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Light is all we see in
Astronomy
Variability
(change with time)
There are three basic aspects of
the light from an object that
we can study from the Earth.
Intensity
(spatial distribution of the light)
Spectra
(composition of the object
and the object’s velocity)
Most kinds of e-m radiation cannot
penetrate the Earth's atmosphere
Imaging
Imaging
An Object’s Spectrum
Encoded in an object’s spectrum
is information about the emitter/absorber.
This is how we learn what the Universe
is made of!
Spectra of galaxies
27 COSMOS BzK
<z>=1.87
The Four Basic Parameters of
Stars
Luminosity
Size
Mass
Surface Temperature

How to measure the surface
temperature of a star?
1.
2.
Overall spectral shape (the peak of
the blackbody continuous spectrum)
More accurately, spectroscopically
Remember: the peak of the B-B radiation lets us know the
temperature of the emitting body.
Spectral Types
A classification of the stellar black body
For historical reasons,
astronomers classify the
temperatures of stars on a
scale defined by spectral
types, called O B A F G
K M, ranging from the
hottest (type O) to the
coolest (type M) stars.
The sun has a spectral type: G2
Temperature, Luminosity, and
Size – pulling them all together
A star’s luminosity, surface temperature, and size
are all related by the Stefan-Boltzmann Law:
Stefan-Boltzmann Law
L=4πR2 σT4
Luminosity
Stellar
radius
Surface
temperature
L=4πR2 σT4
Two stars have the same surface temperature, but
the radius of one is 10 times the radius of the other.
The larger star is
1) 10 times more luminous
2) 100 times more luminous
3) 1000 times more luminous
4) 1/10th as luminous
5) 1/100th as luminous
The Doppler Effect:
other information contained in spectrum


A moving light or sound source
emits a different frequency in the
forward direction than in the reverse
direction.
Take a look at the police car to see
how this works.
In general …



The “native” frequency at which an object
is emitting is called the rest frequency.
You will see/hear frequencies higher than
the rest frequency from objects moving
towards you.
You will see/hear frequencies lower than
the rest frequency from objects moving
away from you.
Doppler Effect
The first crest travels out in
circle from the original position
of the plane
Shorter wavelength
(more blue)
At a later time, a second
crest is emitted from the
planes new position,
but the old crest keeps
moving out in a circle
from the planes original
position
The same thing happens again at
a later time
Longer
wavelength
(more red)
But the spectrum tells us about
the motion of sources
Two identical stars are moving towards the Earth.
Star A’s emission lines are observed to be at
visible wavelengths. The same emission lines
for Star B are observed to be at ultraviolet
wavelengths. From these observations you
conclude that:




Both stars are moving away from the Earth
Star A is moving towards the Earth faster than
Star B
Star B is moving towards the Earth faster than
Star A
Star B is moving away from the Earth while
Star A is moving towards the Earth.
The Doppler shift

An object shining red light with l=656.3
nm is moving at V=5,000,000 m/s
toward you. What is the color of the light
that you see?


V/c = lo- lr/lr
5x106/3x108 = 1.67x10-2 = lo- lr/lr
lr = lox(1+1.67x10-2) = 667.3 nm
Two otherwise identical stars are rotating at
different rates. Star A is rotating slower
than Star B. How do Star A’s spectral
lines appear with respect to Star B’s
lines?
Star A’s lines are narrower than Star B’s
lines.
Star B’s lines are narrower than Star A’s
lines.
There is no difference in the lines of the
two stars.
Star A’s lines are stronger than Star B’s
lines.