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
  • 1. Open Clusters 2. Globular Clusters 3. Associations

Spiral GalaxiesAnatomy: bulge, disk, and halo

Open Cluster

Open Clusters

  • Contains typically 100- 1,000  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
  • Contain higher abundances of heavy elements than globular clusters
  • Widely dispersed due to stellar wind, supernovae, and revolution around galactic center
  • Population I: (in Milky Way) young stars, heavy elements, 900 in galactic disk
  • e.g. Ursa Major cluster (21 pc). Hyades (42 pc), Pleiades (127pc)

Globular Cluster

Globular Clusters

  • Very dense star clusters
  • Lie above and below galactic plane
  • Spherical in shape, more massive, more tightly packed
  • Typically 10,000 to 1 million stars
  • Very old — up to about 12-13 billion years old
  • All bright stars have reddish color, no bright blue stars
  • Too tightly bound by gravity for dispersal
  • Have much lower abundances of heavy elements than the Sun
  • Found in the halo region of galaxies
  • About 150 in Milky Way, 200 in M31
  • Population II: (in Milky Way) almost no heavy elements, only hydrogen and helium
  • e.g. M5, M92


  • Similar to open clusters, fewer stars, larger in size, looser in structure
  • 10 to a few hundred stars
  • Very young O and B stars, T Tauri stars
  • O associations and T associations
  • Break up faster than open clusters
  • e.g. Trapezium Association

*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

Standard Candles

-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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