* Celestial Sphere: * a sphere surrounding, and rotating around, Earth on which the stars and constellations were once thought to be firmly embedded * Observations: * Stars and constellations seem to move from east to west across the sky * Relative positions of the stars (within the constellations, for example) remained the same

* Result of Earth’s rotation around axis * The rotational axis passes through the north and south celestial poles * Polaris (the North Star) is exactly on the North celestial pole (so doesn’t appear to move) * Circumpolar stars: those stars near the celestial poles

* "Circumpolar AZ81" by LCGS Russ - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - commons.wikimedia.org/wiki/File:Circumpolar_AZ81.jpg #/media/File:C ircumpolar_AZ81.jpg

* Interstellar space: the space within a galaxy between stars * Average distance between stars = ~1 parsec (pc) (=3.26 ly = 3.09 x m) * Distance to nearest star (Proxima Centauri) is ~4.3 ly (1.3 pc) * Average density ~1 atom cm -3 * Intergalactic space: the space between galaxies * Average distance between galaxies= ~100 kpc – Mpc * From Milky Way to Andromeda ~ 2.5 million ly * Average density ~1 atom H per m 3

* Luminosity: the amount of energy radiated by the star per second * Depends on surface temperature and surface area of the star * Apparent brightness: the received energy per second per unit area of the detector

* Used to measure apparent brightness * Photosensitive silicon surface * Number electrons released is proportional to the number of photons that hit the surface. Charge is a direct measure of the brightness of the object being observed * Surface divided into smaller areas = pixels * Charge released in each pixel used to reconstruct an image of the object observed * 50 x more efficient than conventional film

* Objects with measurable temperatures will radiate heat in the form of radiant energy * Depending on the temperature, different wavelengths of light will be emitted

* Relates the wavelength of light with the temperature of the Black Body: * For example…determine the surface temperature of an object that predominantly emits in the visible spectrum:

* Relates the total energy being emitted by the blackbody to its temperature:

* 3 clear features: * Most stars fall on a strip called the main sequence * Red giants are large, cool stars—in the top right corner * Small hot stars (white dwarfs) are in the bottom left