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Star Clusters

Star Clusters. Ay 16 Lecture 11 March 6, 2008. Star Clusters. Types Distances (Space Motions, Parallax ++) Dynamics Relaxation Masses (Virial Theorem) Stellar Populations HR Diagrams Pop I and II. Star Clusters. Four Basic Types:

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Star Clusters

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  1. Star Clusters Ay 16 Lecture 11 March 6, 2008

  2. Star Clusters Types Distances (Space Motions, Parallax ++) Dynamics Relaxation Masses (Virial Theorem) Stellar Populations HR Diagrams Pop I and II

  3. Star Clusters Four Basic Types: Globular (round & bound) Open (irregular & bound) Association (irregular, not bound) HII Region (very young, irregular) ( other young eg. GMC clusters)

  4. Cen

  5. G1

  6. Palomar 1

  7. NGC 188

  8. h &  Per

  9. CYG OB2

  10. ORION Trapezium

  11. NGC2237

  12. Globulars (15-20,000) M87 David Malin AAT

  13. Spatial Distribution of Globular Clusters

  14. Distribution of HII Regions in The Milky Way

  15. M31 UV

  16. Stars Move…. v = total velocity vR= radial vel vT = tangential velocity vT = v sin  tan  = vT/vR

  17. Proper Motion  (radians/s) = vT(km/s)/d(km) but radians/sec is a “bad” unit, never mind km! So convert km/s to parsecs/yr and radians to arcsec: multiply by 3.156x107s/yr / 3.086x1013 km/pc and by 206265”/radian vT = 4.74 (arcsec/yr) d(pc) or d = vT/4.74  or, if we know , d = vR tan  / 4.74 

  18. Moving Cluster Method Compare the proper motion of a cluster with the rate at which its size changes. Clusters appear to be heading towards particular points on the sky.

  19. We need the radial velocity, vr, the proper motion, , and the angle to the convergent point, .

  20. Today --- Hipparcos High Precision Parallax Collecting Satellite 2.4 million stars measured, ~118,000 with high precison parallaxes better than 0.001 arcsec Hyades distance = 46.34 +/- 0.27 pc ESA 1993

  21. Also, get distances from HR diagrams. Magnitudes --> apparent luminosity is proportional to 1/d2 Color is essentially distance independent (except when dust gets in the way) So --- slide HR diagrams for similar clusters to get offset between a cluster of a known distance and an unknown.

  22. Cluster Timescales Stars still move! How fast/long? Cluster dynamics can be described in terms of two important parameters:

  23. Timescales: Size r (pc) Stellar Velocity or velocity dispersion (the distribution of velocities w.r.t. the cluster mean)  (km/s) Simplest time scale = crossing time tC = r/ = 106 years r(in pc)/ ( in km/s)

  24. Relaxation Time tr = relaxation time, = time to establish dynamical equilibrium Estimate by “collision” time for stars in the cluster. If n = the stellar density v = average stellar velocity r = “impact parameter” By geometry n (r2) v tr = 1  tr = 1/nr2v

  25. The Virial Theorem For a “relaxed” system, i.e. one that is • bound, time invariant on average • With well defined kinetic and potential energies < K > = -1/2 <U>

  26. HR Diagrams Ejnar Hertzsprung & Henry Norris Russell 1906/1913 Studied star clusters and nearby stars

  27. HR Diagrams Complete Color-Magnutide diagrams can give you cluster distances cluster ages cluster metallicities

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