Announcements

1. Announcements • Test this week (all about stars) • Turn in Homework 11 today • Pick up Homework 12 • Questions on last 3 assignments? • Second project is due December 1

7. Summary of Stellar Properties

8. Course Outline • Naked-eye astronomy • Crash course in physics • Our solar system • The stars • Structure and history of the universe

9. Course Outline • Naked-eye astronomy • Crash course in physics • Our solar system • The stars • Structure and history of the universe You are here

10. Star Clusters 13 November 2006

11. Today: • Fuzzy objects in deep space • Two types of star clusters • Measuring distances beyond the range of parallax (variable stars as standard candles)

12. Fuzzy Objects • Nebulae, star clusters, and galaxies • Most famous list was compliled by Charles Messier, to avoid confusion with comets • William Herschel started what became the NGC list (New General Catalog)

13. Messier Catalog • 27 “open” clusters • 29 globular clusters • 6 diffuse nebulae • 4 planetary nebulae • 1 supernova remnant • 2 small groups of stars • 40 other fuzzy things in which no individual stars are visible

14. Diffuse Nebulae • Huge clouds of gas, mostly hydrogen • Often associated with clusters of young stars that formed from the gas and illuminate it

19. Open Clusters • Irregular in shape • Typically contain a few hundred stars • H-R diagrams indicate that they are fairly young (few or no giant stars) Double cluster in Perseus Pleiades (Subaru)

20. Globular Clusters • Spherical clusters, each containing 105 - 106 stars • About 100 known • H-R diagrams indicate that they are very old (no massive main-sequence stars left) • Nearly all are on one side of the sky, centered on Sagittarius • Most are known by Messier numbers (M4, M13, etc.) or NGC numbers

21. A young star cluster (Pleiades) Main sequence only, no red giants or white dwarfs

22. An old star cluster (Messier 3) Main sequence “cuts off” above a certain point; plenty of red giants and white dwarfs Oldest known cluster ages are about 12 billion years

23. Brightness of Stars • True brightness (or “luminosity”) is a star’s actual rate of energy output, measured (for example) in watts. The sun’s luminosity is about 4 x 1026 watts. • Apparent brightness is determined by the “intensity” of starlight striking a detector. It is measured (for example) in watts per square meter. The sun’s apparent brightness from earth’s location is about 1400 watts per square meter. True brightness Formula: Apparent brightness = 4π(distance)2

24. Brightness of Stars True brightness Formula: Apparent brightness = 4π(distance)2

25. Measuring distances beyond parallax • Principal method is “Standard candles”: Find an object whose true luminosity we can guess, measure its apparent brightness, then calculate distance • Out to a few million light-years, the best standard candles are pulsating variable stars True brightness Apparent brightness = 4π(distance)2

26. Variable stars as standard candles • Longer period of variation implies greater luminosity • Tricky because there are different types of variable stars Henrietta Leavitt

27. The Magellanic Clouds