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ASTRO 1504 MIDTERM EXAM
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ASTRO 1504 MIDTERM EXAMSpectroscopy - ANSWER Light from astrophysical objects that don't produce [ure
blackbody spectrum
- solids or hot dense gases produce a continuous spectrum
- light passing through coll diffuse gas removes photons at specific wavelengths in
a continuous spectrum, called absorption lines
- light passing through hot diffuse gases adds photons at specific wavelengths in a
continuous spectrum, called emission lines
Emission lines: atomic level - ANSWER Moving from a higher energy (N=3) to a
lower energy level (N=2), a photon is created (emitted)
Absorption lines: atomic level - ANSWER Moving from a lower energy (N=2) to a
higher energy (N=3), the photon can be absorbed
Doppler effect - ANSWER Motion of astronomical objects causes their spectra to
shift (uniformly) to different wavelengths
- objects receding from the earth will have spectra "REDSHIFTED" (shift to longer
wavelengths)
- objects approaching Earth will have spectra "BLUESHIFTED" (shift to shorter
wavelengths)
ISM - ANSWER Interstellar medium, recycled material, the gas and dust between
stars
- HII regions (very rare) are very hot and diffuse
HII regions are composed of very hot, diffuse gas. What type of spectral features
should this gas produce? - ANSWER Emission lines
IS dust - ANSWER 1. extinction
- things appear dimmer than they really are
, - absorption by dust grains
- scattering by dust grains
2. reddening
- extraction is wavelength dependant - so blue light is scattered out of the line of
sight preferentially
- objects appear 'redder' than they really are
Scattering in action - ANSWER Blue light is preferentially scattered more
- look in any direction other than the line of sight to the star and you should see the
'excess scattered blue light' REFLECTION NEBULA
What is ISM? - ANSWER Mostly gas, some dust (which is different than 'Earth
dust'
What is ISM mainly made out of? - ANSWER Mostly Hydrogen and some Helium,
fractional amounts of metals - much is recycled material
What is the crucial role in determining whether a cloud is molecular, HI or HII
region? - ANSWER Temperature - also affects the presence of dust (destroyed in
hot regions)
What are the two different ways dust can affect observations of most objects? -
ANSWER Extraction - when things appear dimmer
Reddening - when things appear 'redder'
Star formation - ANSWER Stars form from the gravitational collapse of part of (a
clump of) cold, molecular cloud
Why do ISM clouds collapse? - ANSWER Most clouds are in an equilibrium state,
so they doesn't collapse and there's a battle between 2 forces (gravity and pressure)
when these forces are equal, the cloud is in equilibrium and just stay as a molecular
cloud
Why doe clouds collapse? - ANSWER Contraction happens when gravity is greater
than pressure caused by a random, small perturbation and caused by outside help
(compression and stellar wind)
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