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
- 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
-Angles between planes are measured in a plane perpendicular to the line of intersection of the
-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
-the reference plane and direction of the equatorial system are also changing
-Solar eclipses are extremely useful for:
-exact determination of