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University of South Carolina
ASTR 111

Astronomy Unit 7 -classical Cepheids are also known as Cepheid I stars and W Virginis Cepheids are known as Cepheid II stars -Each of these stars has a fixed period of oscillation -all Cepheid I stars have a period of 1 to 100 day -Cepheid I variables (Classical)- These are very large bright yellow stars (supergiants). -They have absolute magnitudes in the range −1.5 to −5, - with each star’s magnitude varying 0.1 to 2 magnitudes They are extremely rare (~600 in galaxy) and are found only in the spiral arms. -Cepheid II variables (W. Virginis)- These are bluer than Cepheid I stars and have different ligh tcurves. -- -Their periods lie in the range of 3 to 50 days. -found only in globular clusters. -absolute magnitudes in the range −0.5 to −4 A few dozen are known. -Globular clusters- These are symmetric clusters of hundreds of thousands of stars scattered spherically, uniformly about the galaxy. - The stars are different than spiral arm stars since they have a very low metal content. -RR Lyrae variables- These stars are the color of our Sun but much more luminous -with an absolute magnitude of about +0.5; their periods are 0.3 to 0.7 days. -There are 2,500 known, almost all of them are in globular clusters -galaxies often cluster with several hundred members and that these clusters form super clusters -where we can independently, and reliably measure distances to clusters of galaxies, the red shift is larger the farther away the cluster lies -big-bang theory is both a cosmology and a cosmogony -The galactic coordinate system has the same basic properties as the horizon and equator systems: - reference plane (the mean plane) of the Milky Way galaxy, called the galactic equator - There is a reference direction (the direction to the center of the galaxy). - There is a coordinate along the reference plane ( galactic longitude) - a coordinate perpendicular the reference plane ( galactic latitude). -Hubble’s relation is given in the text as: recession velocity is proportional to distance -This description is valid only when the recession velocity is much less than the speed of light. Astronomy Unit 9 -The meridian may also be referred to as the celestial meridian or the local meridian -The ecliptic poles are points on the celestial sphere 90° away from the ecliptic. -They have the same relation to the ecliptic that the celestial poles have to the celestial equator -Angles between planes are measured in a plane perpendicular to the line of intersection of the two planes -Columbia is at a longitude of 81° W and latitude of 34° N. -Precession of the Earth’s axis is a wobbling of the polar axis with respect to the external stars caused by the other planets and the Moon -Earth’s wobble has a 26,000 year period, so the R.A. and declination of stars changes slowly -the reference plane and direction of the equatorial system are also changing -Solar eclipses are extremely useful for: -exact determination of
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