Star Clusters: An Overview

Star Cluster


  • 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

What are White Dwarfs?

White Dwarf

White dwarfs are the bare cores of low-mass stars such as the Sun. A low-mass Main Sequence star becomes a white dwarf when the star uses up all its hydrogen, swells, and ejects its outer layers.

If a white dwarf has a binary companion…

  • Mass-Transferring Binary Star System (e.g. white dwarf and red giant)
  • Gas “spills over” from red giant’s atmosphere and is gravitationally pulled into the white dwarf
  • Nova– explosion powered by fusion of hydrogen to helium on the surface of a white dwarf star; caused by matter spilling onto the star from its binary companion
    • Star brightens rapidly, then fades over weeks or months
    • Nova explosions can recur in the same binary system

Maximum Mass

  • S. Chandrasekhar(1930): calculated the maximum mass of white dwarf
    • Electron degeneracy pressure can only support a white dwarf less than 1.4 M☉
    • If a white dwarf accreted enough mass that overcomes this limit, gravity would win and something dramatic would happen
    • Chandrasekhar’s Limit: a white dwarf’s mass can only be less than 1.4M☉

After the Type Ia Supernova, the white dwarf is completely destroyed; no solid remnant is left, although the companion star might remain.

Type Ia Supernovae

  • Brightens over 2 weeks, reaches a peak, and then fades
  • At its peak, the supernova is 10 times more luminous than the Sun (e.g. 1994 D in galaxy NGC 4526)
  • Composed of mostly iron and other heavy metals
  • Core collapse supernova produces carbon, oxygen, neon, magnesium, silicon, and other lighter elements, and iron and other heavy elements
  • The ejected material is “recycled” into new generations of stars and planets

*Note: Without supernova explosions, earth-like planets, organic chemistry, and life wouldn’t exist.

  • All heavy elements were created inside stars or during supernova explosions, and then expelled into interstellar space
  • Heavier elements in supernova form by neutron capture: in an dense environment of  free neutrons, atoms absorb neutrons, beta (β) decays, and a proton forms — when an atom gains a proton, its identity changes and it moves one atomic number on the periodic table
  • Carbon, nitrogen, and oxygen are winners in the burning (release tons of energy)
  • Lithium, beryllium, boron are destroyed and not created in stars
  • Iron, the most stable element, is the end of nuclear burning
  • More massive elements formed by neutron capture followed by β-decay