* celestial sphere: * a sphere surrounding, and rotating around, earth on which the stars and...
Post on 27-Dec-2015
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*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
𝑏=𝐿
4𝜋 𝑑2
*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
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