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Star Clusters: Types, Dynamics, and Distances in Space

Explore the classification, movement, and mass calculation of star clusters, including Globular and Open types. Learn about velocity, relaxation times, and the Virial Theorem. Discover the significance of HR Diagrams in studying Stellar Populations. Dive into topics like Proper Motion, Hipparcos Parallax, and Cluster Timescales.

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Star Clusters: Types, Dynamics, and Distances in Space

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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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