AST- 105 1st EditionLecture 17Outline of Last LectureI. Main Sequence EvolutionII. Red Giant PhaseIII. Helium Flash PhaseIV. Horizontal Branch PhaseV. Second Red Giant PhaseVI. Planetary Nebula PhaseVII. White Dwarf PhaseOutline of Current Lecture I. Open Star ClusterII. Globular Star ClusterIII. Star Cluster AgesIV. The EquinoxesCurrent Lecture I. Open Star Clustera. Found in the disk of the Galaxyi. Ex. The Pleiadesb. Stars are young and recently formed.c. Most stars are near main sequence.d. The most massive stars are just leaving the main sequence.e. Age is approximately 2 x 10^7 years.II. Globular Star Clustera. Found in the bulge/halo of our Galaxyb. Contain only old stars.c. Age is approximately greater than 10^10 years.d. Almost no stars are present on upper main sequencei. These stars have moved on to the next phase.III. Star Cluster Agesa. Main Sequence Turnofi. Main Sequence Turnof- location in H-R diagram of stars just becoming red giants.b. The younger the cluster, the higher the mass of the turnof stars.c. The location of the turnof determines the age of the cluster. IV. The Equinoxesa. Earth’s axis is tilted by 23.5 degrees away from and perpendicular to Earth’s orbital plane.b. At Equinoxes: i. Equinox- When day and night have equal length (i.e. 12 hr passes between sunrise and sunset).1. Day is longer in the summer.2. Day is shorter in the winter.ii. The Sun rises due East and sets due West.iii. Autumnal (fall) Equinox is approx. on Sept. 22iv. Vernal (spring) Equinox is approx. on March 211. At Autumnal and Vernal Equinox, the Earth’s axis tilts neither toward or away from the Sun. 2. The Sun is overhead at the equator on the
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