sign in sign up
<<
Home , IC 2391, IC 4499, Jewel Box (star cluster), NGC 2516, NGC 2808, NGC 3201

Milky Way Globular Clusters (Rob Horvat)

47 Tucanae (47 Tuc). Credit: Gerry Aarts (WSAAG). This has more than one million . Mulple Systems A small ght group of stars (usually 2 or 3) that are bound together by gravity form a mulple star system. A two-star system is simply referred to as a . Visually, one star might be seen to orbit another over a period of decades or centuries.

Most binary stars are separated by a lot less than 200” and 0.3 light .

Star Distance (Lt Yrs) Separaon Sep (AU) Sep (Lt Yrs) AB 4.3 5” 7 0.0001 322 4” 390 0.006 Beta Monoceros (A-BC) 680 7.4” 1,540 0.024 Beta Cygni (Albireo) 430 35” 4,067 0.064 Beta 1,2 Capricorni 340 205” 21,300 0.338

Alpha Monoceros Open Clusters An is a loosely bound group of a hundred, a few hundred or a few thousand stars (M11 has 2,900 stars).

Most of the well-known open clusters are somewhere between 10’ and 100’ in size and 10 to 30 light years across. About 2-3 light years between each star.

The of the system is less than the average velocity of its stars so open clusters tend to disperse aer a few 100 million years.

Buerfly Cluster (M6). Credit: Narayan Mukkavilli (WSAAG). Approximately 100 stars. Cluster is 50-100 million years old. Open Cluster Distance (Lt Yrs) Angular Size Size (Lt Yrs) M45 (Pleiades) 430 120’ 15 IC 2602 (Southern Pleiades) 530 100’ 15 IC 2391 ( Cluster) 570 60’ 10 M44 (Beehive) 610 90’ 16 M7 (Ptolemy Cluster) 980 80’ 23 NGC 2516 (Southern Beehive) 1,300 30’ 11 M6 (Buerfly Cluster) 1,600 30’ 14 M47 1,600 25’ 12 M41 2,300 40’ 27 M50 3,100 15’ 14 NGC 6231 (Northern ) 4,100 14’ 17 M46 4,900 20’ 29 M11 () 6,100 14’ 25 NGC 4755 (Jewel Box) 6,400 10’ 19 NGC 3766 (Pearl Cluster) 7,200 9’ 19 NGC 3293 (Gem Cluster) 7,600 6’ 13 Most of the known open clusters are less than 200 million years old. The Jewel Box (NGC 4755) is one of the youngest at only 7 million years old.

Ptolemy Cluster (M7). Credit: Narayan Mukkavilli (WSAAG). Approximately 100 stars. Cluster is 220 million years old. There are over 1000 known open clusters in our but there could be 10 mes this number. Open clusters are found in the galacc disk and move in rotaon with the disk. Younger open clusters are more densely concentrated in the spiral arms of the galaxy where the levels of hydrogen gas are higher.

NGC 1566. Credit: ESA/ Hubble & NASA Globular Clusters A globular cluster is a ghtly bound group of tens of thousands, oen 100,000s and occasionally millions of stars. They are spherical in appearance.

The typical distance between stars in a globular cluster is about one light .

In the core of , the average distance between any two stars is about 1/3 of a light year.

Omega Centauri. Credit: Charles Yendle (WSAAG). The cluster contains perhaps 5 million stars. The difference in shape between an open cluster and globular cluster is somewhat like the difference between an asteroid and a spherical in the .

Saturn’s moon Mimas. diameter 396km. Large Herschel Crater. Taken by Cassini 2010. Smallest known round Asteroid 243 Ida. astronomical body. 60km x 25km x 19km. “The potato radius”, Taken by Galileo 1993. approximately 200km. Credit: NASA/JPL Credit: NASA/JPL/SSI.

Enough / stars and it pulls into a sphere.

Open cluster NGC 6231. Credit: 47 Tuc. Credit: Ted Dobosz (WSAAG) Narayan Mukkavilli (WSAAG) William Herschel was the first to use the name globular cluster for his descripon of them in his 1789 catalog of deep sky objects.

Very lile is known about how globular clusters are formed or why they have orbits outside the galacc disk.

M22. Credit: Narayan Mukkavilli (WSAAG) Age of Globular Clusters

The is about 13.8 billion years (or 13.8 Gyr) old. Globular clusters are some of the oldest objects in the Universe and were probably formed before the material of the Galaxy flaened into the present thin disk.

Globular Cluster Age (billions of years) Omega Centauri 11.5 47 Tuc 13 M2 13 M3 11.4 M4 12.2 M5 10.6 M10 11.4 M12 12.7 M13 11.7 M15 12 M22 12 Most globular clusters in our galaxy show a lack of O and B type blue stars.

Globular clusters contain many low-mass red stars and intermediate-mass yellow stars < 0.8 solar masses. (Source – Hyperphysics, Dept of Physics and , Georgia State University).

Star formaon should have stopped in these clusters almost 13 billion years ago, so only old stars are expected to be found there.

Stars are composed mostly of Hydrogen and Helium. The is 73% Hydrogen and 25% Helium. Heavier elements (oxygen, carbon, iron etc), called metals in astronomy, only account for 2% of its mass.

Hydrogen Helium Metals

Metallicity [Fe/H] = log(Fe/H)STAR - log(Fe/H)SUN compares the rao of iron to hydrogen in a star with the Sun. By definion, the Sun (4.6 billion years old) has metallicity [Fe/H]=0.

Spectral analysis for thousands of stars gives the range for metallicity to be between -4 and +1. Populaon I Stars – in the galacc disk (e.g. open clusters), [Fe/H]>-1. These stars were formed from an interstellar medium progressively enriched with the ejected materials of red giants and supernovae. It is therefore assumed that the higher the rao, the more recently the star was formed. A star with metallicity +1 would have 10 mes the iron content of the Sun.

Populaon II Stars - in globular clusters & galacc bulge, [Fe/H]<-1. Halo stars have not had access to the heavier elements of the disk. A star with metallicity -2 would have 100 mes less iron than the Sun.

The belief was that globular clusters consisted of a single populaon of metal poor stars that formed together. Globular clusters should consist predominantly of older “populaon II” yellow to red stars. Globular Metallicity Globular Metallicity Globular Metallicity Cluster Cluster Cluster M15 -2.37 M19 -1.74 M12 -1.37 M92 -2.31 M22 -1.70 M28 -1.32 NGC 5053 -2.27 M2 -1.65 M5 -1.29 M30 -2.27 Omega Cen -1.53 M75 -1.29 M68 -2.23 M70 -1.62 M14 -1.28 NGC 2419 -2.15 M79 -1.60 M62 -1.18 M53 -2.10 M10 -1.56 M4 -1.16 NGC 6397 -2.02 NGC 6752 -1.54 NGC 2808 -1.14 M56 -1.98 M13 -1.53 M107 -1.02 M55 -1.94 M3 -1.50 M71 -0.78 M9 -1.77 M54 -1.49 47 Tuc -0.72 M80 -1.75 M72 -1.42 M69 -0.64 However, many globular clusters such as Omega Centauri have complex formaon histories and consist of at least two disnct populaons of stars.

Omega Centauri. Credit: Ted Dobosz (WSAAG) Blue Stragglers

Blue stars have been found in the central regions of many globular clusters. One explanaon for these is the formaon of blue stragglers.

The stars in the core of a globular cluster can be so close that occasionally two will collide, forming a larger and hoer star ().

Due to the old age of globs, many thousands of collisions may have occurred since the cluster formed.

A near miss could also result in a close binary where a more massive star siphons hot gases off its companion. The increased mass makes this star hoer or bluer. 100,000 Stars Pictured in the Centre of Omega Centauri

Core area of Omega Centauri (1.3’ x 1’). Credit: NASA, ESA, and the Hubble SM4 ERO Team. Blue Stragglers in NGC 6362 (). Credit: ESA/Hubble & NASA Galacc Cannibalism A significant number (?) of globular clusters may have come from dwarf satellite that were absorbed by the Milky Way Galaxy.

The high density of the globular clusters would allow them to survive in our galaxy’s halo.

There is also some debate as to whether globular clusters are disnct and separate objects to dwarf spheroidal galaxies. Omega Centauri is thought to be the remnant nucleus of a dwarf satellite galaxy captured and stripped of its outer stars.

Of similar to 47 Tuc, M54 may once have been the nucleus of the Sagiarius Dwarf Ellipcal Galaxy (SgrDEG). At 86,400 ly distance, it becomes the first extra-galacc globular cluster discovered (Charles Messier 1778).

M54. Credit: Jim Mis (Mis Mountain Observatory) Like 47 Tuc, NGC 2808 is thought to contain over one million stars. Analysis of HST data for this massive glob provides evidence that star birth occurred over 3 generaons early in the cluster's life. Each successive generaon appearing slightly bluer.

It is thought that radiaon pressure from component stars should drive interstellar gases out of globular clusters early in their formaon.

Perhaps massive clusters like NGC 2808 exert enough gravity to hold onto gases early in their formaon. As a result, a second and a third generaon of stars might form.

Or like Omega Centauri, it could be masquerading as a globular cluster … a dwarf satellite galaxy stripped of most of its material by the Milky Way.

NGC 2808. Credit: NASA, ESA, A. Sarajedini (University of Florida) and G. Pioo (University of Padua (Padova)) Any Planets?

A search for Jupiter-sized planets in 47 Tuc, carried out by a team of using the , came up empty-handed. The work involved checking 34,000 stars in the cluster for signs of large transing planets.

The absence of any posive results strengthens the argument that planets are rare or nonexistent in globular clusters because of their very low concentraon of heavy elements. Diameters of Globular Clusters

Omega Centauri is considered to be the most massive of the Milky Way globs. However, if you do some calculaons based on angular size (visual) and distance: NGC 2419 is 321 light years in diameter. M54 is 229 light years in diameter. Omega Centauri is 6th at 178 light years in diameter!

At the opposite end, Terzan 9 is only 1 light year in diameter.

The average diameter of Milky Way Galaxy globular clusters is about 70 light years. The median diameter is about 60 light years. Diameters are calculated from angular size and distance. Given angular sizes are usually visual and rather subjecve e.g. where does the cluster end? This will also depend on the size of the aperture.

The following cropped images of Omega Centauri simulate how its extent can vary under different circumstances.

Credit: Ted Dobosz Credit: Charles Yendle Credit: Gerry Aarts

Visual: 10x60 18’ Visual: 30inch Telescope 36’ Photographed 65’

On the whole, calculated diameters are considerably under-esmated. Omega Centauri’s diameter calculated from a visual 36’ is 178 ly. From a photographic diameter of 65’ it is 320 ly. The Orbits of Globular Clusters Globular clusters do not move in lock with the rotaon of the galacc disk but orbit through the halo of the galaxy around the galacc core. Many orbit in the opposite direcon (retrograde moon) to the galacc disk. The orbits of globular clusters are likely to be in the form of a precessing ellipse.

There are 17 globs less than 5000 ly from the galacc centre. If the central bulge is 5000 ly in Rosee orbit radius then these inner globs are actually passing through the bulge. Distribuon of Globular Clusters

There are 157 Milky Way globular clusters in the Catalog of Professor William Harris, McMaster University, Canada. There are more candidates as yet unconfirmed.

The diameter of the Milky Way galaxy is about 100,000 light years. The Sun is about 26,000 light years from the galacc centre or about half way out from the centre of the galaxy. There are roughly an equal number of globs above and below the galacc disk.

The average distance of globular clusters from the Galacc Centre is about 40,000 light years. However, the median is only about 16,000 ly.

98 globular clusters are within 25,000 ly of the galacc centre.

There are 13 globs further than 100,000 light years.

The Intergalacc Wanderer

NGC 2419 was discovered by William Herschel in 1788.

It is also known as the Intergalacc Wanderer, a name given when it was erroneously believed not to be in orbit about the Milky Way.

NGC 2419 lies about 270,000 ly from the Sun. Nearly twice as far as the LMC. At this distance, it takes some 3 billion years to make one trip around the galaxy.

This dense glob is very luminous and ranks with Omega Centauri and 47 Tuc in absolute brightness.

NGC 2419. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona SkySafari screenclip for 9pm on 15th March 2016. NGC 2419 altude 18 degrees. Observing Globular Clusters

Of the 157 Milky Way globular clusters in William Harris’ catalogue half (79) are found towards the galacc centre in just 3 constellaons:

Sagiarius: 35 globular clusters. 7 Messier: M22, M28, M54, M55, M69, M70, M75.

Ophiuchus: 25 globular clusters. 7 Messier: M9, M10, M12, M14, M19, M62, M107.

Scorpius: 19 globular clusters. 2 Messier: M4, M80.

There are 29 Messier globular clusters: M2, M3, M4, M5, M9, M10, M12, M13, M14, M15, M19, M22, M28, M30, M53, M54, M55, M56, M62, M68, M69, M70, M71, M72, M75, M79, M80, M92, M107. Sagiarius has 22 globular clusters brighter than 10 Globs Nearest to the Galacc Centre

NGC 6522 and NGC 6528 in Sagiarius are probably the two globular clusters closest to the Galacc Centre as the closer posion of Dufay 1 (mag 11.6) is now in queson.

The pair are separated by only 16’ and are only 710 light years apart.

Globular Cluster Galacc Galacc Distance from Distance from Longitude Latude Sun (ly) Galacc Centre (ly) Dufay 1 357.44° 2.12° 26,700 1,600 NGC 6522 1.02° -3.93° 25,100 2,000 NGC 6528 1.14° -4.17° 25,800 2,000 UKS1 5.13° 0.76° 25,400 2,300 Terzan 2 356.32° 2.3° 24,400 2,600 Liller 1 354.84° -0.16° 26,700 2,600 M69 and M70 are separated by 2.5° and are only 1400 ly apart.

M22 and NGC 6544 are separated by 6.7° and are only 1320 ly apart! has 18 globular clusters brighter than magnitude 10.

M4 and M80 are in .

M10 and M12 are separated by 3.3° and are only 1,560 ly apart. From a planet around a star near M12, the globular cluster M10 would look six mes bigger* in the sky than the ’s Full Moon. (*diameter). Scorpius has 7 globular clusters brighter than magnitude 10.

NGC 6453 has mag 10.08 and is also shown. Coincidently, NGC 6441 and NGC 6453 lie at the same distance from the Sun, separated by 2.5° and 1,620 ly apart. The Earth’s orbit about the Sun (Eclipc shown in blue) is lted to the plane of the galaxy at about 60 degrees. The normal (axis of orbit) is therefore lted to the plane of the galaxy at 30 degrees.

The Earth’s axis is lted away from the normal at 23.5 degrees and somewhat towards the galacc plane. The Earth’s axis makes an angle of about 27 degrees to the plane of the galaxy. The Earth’s axis keeps the same orientaon in space as the Earth orbits the Sun. As the Earth rotates, the North Celesal Pole (NCP) and South Celesal Pole (SCP) are fixed points in the sky (ignoring precession). The NCP has galacc coordinates: longitude 123°, latude +27°. The SCP has galacc coordinates: longitude 303°, latude -27°.

Note that the NCP faces away from the Galacc Centre but the SCP faces more towards the Galacc Centre. Because the NCP faces away from the Galacc Centre, there are only 27 globular clusters in the northern hemisphere but 130 in the southern hemisphere.

Only 13 northern globulars are brighter than magnitude 10, compared to 81 in the southern hemisphere. There are actually only 8 Messier globs in the northern hemisphere: M3 (CVn) M5 (Ser) M13 (Her) M15 (Peg) M53 (Com) M56 (Lyr) M71 (Sge) M92(Her)} 21 Messier globs are in the southern hemisphere but none are below DEC -33°.

M2 (Aqr) is the closest Messier glob to the Celesal Equator at DEC -0.8°.

There are only two Milky Way globs are above DEC +45°: NGC 6229 (mag 9.4) in at DEC +47.5° Palomar 1 (mag 13) in Cepheus at DEC +79.6°.

There are 25 Milky Way globs below DEC -45 degrees: the lowest being IC 4499 (mag 9.8) in at DEC -82°. Brightest by Visual Magnitude

There are 94 globular clusters brighter than visual magnitude 10. All within range of an 8 inch telescope. There are 24 brighter than magnitude 7 and viewable with binoculars. Only two are really : Omega Centauri and 47 Tuc.

You may be surprised to know that NGC 6752 in and NGC 6397 in Ara are in the top 10.

Globular Cons VMag Globular Cons VMag Omega Cen Cen 3.7 M55 Sgr 6.3 47 Tuc Tuc 4.0 NGC 362 Tuc 6.4 M22 Sgr 5.1 M92 Her 6.4 NGC 6752 Pav 5.4 M62 Oph 6.5 M4 Sco 5.6 M2 Aqr 6.5 M5 Ser 5.7 M10 Oph 6.6 NGC 6397 Ara 5.7 M12 Oph 6.7 M13 Her 5.8 NGC 6388 Sco 6.7 M3 CVn 6.2 NGC 3201 Vel 6.8 M15 Peg 6.2 M19 Oph 6.8 NGC 2808 Car 6.2 M28 Sgr 6.8 NGC 6541 CrA 6.3 NGC 4833 Mus 6.9 Given visual magnitudes for larger objects, including globular clusters, are rather errac and vary from source to source.

For Omega Centauri:

Source Vmag SIMBAD (Astronomical Database) 5.33 NGC/IC Project 3.9 SkySafari Pro 3.68 Atlas of the Universe by Patrick Moore 3.6 Atlas of the Night Sky (Massey & Quirk) 3.5

Stephen O’Meara (The Caldwell Objects) states: “Although many sources show it as bright as magnitude 3.7 or 3.5, my naked-eye esmate of magnitude 3.9 is not so opmisc”.

I find it laughable that the visual magnitude can be quoted to two decimal places when in fact different sources can’t even agree on the first decimal place! and Luminosity The absolute magnitude of a star is what its would be at a standard distance of 10 (32.6 light years). Absolute magnitude allows the intrinsic brightness of stars to be compared directly.

Star Distance (ly) Vmag Abs Mag Sun 0.000016 -26.7 +4.83 Sirius 8.7 -1.44 +1.43 Vega 25 +0.02 +0.60 Aldebaran 67 +0.99 -0.57 Achernar 139 +0.54 -2.61 309 -0.62 -5.50 Antares 550 +1.07 -5.07 Luminosity is the total amount of energy radiated by a star (or some other celesal object) per unit me. Basically, the power output of the star. Luminosity can be stated as so many mes brighter than the Sun and can be calculated from the Abs Mag by the formula: L = 2.5124.83-Abs Mag where 2.512 = Pogson’s rao.

Absolute Magnitude Luminosity x Sun -10.17 1,000,000 -7.67 100,000 -5.17 10,000 -2.67 1,000 -0.17 100 +2.33 10 +4.83 1 +7.30 0.1 +9.80 0.01 Most Luminous Globs Luminosity has been calculated off absolute magnitude, which in turn has been calculated off distance and visual magnitude.

Glob Cluster Con Distance VMag Abs Mag Luminosity Omega Centauri Cen 17,000 3.7 -9.91 785,000 M54 Sgr 86,400 7.6 -9.52 548,000 47 Tuc Tuc 14,700 4.0 -9.32 458,000 NGC 2419 Lyn 269,300 10.4 -9.18 400,000 M15 Peg 33,900 6.2 -8.88 306,000 M3 CVn 33,300 6.2 -8.86 298,000 M2 Aqr 37,500 6.5 -8.83 292,000 M5 Ser 24,400 5.7 -8.72 263,000 NGC 2808 Car 31,300 6.2 -8.71 261,000 M53 Com 58,400 7.6 -8.66 248,000 M13 Her 23,100 5.8 -8.47 209,000 NGC 5824 Lup 104,600 9.1 -8.44 204,000 Classificaon of Globular Clusters

A Classificaon of Globular Clusters by Harlow Shapley and Helen B. Sawyer 1927. Globular clusters were placed in 12 classes on the basis of apparent concentraon of stars to centre. Class 1 globs are highly concentrated towards centre, whilst class 12 globs appear fairly homogeneous with lile central concentraon.

Class 1 Globular Cluster Class 12 Globular Cluster Simulaon showing how the proporon of stars shis gradually from the centre into the halo from class to class. Note: images may not exactly reflect actual observaons. Glob Class Table by Harlow Shapley and Helen B. Sawyer 1927.

Total of 95 globs. Asterisked (*) globs (usually bright) were chosen as being representave of their class.

The classes for some globs were uncertain - indicated by a colon.

Daggered NGCs had quesons about their status as globs. Shapely and Sawyer used photos of the clusters taken with the Bruce 24inch photographic refractor at Arequipa in Peru (southern staon of the Harvard College Observatory) to derive the following data.

Class 1 2 3 4 5 6 7 8 9 10 11 12 Mean Vmag 8.7 7.5 6.8 8.6 7.4 8.4 8 8.2 8.8 8.9 9.4 9.6 Frequency 3 6 7 10 10 9 8 11 10 9 8 4

Their comments …

“The various classes are widely spread in apparent brightness and diameter and do not depend on integrated magnitude, except for a slight tendency of the least condensed clusters in the mean to be faint. In apparent galacc distribuon the classes are thoroughly mixed.

The classes of globular clusters are probably an indicaon of developmental age.” Class Globular Clusters 1 M75, NGC 2808, NGC 7006 2 M2, M80, NGC 1851 (Col), NGC 2419 (Lyn) 3 47 Tuc, M54, NGC 362 (Tuc), NGC 6388 (Sco), NGC 6541 (CrA) 4 M15, M28, M62, M92 5 M5, M13, M30, M53, M69, M70, M79, NGC 5286 (Cen) 6 M3, NGC 6752 (Pav) 7 M10, M22 8 Omega Centauri, M9, M14, M19, NGC 4833 (Lup) 9 M4, M12, M72, NGC 6397 10 M56, M68, M107, NGC 288 (Scl), NGC 3201 (Vel) 11 M55, M71, NGC 6352 (Ara) 12 NGC 4372 (Mus) Class 2: M80 (Scorpius). Credit: The Hubble Heritage Team (AURA/STScl/NASA) Class 4: M15 (Pegasus). Credit: ESA/Hubble & NASA Class 6: M3. Credit: Jim Mis (Mis Mountain Observatory) Class 8: ω Centauri. Credit: Gerry Aarts (WSAAG) Class 10: M107 (Ophiuchus). Credit: ESA/NASA Class 12: Palomar 1 (Cepheus). Credit: ESA/Hubble & NASA Most of the data used for globular clusters was taken from the SIMBAD database, operated at CDS, Strasbourg, France and from the …

CATALOG OF PARAMETERS FOR MILKY WAY GLOBULAR CLUSTERS: THE DATABASE Compiled by William E. Harris, McMaster University This revision: December 2010

Except where otherwise indicated (pages 33 and 53) , all tables, charts and diagrams in this presentaon were created by me. The maps of globular clusters in Sagiarius, Ophiuchus and Scorpius were ploed from computer programs created by me using J2000.0 coordinates of stars and clusters from SIMBAD. Labels and other informaon on charts were added using EazyDraw on a Mac.

The celesal (RA, DEC) and galacc charts for globular clusters were ploed from computer programs created by me using equatorial and galacc coordinates from Harris’ database.

Simulaons for classes and orbits of globular clusters were also ploed from computer programs.

All other diagrams were created in EazyDraw.

Rob Horvat (WSAAG) November 2015