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Lecture 6

Lecture 6--11, Study Guide


Department
Astronomy
Course Code
ASTA01H3
Professor
Rick Halpern
Lecture
6

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Lecture 6
Escape velocity: the speed necessary for a projectile to completely escape a
planets gravitational field
Radiation
oEM Radiation
Visible light is the particular form of em radiation that our
eyes espond to
Invisible radiation: infrared, ultraviolet, x rays and gamma
rays
All travel at speed of light and travel in form wave through
space
oWave Properties
Period is amount of time required wave pattern to repeat
itself
Wavelength measured in nanometer (10-9m)
The Angstrom is 0.1nm or 10-10m
oCharged Particles
Changes in the electric field of the particle travel out
through the electric filed as em waves
The frequency at which particle vibrates determines the
frequency of the EM waves
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If a charged particle begins to vibrate its electric field
changes and the resulting disturbance travels through
space as wave
oEM waves
Electric fields propagates with magnetic fields
A change in one field causes change in the other
These fields are always perpendicular to eachother
Light travels at 300 000km/s in vacuum
oHigh Speed observers / Doppler Effect
The Doppler effect refers to a change in the perceived
frequency of a wave that occurs when the source is moving
towards or away from the observer
Observers in fast moving craft will see stars ahead bluer
than normal and those behind are reddened
The front source wave becomes shorter wavelength and
appear blue shifted
The back source wave become stretched and wavelength
becomes longer so appears red shifted
oDoppler Effect Quantified
Two objects moving apart is receding and they have a
recession velocity
Object approaching eachother have negative recession
velocity
C=lambda*f
Lecture 7
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273.15K = 0oC
Blackbody Curves, Ideal vs Reality
oA blackbody absorbs all radiation falling on it but then
reemits otherwise temperature would not change
oBlackbody curve represents intensity of radiation emitted
by any object over all possible frequencies
oThe peak emission wavelength decreases with temperature
oObject with the suns surface temperature peaks in the
visible spectrum
oThe total amount of energy emitted per unit area increases
temperature as fourth power
oComparison of blackbody curves of four cosmic objects. The
frequencies and
owavelengths corresponding to peak emission are marked.
oA cool, invisible galactic gas cloud called Rho Ophiuchi. At a
temperature
oof 60 K, it emits mostly low-frequency radio radiation.
oA dim, young star (shown red in the inset photograph) near
the center of
othe Orion Nebula. The stars atmosphere, at 600 K, radiates
primarily in the
oinfrared, here falsely coloured to represent differences in
temperature.
oThe Suns surface, at approximately 6000 K, is brightest in
the visible
oregion of the electromagnetic spectrum.
oSome very hot, bright stars in a cluster called Omega
Centauri, as
oobserved by a telescope aboard a space shuttle. At a
temperature of
o60,000 K, these stars radiate strongly in the ultraviolet.
(Harvard College
oObservatory; J. Moran; AURA; NASA)
Wiens Law
oT in kelvins
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