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14 Cards in this Set

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telescopes
reflecting: mirror to focus light
refracting: lenses to focus light
which preferred/why?
reflecting
1. refracting have chromatic aberration (diff wavelegnths; focuses red and blue light differently)
2.mirrors do not absorb light
3.mirrors can be supported by back surface, not edges like lenses
4.mirror only has 1 surface, lenses have 2
why larger telescope?
1.light-gathering power; the larger the opening, the more light will gather
2.resolving power: the bigger the opening, the more 2 close objects will look separated (angular resolution)
limits to angular resolution
size of telescope- need larger one for better resolution
wavelength of light- shorter wavelegnth, the better the resolution
earth's atmosphere limits ground based resolution
CCD
cells which collect light in form of electrical charge

more efficient than photographic plates (less info lost)
more precise than photographic plates (better resolution)
can be easily converted into data analysis
observing at wavelegnths
earth's atmosphere is transparent to radio and visual wavelegnths
is opaque to most other wavelegnths
interstellar medium
matter b/w stars, gas and dust
75% hydrogren
25% helium
trace amounts of metals, microscopic dust
nebula
cloud of gas and dust
reflection nebula
clouds scatter light from nearby cooler stars
blue light scattered more than red, so looks blue (same reason sky looks blue, particles are scatter away in atmosphere)
dark nebula
clouds dense enough to block background light
cooler than surroundings
emission nebula
clouds ionized by nearby hot stars, emit an emission spectrum

H II- ionized hydrogen (hydrogen w/ electron stripped away)need nearby hot stars to ionize; pink
interstellar reddening
dimming of starlight by interstellar matter
tends to scatter higher-frequency (blue) more efficiently than lower-frequ (red) components of radiation
21 centimeter radiation
from hydrogen spin-flip
small energy (21 cm), long wavelegnth
isnt observed by dust
molecular cloud
in coldest regions, hydrogen exists as molecular hydrogen which does not give off 21 cm radiation
use tracers (other elements less abundant than hydrogen) to locate molecular hydrogen