Open And Globular Star Clusters

OPEN AND GLOBULAR STAR CLUSTERS Roberto Bartali Introduction In this Essay I explain the characteristic, in general terms and the difference of two types of stellar clusters: Open and Globular, making sometimes direct reference to a particular cluster. These groups of stars are located on well determined location in the Milky Way and was born in different time and are evolved in different way, this information can be extrapolated plotting star luminosity on Hertzsprung-Russel Diagram (HR) and also the on Color Magnitude Diagram (CMD). The description of each type of cluster came in the same structure, so readers can easily compare one to each other; also, see the difference in the appearance of each cluster types in the sky. Open clusters An Open Cluster (OP) is a group containing from a few stars to some thousand stars joined together by gravitational forces and at relatively great angular distance one from the other making them easy to resolve with relatively small telescope and even the naked eye. At first look, stars appear dispersed and without an order, but all share the same motion toward some place in the sky (Figure 4). Figure 1 NGC4755 in Crux “Jewel Box Cluster”

Stars inside OP are moving away from the mass center of the cluster, due to centrifugal forces from the center of the Galaxy and due to perturbation from other objects, so they show a tendency to desegregate. The rate of desegregation of an OP is inversely proportional to its density, because of less gravitational forces acting over them. Star density and separation between stars are very different (Figure 1 and 2). We know some very dense clusters like M11 (containing about 80 stars per cubic parsec in the center) and others like the Big Dipper group whose stars are many parsec away from each other. Apparent size of OP varies from 2´ to more than 330´ like The Hyades in Taurus, and real sizes in the range from 2 and 50 parsecs. Some of the smallest are NGC6603 in Sagittarius (4’ apparent diameter) and NGC7092 in Cygnus (2 parsec). Pleiades in Taurus (Figure 2) and M35 in Geminis are about 100’ in diameter.

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All OP, with some exception, are orbiting the center of the Galaxy near its equatorial plane. Figure 2 M45 Pleiades in Taurus Type “c” open Cluster

We know about one thousand clusters, but interstellar absorption can occult many more at greater distance from us. The nearest OP to us is The Hyades in Taurus, around the red giant star Aldebaran, but it is not a component of the cluster. Some OP are associated to an emission nebula (a cloud of gas containing atoms exited by the radiation of nearby stars, like M45, Figure 2) and others are bounded to a diffuse nebula (a gas cloud where stars are actually in the incubation stage, like M16, (Figure 3). It seems to be common the presence of an emission nebula in young clusters with age no more than 200 million years like M45, M6, M16 and NGC6231. Figure 3 M16 and Eagle Nebula in Serpens Type “e” open cluster

We know now that stars born in a nebula by accretion of the cloud gas and particles so all share the same chemical composition and we can suppose them of same age. This is very interesting, because observing the cluster members we can see stars at different evolution stage, but this is due only to the mass of each star when it born (Figures 1). As all stars came from the same nebula we suppose them at same distance from us, because distances between stars are much less than our distance to the cluster, so the apparent magnitudes are related to absolute magnitude by a constant.

OP are classified using one of two scheme (10). The Harlow Shapley Scheme uses the number of stars in the OP as: • c = very loose and irregular • d = loose and poor • e = intermediately rich • f = fairly rich • g = considerably rich and concentrated The R.J.Trumpler Scheme is more complete and descriptive; it uses the concentration (class), the bright and the number of stars in the OP as: Concentration

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• I = Detached; strong concentration toward center • II = Detached; weak concentration toward center • III = Detached; no concentration toward center • IV = Not well detached from surrounding star field Range in Brightness • 1 = Small difference • 2 = Moderate difference • 3 = Large difference Richness • p = Poor: <50 stars • m = Moderately: 50< stars <100 • r = Rich: >100 stars If the OP has a nebula associated, there is an “n” following the concentration roman number. Figura 4 Relative motion of Hyades

Most of the OP are young, with an average age of 30 to 300 million years old. Some are less than a million years, one of the youngest is NGC6231 (3.2 million years old), and the oldest are NGC6791, NGC188 and M67 (7, 5 and 4.5 billion years old respectively). We can calculate the age of a cluster plotting their stars on an H-R diagram. The H-R diagram correlates the luminosity with the spectral class of stars, but we know that the color of the star is a function of the temperature (stars acts like blackbody), the absolute and the visual magnitude are a function of luminosity and distance. But, also, luminosity and color are a function of the star mass. The points at which a Figure 5 star reaches the main sequence (Zero Age Main Sequence) H-R diagram of star clusters depend on the mass it has when start nuclear reaction in its core, and, the time it stay on the main sequence, too. There is a point (turn off point) where stars leave the main sequence because they end the burning Hydrogen process. Younger Clusters has many O and B stars on the main sequence and, older cluster has many K and M stars on it. As we see in pictures, younger cluster are white and blue stars rich, spectral type O and B (Figure 2 and 3); medium age are filled of orange, yellow and white stars, spectral type G, F and A (Figure 1); older ones have orange and red stars, spectral type K and M. Younger clusters contains stars with metallic elements and older ones are similar to the Sun, very few metallic elements. Aged clusters contains

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Figure 6 UBV Color Magnitude Diagram (old cluster)

Figure 7 UBV Color Magnitude Diagram (young cluster)

also more dwarf stars. The H-R diagram tell us about the age of the cluster; it means the time when stars leave the main sequence. Figure 5 shows the turn off point on the main sequence of many clusters plotted together. Figure 6 and 7 shows the CMD of and old OP and for a young OP. These diagrams are equivalent to the H-R diagram, but correlate the visual magnitudes (V), and the color index (B-V is a linear function of spectral type) younger the cluster, lesser de color index value; negative color index represent even younger clusters. Normally in OP there are not variable stars (with the exception of eclipse variables, but they are not physic variables). Due to relatively spreading of stars in the cluster, it is easy their observation through small telescopes and binoculars. TABLE 1 Data and position of most important clusters Coordinates are for epoch 2000.0

NAME

CONST.

R.A.

DECL.

M45 Mel 25 M44 M11 NGC869 NGC884 M35

TAURUS TAURUS CANCER SCUTUM PERSEUS PERSEUS GEMINI

3h 47m 4h 27m 8h 40m 18h 51m 2h 22m 2h 22m 6h 09m

+24° 07’ +16° 00’ +19° 59’ -06° 16’ +57° 09’ +57° 07’ +24° 20’

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My observation of OP:

Figure 9 NGC 869-884 September 1974

M11

Figure 8 M45 Open Cluster Visual observing Refractor 60 mm diam. F/6 80X January 1978

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Globular Clusters Globular Clusters (GC) are beautiful set of stars very closely one to the other likes a snow ball in the sky. Figure 10 M3 in Canes Venatici

Globular Clusters (GC) are agglomerations of some thousand to 1 million stars joined together by gravitational forces. We know about 150 GC; all belongs to the Milky Way Galaxy except for one, M54, which is presumably a member of Sagittarius Dwarf Elliptical Galaxy in the Local Group.

Star density in GC is very high, because the diameter varies from 20 to 200 light years. Half of GC known, are located in a particular area of the sky, in Sagittarius, Scorpius and Ophiuchus, the center of Milky way is also in Sagittarius, as we are far from galactic center we see the 90% of them toward this constellation; this implies that they are physically around the galaxy core. GC are all moving at roughly the same speed of about 100 to 150 Km/s with respect to us, in very eccentric elliptical orbits. With orbits like these, GC can leave the Galaxy and really they form a spherical bubble around its center, their orbital periods are in the range of billion years.

Figure 11 M80 in Scorpius Figure 12 NGC104 in Tucana

This implies they suffer desegregation due to encounter of massive zones around galaxy center, tidal forces and acceleration due to movement and encounters, can act on peripheral stars and then spread them out; each time they cross the plane of the galaxy, this gravitational forces and perhaps closer encounters or collisions, can increase star loosing. The rate of desegregation of GC is much lesser than of OP because they are much more massive and denser than OP. Plotting stars on the H-R diagrams and on CMD, it is clear that they are older than OP, so they have been born in the first phase of the galaxy life, just after or before; like stars near the nucleus of the galaxy. The main sequence of GC denotes the age (Figure 13), because it is very short and most the stars evolved to giant branch, so only less massive are still on it. GC seems to be born at the same time about 12 to 16 billion years ago. Stars in GC are poor of metallic, or heavy, elements, normal in older stars. There are very few interstellar matter inside a GC, so it is impossible to have star formation in it.

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A typical characteristic of GC is the presence of many variable stars of RR Lyr and many white dwarfs. Only 4 GC contains planetary nebulae, this is very strange because their life is much shorter than the age of the cluster itself, but it is extremely difficult to observe such faint objects, so I think there are much more than we suspect. When massive and luminous stars of GC ends its life on the Main sequence (stop burning Hydrogen) they reach the giant branch, so from blue-white, they are now red. When contraction of the star toward its core reach a particular density and temperature (100 millions K), it start the Helium burning phase. Now GC stars go backward to the main sequence, after that phase, stars go to the left of the main sequence and stay on the horizontal branch.

Figure 13 CMD for M3 Globular Cluster

Figure 14 CMD for NGC5466 Globular Cluster TABLE 2 Data and position of some Globular Clusters Coordinates are for epoch 2000.0

NAME

CONST.

M3 M5 M13 M22 M55 M15 M12

CANES VEN. SERPENS HERCULES SAGITTARIUS SAGITTARIUS PEGASUS OPHIUCHUS

R.A.

DECL.

13h 42m +28° 23’ 15h 18m +2° 05’ 16h 41m +36° 28’ 18h 36m -24° 52’ 19h 40m -30° 58’ 21h 30m +12° 10’ 16h 47m -1° 57’

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My observation globular cluster

Figure 15 M13 Refractor 60 mm diam f/6

Conclusion This essay I tried to explain in plain text form which is the relationship of stars in clusters and how those evolved. Studying star clusters we can model the evolution of stars with different masses born also simultaneously and all in the same field of view. The OP reveals to us the first half of stellar evolution and GC the other half. References 1) ALMANACCO ASTRONOMICO COELUM, ed. 1977 2) Cecchini G., IL CIELO VOL Il, UTET, section 5, chapter 1, ed.1969 3) Freedman R., Kaufmann William III, UNIVERSE, Freeman, chapter 20,21, ed. 2002 4) Hack M., L´ASTRONOMIA, num 11,12, ed.1981 5) Holliday K., INTRODUCTORY ASTRONOMY, chapter 13, ed.1998 6) Kaufmann, William III, STARS AND NEBULAS , Freeman, chapter 5,6, ed.1978 7) Karttunen H. et al, FUNDAMENTAL ASTRONOMY, Springer, chapter 6,9,17, ed. 2000 8) http://zebu.uoregon.edu/~js/ast122/lectures/lec12.html 9) http://www.allthesky.com/clusters/clusters.html 10) http://www.seds.org/messier/open.html 11) http://homepage.interaccess.com/~purcellm/jewel.htm 12) http://www-astro.phast.umass.edu/~jamesm/cluster.html 13) http://obswww.unige.ch/webda/cgi-bin/ocl_page.cgi?dirname=mel025 14) http://aida.astroinfo.ch/schirmer/NGC6441.jpg.html 15) http://www.seds.org/messier/xtra/supp/gc_pn.html 16) http://www.seds.org/messier/more/m022_pn.html 17) http://www.seds.org/messier/glob.html

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