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Milky Way Globular Clusters (Rob Horvat)

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Milky Way Globular Clusters (Rob Horvat) Milky Way Globular Clusters (Rob Horvat) 47 Tucanae (47 Tuc). Credit: Gerry Aarts (WSAAG). This globular cluster has more than one million stars. MulCple Star Systems A small Cght 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 binary star. 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 years. Star Distance (Lt Yrs) Separaon Sep (AU) Sep (Lt Yrs) Alpha Centauri AB 4.3 5” 7 0.0001 Acrux 322 4” 390 0.006 Beta MonoceroCs (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 Beta Centauri MonoceroCs Open Clusters An open cluster 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 escape velocity of the system is less than the average velocity of its stars so open clusters tend to disperse aer a few 100 million years. BuXerfly 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 (Omicron Velorum Cluster) 570 60’ 10 M44 (Beehive) 610 90’ 16 M7 (Ptolemy Cluster) 980 80’ 23 NGC 2516 (Southern Beehive) 1,300 30’ 11 M6 (BuXerfly Cluster) 1,600 30’ 14 M47 1,600 25’ 12 M41 2,300 40’ 27 M50 3,100 15’ 14 NGC 6231 (Northern Jewel Box) 4,100 14’ 17 M46 4,900 20’ 29 M11 (Wild Duck Cluster) 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 galaxy but there could be 10 Cmes this number. Open clusters are found in the galacCc 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 Cghtly bound group of tens of thousands, o\en 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 year. In the core of Omega Centauri, 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 moon in the solar system. 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 mass/ 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 descripCon of them in his 1789 catalog of deep sky objects. Very liXle is known about how globular clusters are formed or why they have orbits outside the galacCc disk. M22. Credit: Narayan Mukkavilli (WSAAG) Age of Globular Clusters The universe 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 Astronomy, 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. Metallicity Stars are composed mostly of Hydrogen and Helium. The Sun 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 definiCon, 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. Popula3on I Stars – in the galac3c 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 Cmes the iron content of the Sun. Popula3on II Stars - in globular clusters & galac3c 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 Cmes 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 Milky Way globular clusters such as Omega Centauri have complex formaon histories and consist of at least two disCnct 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 hoXer star (blue straggler). 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 hoXer 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 (Ara). Credit: ESA/Hubble & NASA GalacCc Cannibalism A significant number (?) of globular clusters may have come from dwarf satellite galaxies 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 disCnct 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 luminosity to 47 Tuc, M54 may once have been the nucleus of the SagiXarius Dwarf EllipCcal Galaxy (SgrDEG). At 86,400 ly distance, it becomes the first extra-galacCc globular cluster discovered (Charles Messier 1778). M54. Credit: Jim MisC (MisC 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.
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