Globular Cluster Talk (Outline)

The following is the outline of the presentation I gave to the Northern Colorado Astronomical Society (NCAS) on 07/05/2001.
Modifications to this outline shown in Green, are the result of the updated/expanded presentation to the Aurora Astronomical Association (A-cubed) on 08/01/2001.

100 Globular Clusters
Plus or Minus 50

Introduction

Probably 50 are visible in a 2.4" refractor, 100 in typical (6"-8") amateur scopes, and 150 in large amateur scopes
Herschel 400 List (more than just the 29 Messiers and a few NGCs), there are 33 Herschels and 105 total with NGC numbers, 3(4) IC, and still another 75 others
Two named as stars (Omega/47 Tuc), and M 22 was first thought to be a star or nebula
Observing tip: use same eyepiece consistently for comparing scale of apparent sizes (NGC 288)
http://home.den.iccx.net/~lwlg/SCI/catalogsG.html
The above link has links to seds.org, Brian Skiff's ftp site, Bill Harris's web page/ftp, and messier45.com
Academic bibliography may be found from Skiff's and Harris' sites

What are GC's?

Most if not all galaxies have them
Globular shaped, dense stellar masses, 100,000's to millions of stars
Most around the Milky Way are >10 billion years old
Some Magellanic GC's are younger in the tens of millions of years old (LMC NGC 1818 and NGC 1850 are only 40 million years old)
MWGC's centered on a point 8 kpc toward Sagittarius (Shapley: 15 kpc)
some 20 GC's have undergone core collapse
4 GC's in MW and a number in M 31 are X-ray sources, but may not be BH's
Blue Stragglers colliding(?) in NGC 6397, slow spiral in NGC 104, and few in M 13
Thick disk Population II rotate with the galactic disk and are more circular, and slower orbits
Halo Population II do not rotate with the galactic disk, and some even are in the opposite direction of rotation
Helped to measure MW mass at one-half trillion solar masses and has lead to the theory of dark matter
Evolutionary model has the MW forming out of a sphere of matter and due to increased spinning as it collapsed, a disk formed. GC's seem to be the "eddies" of this process
An interesting analogy to Gas Giant planets, Brown Dwarfs, and stars
Dwarf Spheroidal Galaxies (dSph's) are super globulars and may just be an upper bound on the definition of a GC, and elliptical galaxies may be their upper bound
Omega Centauri (NGC 5139) w/1-2M*, 5M SM @ 10x other GC masses, and multiple * populations may just be a dSph
GC's are distinguished from Open clusters by their Color-Magnitude Diagrams (observational H-R Diagram)
M 11 was once thought to be a GC and its CMD disproved this
Shapley and Sawyer-Hogg classifications I-XII (densest to sparsest)
My extension to the SS-H class is a galactocentric description
There may be more in/around the MW that we can't see (lead in to next section's first bullet)

Where are the GC's in the sky?

Hidden behind the hub/bulge of the MW
Southern hemisphere
I have traveled twice to Tucson to observe, and once to Big Bend National Park
10-12.5 degrees further South adds perhaps as many GC's to your reach above the horizon
You can pretty much see GC's all year, but you have to work at it
Much sparser in the winter time (NGC 1049/Fornax, M 79/Lepus, NGC 2298/Puppis, NGC 2419/Lynx, NGC 4147/Coma Berenices, M 68/Hydra, and M 53/Coma Berenices) but viewable
Concentrated most heavily in the galactic core region, Sagittarius, Ophiuchus, and Scorpius, are the three most populated constellations with 71 of my list of 141 MW/SagDEG
Many more scattered around late Spring and Summer skies
Associated with the Magellanic Clouds
NGC 7006 in Delphinus is a good test for observing Magellanic GC's—they are two-thirds its apparent size

Other Galaxies with GC's

MW Satellite Galaxies (Magellanic Clouds, SagDEG)
SMC 121
M 54 is associated with the Sagittarius Dwarf Elliptical Galaxy (SagDEG) along with three other GC's orbiting around/with SagDEG
NGC 1049 mentioned above is with the Fornax Dwarf, as well as four other GC's—Fornax 1-5 (NGC 1049 is Fornax 3)
G1 in the Andromeda Galaxy M 31, G1 is the brightest of 300 that were cataloged in that galaxy with the Mt. Wilson 100" or Palomar 200" telescope
The Ellipsoidal satellite galaxy of M 31, M 110 (NGC 205) has 8 cataloged GC's
Giant Elliptical galaxy M 87 has at least 4,000, and perhaps 15,000
There are several others in Local Group galaxies that are within reach of the bigger amateur telescopes such as M 33's young clusters
UGC 9799 has 48,000 ± 16,000 GC's

Other catalogs/lists

Palomar Sky Survey (15 discoveries from the plates taken with 48" Schmidt Telescopes)
Arp & Madore
Arp & Van den Bergh
Two by constellation (Pyxis and Eridanus)
G for Andromeda
Fornax
ESO survey
Ruprecht (list of open clusters)
Van den Bergh-Hagen, sometimes abbrev. B-H
Lynga
Lund (Swedish list of Open Clusters)
Terzan (red galactic core survey turned these up)
Haute-Provence
Grindlay (never confirmed)
Liller
Tonantzintla
Djorgovski
UKS (UK Schmidt Telescope)
Completely Overlapping the above lists/catalogs
GCL
Dunlop
Melotte
Berkeley
Lacaille
Collinder

Some of my favorites

Messiers
NGC 5139 (THE BEST!!!)
NGC 288 (as big as M 13, but dimmer)
NGC 5466 (near M 3 in the sky and as big, resolved stars in my 12" when my 10" couldn't)
NGC 2419 (cool and fourth brightest near MW)
NGC 6522/6528 (double GC)
M 13 (of course)
M 4 (nearest to Earth)
M 22 (brightest and first discovered)
Palomar 14 (faintest I have observed at magnitude 14.7)

Questions & maybe Answers???

Q: I can understand how they would measure radial velocity, but what about proper motion?
A: Well, I kind of hedged when I said that. I actually have not seen any mention of studies of proper motion. I have come across a few whose position data seemed way out of line, so I have wondered. My own speculation about the statements that researchers make about circular and elongated (note that it is not elliptical) orbits, may be application of statistics. If many GC's were in elongated orbits, because they would be moving slower further from the center of the MW, you would statistically expect to see more in the halo, but this is not the case. Thus, I have considered it possible that researchers have just applied a shotgun to their conclusions about how the halo GC's are distributed, and what types of orbits they may have.

Q: More of a comment really, was made about my calculation of the diameter of the Andromeda Galaxy, M 31.
A: It was based upon the photograpic size of M 31 as stated by Hans Vehrenberg in his book, Atlas of Deep Sky Splendors, 4th ed., and the older distance data of 2.3 million light-years. This calculation yielded a diameter of 181,000 l.y. This calculation revised for today's distance of 2.9 million l.y. is 227,000 l.y. I mis-spoke when I said the diameter was 300,000 l.y. 300,000 l.y. is my estimate of the diameter of M 31's halo of GC's! So you can see that if the MW has a diameter of at least 200,000 l.y., it and M 31 are nearly twins.

This brings up another point I didn't make in my talk. M 31 may have more GC's than the MW due to the presence of more massive (by comparison to the Magellanic Clouds, SagDEG, etc.) elliptical satellite galaxies (M 32, M 110, NGC 147, NGC 185, etc.). It is also possible with GC's as faint as AM-1 at 300,000 l.y. distance, these outer halo GC's may be harder to find.

So, the MW and M 31 may be more like twins than different.

Last Update: 12 November 2002

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