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Lecture 7:
The Physics of Light, part 2
Astronomy 111
Spectra
“Twinkle, twinkle,
little star,
How I wonder
what you are
are.”
ASTR111 Lecture 7
Every type of atom, ion, and
molecule has a unique spectrum
Ion: an atom with electrons added
(negative ion) or taken away (positive
ion).
ion)
Molecule: two or more atoms bonded
together.
The spectrum of each atom, ion,
and molecule is a distinctive
“fingerprint”.
The more
complicated
li t d
the atom, ion
or the
th
molecule, the
more complex
l
the spectrum.
electron
neutron
t
proton
ASTR111 Lecture 7
From emission
F
i i or absorption
b
ti lilines, we kknow:
1) which elements are present;
2) whether they are ionized;
3) whether they are in molecules
molecules.
emission spectrum of the Carina Nebula
ASTR111 Lecture 7
Kirchoff’s Laws of
Spectroscopy
1) A hot solid or hot, dense gas produces
a continuous spectrum.
2) A hot, low-density gas produces an
emission-line
i i li spectrum.
t
3) A continuous spectrum source viewed
through a cool, low-density gas
produces an absorption-line
p
p
spectrum.
p
ASTR111 Lecture 7
Continuum
Source
Cloud
ASTR111 Lecture 7
ASTR111 Lecture 7
A hot, low density cloud of gas
produces an emission line spectrum
Light is emitted only at wavelengths
corresponding
co
espo d g to e
energy
e gy d
differences
e e ces
between permitted electron orbits.
Results: an emission line spectrum
spectrum.
Hydrogen emission spectrum
ASTR111 Lecture 7
The Carina Nebula
Neb la
A cloud of hot, low
density gas about
7000 light years
away.
y
Its reddish color
comes from the
656.3 nm emission
line of hydrogen.
ASTR111 Lecture 7
A cool, transparent gas produces
an absorption line spectrum
Consider
C
id a cold,
ld llow d
density
it cloud
l d off
hydrogen in front of a hot blackbody.
Light is absorbed only at wavelengths
corresponding to energy differences
between permitted electron orbits.
Result: an absorption
p
line spectrum.
p
ASTR111 Lecture 7
Absorption spectra can tell us
about extrasolar planets
• A planet’s atmosphere is a cold, low
density
y cloud of gas
g illuminated by
ya
background source (its star)
ASTR111 Lecture 7
The most abundant elements in the
Universe are hydrogen and helium
It is fairly easy to determine which
elements are p
present in a star.
It is much harder to determine how much
of each element is present
present.
Strength of emission and absorption lines
depends on temperature as well as on
the element’s abundance.
ASTR111 Lecture 7
Abundance of elements
i the
in
th S
Sun’s
’ atmosphere:
t
h
Hydrogen (H): 75%
Helium (He): 23%
Everything else: 2%
As discovered in 1920’s,
1920 s, other stars are
mostly hydrogen and helium, too.
ASTR111 Lecture 7
Cecilia
C
ili PayneP
Gaposchkin (19001979) was a BritishAmerican astronomer.
She left England
g
in 1922.
In 1925, she became the
first ever Ph.D. in
astronomy from Harvard.
Her thesis established
that hydrogen was the
overwhelming constituent
of the stars.
ASTR111 Lecture 7
Temperature Scale
I physics
In
h i and
d astronomy,
t
we use th
the
Kelvin scale, which has a zero at
absolute
b l t zero.
Kelvin = Celsius + 273
Water boils: 373 Kelvin
Water freezes: 273 Kelvin
Ab l t zero: 0 K
Absolute
Kelvin
l i
ASTR111 Lecture 7
An object is hot when the atoms of which it is
made are in rapid random motion
motion.
The temperature is a measure of the average
speed of the atoms
atoms.
Random motions stop at absolute zero
temperature.
temperature
A hot, opaque object produces a
continuous blackbody spectrum
of light
The universe is full of light of all different
wavelengths. How is light made?
One way
y to make objects
j
emit light
g is to
heat them up.
ASTR111 Lecture 7
Blackbody Radiation
A Blackbody is an object that absorbs all light
light.
• Absorbs at all wavelengths
• Characterized by its Temperature
It is also the perfect radiator:
• Emits
E it att allll wavelengths
l
th ((continuous
ti
spectrum)
• Total Energy emitted depends on
Temperature
• Peak wavelength also depends on
Temperature
ASTR111 Lecture 7
Wien’s
Wien
s Law
Wavelength
W
l
th off maximum
i
emission
i i
is inversely related to temperature
max
2,900,000 nm
max 
T
 wavelength of maximum emission
T  temperature (in Kelvins)
Stefan-Boltzmann Law
Energy emitted
E
itt d per second
d per area b
by a
blackbody with Temperature (T):
E = T
4
 is
i B
Boltzmann's
lt
' constant
t t (a
( number)
b ).
In Words:
“Hotter objects are Brighter at All
Wavelengths”
a e e gt s
ASTR111 Lecture 7
Blackbody curves:
ASTR111 Lecture 7
Solar spectrum:
ASTR111 Lecture 7
Taking the
temperature of stars
Betelgeuse:
g
a
reddish star
(
(cooler).
)
Rigel: a bluish
star (hotter).
(hotter)
ASTR111 Lecture 7
Stellar spectra in
order from the hottest
(top) to coolest
(
(bottom).
)
ASTR111 Lecture 7
Inverse-Square Law of
Brightness
Luminosity is not the same
as Brightness
B i ht
!
Luminosity is how much light
leaves a source (it does not
depend on your location).
location)
Brightness is how much light
arrives at a particular
location (it depends on how
far away you are).
ASTR111 Lecture 7
Doppler shift
• Light can experience a Doppler shift
much like the change in frequency of a
train whistle as it passes an observer
ASTR111 Lecture 7
Th reason ffor D
The
Doppler
l shifts:
hift
Wave crests are bunched up ahead of the
light source, stretched out behind.
ASTR111 Lecture 7
The Doppler effect in light
Amount of Shift depends upon the emitted
wavelength (em) and the relative speed v:
• If the motion is away from observer
Wavelength gets longer = REDSHIFT
• If the
th motion
ti is
i towards
t
d the
th observer
b
Wavelength gets shorter = BLUESHIFT
ASTR111 Lecture 7
If a light source is moving toward you, the
wavelength is shorter (called a “blueshift”)
blueshift ).
If a light source is moving away from you, the
wavelength is longer (called a “redshift”).
ASTR111 Lecture 7
The radial velocity of an object is
found from its Doppler shift
Radial
R
di l velocity
l it = how
h
ffastt an object
bj t iis
moving toward you or away from you.
If a wave source moves toward you or
away from
f
you, the
th wavelength
l
th is
i
changed.
ASTR111 Lecture 7
Size of Doppler shift is
proportional to radial velocity

vr

0
c
  observed wavelength shift
0  wavelength if source is not moving
vr  radial
di l velocity
l i off moving
i source
p
of light
g
c  speed
ASTR111 Lecture 7
Example:
p
Hydrogen absorbs light with
λ0  656.3 nanometers
t
But we observe a star with absorption line at
λ  656.2 nanometers.
Δλ  0.1 nm
  
c
v r  
 0 
  0.1 nm 
vr  
 300,000 km/sec
 656.3 nm 
v r  46 km/sec
ASTR111 Lecture 7
Way to Measure Speeds
Observe the wavelength (obs) of a source
g ((em)
with a known emitted wavelength
The difference is directly proportional to
the speed of the source,
source v:
(  obs
v
  em )   em   
c
ASTR111 Lecture 7