Star Clusters: An Overview

Star Cluster

STAR CLUSTERS

  • Contain hundreds up to millions of stars
  • Held together by gravitational pull of the stars on one another
  • Stars formed nearly at the same time and the same age

Spiral GalaxiesAnatomy: bulge, disk, and halo

Open Cluster

Open Clusters

  • Contains typically hundreds of stars
  • Irregular shapes
  • Found in the disk region of our galaxy
  • Ages range few million years to few billion years
  • Some young clusters still contain diffuse gas and dust — the material from which the cluster formed

Globular Cluster

Globular Clusters

  • Very dense star clusters
  • Typically 10,000 to 1 million stars
  • Very old — up to about 12-13 billion years old
  • Have much lower abundances of heavy elements than the Sun
  • Found in the halo region of galaxies

*When plotted on the H-R Diagram, star clusters have different turnoff points, or the point where stars being to evolve and die; the turnoff point determines the age of the galaxy

  • Young clusters = turnoff point higher
  • Old clusters = turnoff point lower

Distance to Star Clusters

  • Apparent magnitudes and colors for many stars used to compare with a H-R Diagram that’s calibrated in terms of absolute magnitude

Variable Stars

  • Apparent brightness changes over time
  • Caused by eclipsing binaries or physical condition within a star itself
  • Certain kinds of stars pulsate, or regularly glow and go dark
  • In the “instability strip”: changes in temperature and luminosity, pulsating period ranges from hours to months
  • Light curves: used to plot a star’s luminosity
  • e.g. Mira: long period variable red giant – M3 to M9

Changes in Apparent Brightness of a Cepheid Variable

Cepheid Variables

  • Important class of variables
    • Very luminous super giants
    • Regular light curves with repetition periods of days or weeks
  • Henrietta Leavitt
    • Pulsation period is proportional to the mean absolute magnitude of the star
    • log P α absolute magnitude
      • More luminous Cepheids have larger pulsation periods

Useful in Determining Properties of Star Clusters

  • b = L/ (4∏d²) , where b = apparent brightness, L = intrinsic luminosity, d = distance
  • RR Lyrae Stars: metal-poor horizontal branch stars in the instability strip; common in globular clusters; average absolute magnitude = +0.6
  • Cepheid Variables: period-luminosity relationship; absolute magnitude = -2 to -8 magnitude
  • Type Ia Supernovae: peak luminosity related to the slope of the declining part  of the light curve; at peak of luminosity, absolute magnitude ranges from -17 to -19 magnitude
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