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 Galaxies: Anatomy: bulge, disk, and halo

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 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

Associations

• 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

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