. My 12.5-inch Dob achieves at least partial resolution of almost all the Messier globulars and many non-Messier globulars.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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starrancher
professor emeritus
Reged: 06/09/09
Posts: 585
Loc: Northern Arizona
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C'mon guys , I'm resolving to the core on some of the better ones . M3 , M13 , M22 etc . In my AR5 .
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LXD75 AR5
LXD75 SN8
Series 4000 Plossls
Misc. other stuff
Fort Rock , Az .
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4968
Loc: Zuni Mtns, NM
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That northers AZ location? I dunno, with any sort of light pollution around it takes aroudn 12".. I mean, I can sort of do it in a 6" refractor, but it's tough. 8" a little better maybe, 10" a lot better.
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8275
Loc: Beatrice, Nebraska
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I have seen a few stars in M22 using only a 2.4 inch refractor! M4, M5, M13, and a few others show quite a number of faint stars in my 100mm f/6 refractor. In an 8 inch, many globulars are spectacular, showing hundreds of faint stars. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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HfxObserver
scholastic sledgehammer
Reged: 11/12/04
Posts: 784
Loc: Regina, SK, Canada
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Thanks David, that makes sense.
-Chris
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Chris
7X50 Vixen,22X100 Antares
80mm William Optics Megrez II ED
Santel MK6
Borg 125SD f6 (Pentax/Oasis version)
Tak-Lapides
Pentax XW's 40,20,14,10,5mm XO, 3.5, 3.8XP, Speers 5-8, 30mm Widescan III
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auriga
scholastic sledgehammer
Reged: 03/02/06
Posts: 794
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Hi,
I think the simulation of globular cluster resolution on the Obsession Telescopes web site is not far from the truth. Notice that the 8 inch view shows lots of stars, but faintly.
If you enduring interest is resolving globulars, I would recommend 10 inches or perhaps a bit more for a view in which the component stars don't seem faint.
But globulars are beautiful in almost any size scope, even if not resolved. I remember seeing M22 in my old ANRA 4.25 inch reflector handheld at 17x, and it was granular but beautiful, set against the star fields of the Milky Way. The component stars of M22 were a bit too faint to be seem by me in my sky conditions at that time and at that low magnification, but M22 itself was quite bright.
Bill Meyers
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Not just the red giants are accessible in moderate instruments; so too are the horizontal branch stars. Indeed, it's really at the point where one's scope/eye begin to pick up the latter when a globular starts to appear pretty well resolved.
A glance at a typical color-magnitude or Hertzsprung-Russell diagram will show why. The giants occupy a relatively narrow band running upward and to the right at the top of the cluster sequence. Because of the rapidity with which giants evolve, this portion is not terribly well populated.
After 'zooming' up the giant branch, a giant will move back downward to occupy a position on the horizontal branch, where it undergoes a relatively stable period of core helium burning. All stars on the horizontal branch have rather similar magnitudes, their position color-wise depending sensitively on the mass.
Depending on the metallicity of the cluster members, the horizontal branch may extend quite far blueward (hotter). At any rate, the longer period of core helium burning, as opposed to the brief red giant phase, means there are usually more stars on the horizontal branch. And their similarity in brightness means that once sufficient aperture is brought to bear, these stars will begin to appear 'suddenly', if you will, in significant numbers.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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Dain
Carpal Tunnel
Reged: 03/24/05
Posts: 1596
Loc: N.Y. Adirondack Mnts. NGC 4565...
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My 8-inch dob ( old 'Hopper') has shown me many fine globulars resolved into tiny stars. I've been able to bust to the core on alot, and alot I haven't. It depends on which GC since they're all different. Actually, some of my favorites are ones you can barely detect at all just cause finding them is a blast and knowing what your looking at through the eyepiece at that distance from us is hard to believe.
Clear Skies to All!
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Best,
Dain
Adirondack Mountains (my true dark sky site)
@ Cedar River Flow
Local Site
Clear Skies?
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