* Celestial Sphere: * a sphere surrounding, and rotating around, Earth on which the stars and constellations were once thought to be firmly embedded *

*Motion of the Stars—historical observations

*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

*Motion of the Stars

*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 Stars

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

*Distances and Densities

*Interstellar space: the space within a galaxy between stars

*Average distance between stars = ~1 parsec (pc) (=3.26 ly = 3.09 x 1016 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 m3

*Luminosity

*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

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*Charged Coupled Device (CCD)

*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

*Black Body Radiation

*Objects with measurable temperatures will radiate heat in the form of radiant energy

*Depending on the temperature, different wavelengths of light will be emitted

*Wien Displacement Law

*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:

KmT 3max 109.2

*Stefan-Boltzmann Law

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

*HR Diagram

*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