Download Light and Atoms • The only thing we can get out of stars (and most

Light and Atoms
• The only thing we can get out of stars (and most other objects in the
heavens) is LIGHT! (Electromagnetic (EM) Radiation)
• We need to understand what this stuff is better.
What is Light?
• A struggle for many years. What IS this stuff?
• Light understood at one level due to James Clerk Maxwell who worked
out equations of light in the 1860s.
Maxwell showed that light is just a very very small piece of a larger whole:
the Electromagnetic (EM) spectrum.
The Wave Picture of Light and EM Radiation
Light viewed as a wavelike quantity.
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• Wavelength λ measured in meters
• Need to measure the frequency f , which is in Hz=1/second
• All EM waves in vacuum obey λf = c (c is the speed of light, 3 × 108
m/second).
• As the wavelength decreases, we go from Radio Waves to Microwaves to
Infrared to Visible to Ultraviolet to X-rays to Gamma rays.
• Astronomers use all of these to look at objects.
What parts of the EM spectrum can we look at here on Earth?
Earth based systems can see visible and Radio only.
• To see the other types of EM radiation, we need to put our observers onto
satellites.
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• Satellites, like Chandra (X-rays), ISO (Infrared), IUE (Ultraviolet), CGRO
(Gamma rays).
• We will talk about all of these. Only visible images ”look” like we expect. All the others use false color to indicate how ”much” radiation they
measured (like Infrared)
EM Waves
What is actually “waving” in an EM wave?
EM Waves
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• Lots of experiments show that light behaves as a wave
• Yet, there was a problem. . .
•
Blackbody Radiation
Blackbody Radiation
• Noticed by us as the reddish glow of hot objects (stoves, candle flames,
...)
• Every object gives off some amount of blackbody radiation
• Called “Blackbody” because the better absorber a material is (the blacker)
the more radiation it puts out.
• Can study it just like Newton did with sunlight.
Blackbody Radiation
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What do we find with Blackbody Radiation?
Continuous range of wavelengths given off, with a maximum wavelength
where the intensity is greatest, λm .
Location of maximum follows Wien’s law, or
λm T = constant
Total amount of power that is radiated away:
P ower ∝ T 4 .
Notice that T must be expressed in the absolute temperature scale where T(K)=T(◦ C)+273.15,
and T(◦ C)= 59 (T(◦ F)-32).
Blackbody Radiation
Can we predict the shape of the curve with light as a wave?
Blackbody Radiation
• Didn’t work so well.
• This and other results showed that although Light behaved as a wave in
some ways, in other ways it behaves as a particle
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• Max Planck and Albert Einstein: The idea of a “Quanta” of light: E =
hf = hc/λ
• Explains the shape of the curve and why hotter objects go from red to
blue, since blue light has more energy/chunk than red.
• Does make understanding what light actually “is” even more difficult...
We use the phenomena of Blackbody radiation to measure the temperature
of stars, since they behave like “blackbodies”.
How does this connect with atoms?
• We need to get our length scales straight
• Galaxies are about 100,000 light years or 1021 meters in size
• Atoms are about 10−8 meters in size
• Nuclei are about 10−15 meters in size
• We need to understand the interaction between atoms and light, so let’s
work on our picture of the atom.
The Atom
• How to picture the atom? Start with Hydrogen, 1 proton and 1 electron.
• Earliest thought was a hard sphere, like a billiard ball
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• When electrons discovered in late 1890’s, struggle was to put them in.
“Plum Pudding Model”
This did not fit results from Rutherford which showed mass of atom concen-
trated. His solar system model:
• To get an idea of the scale, imagine that a Hydrogen atom is the size of a
football field.
• How big is the nucleus (here a proton)?
• The proton in the nucleus is the size of a sesame seed at the 50 yard line,
and the electron is at least 1000x smaller still (maybe more so!) Lots of
empty space.
• Further work showed that the electrons do not “orbit” the nucleus, but
exists as waves (like light!) and are not easily localized in the atom.
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Although the location of the electrons are not well defined, the allowed
energies of the atom have definite values. Each atom has its own energy levels,
its own fingerprint. This gives the spectra for that type of atom. (Hydrogen)
• Many different atoms, corresponding to the elements in chemistry. Now
up to 118 different ones.
• Atoms combine in different ways to form compounds, like H2 O, CO2 , NO2 .
• We draw the atoms as having definite energy levels, not locations of electrons.
• More protons in the nucleus, then more strongly the atom holds onto the
electrons.
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Atoms and Light
Since energy levels are fixed for atoms, only specific wavelengths of light are
absorbed or emitted. (Quantum Mechanics)
• Atoms will emit light if they are given energy (Thermal excitation)
• Go up in energy for brief time and then drop down in energy and give off
light.
• This light identifies which atoms are present.
• Atoms also will only absorb very specific wavelengths of light
• How does this all tie into blackbody radiation which is continuous?
Two kinds of light produced by objects:
Gaseous (not dense) give off discrete spectra and can absorb discrete types
of light.
Solids, liquids, dense gases at a temperature T give off a continuous blackbody spectrum.
Interaction of light with gases in space
• When light is created and then interacts with gases in space several things
can happen.
• Kirchoff’s Laws (K & K page 110)
• A heated solid, liquid, or dense gas will produce a continuous spectra.
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• A low density gas when excited will produce a discrete emission spectra.
If continuous light goes through a low density gas the light will appear as
an absorption spectra.
Every atom has its unique fingerprint:
Stellar Spectra
• We can tell the general temperature of a star by its color via blackbody
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radiation
• Which is hotter, a red or blue star?
• From blackbody radiation we know that λm T =constant, so this will give
us the temperature of the outer layers of the star where the radiation
comes from.
• But this is not always as accurate as we would like.
• So we use another way to get the temperature of a star.
Balmer Thermometer
Hydrogen has a series of lines it will absorb and emit. The ones we can see
are the Balmer series.
Balmer Thermometer
• In order to see absorption by Hydrogen in the Balmer lines:
• Hydrogen must have its energy at the second level already (T > 4000 K)
and
• the Hydrogen atoms must still have electrons attached to them! (T <
20, 000 K)
In that case we can measure how strongly the light is absorbed and compare
to measurements here on Earth.
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The problem is we get two different temperatures!
What we need are more counselors...
• With a number of measurements we can pin down the surface temperature
of a star to good precision.
• The hottest surface star temperatures are around 40,000 K
• The coolest are down to 2000 K
• The Sun comes in at about 5800 K
• We will see later that we classify stars by their temperatures (OBAFGKM)
Doppler Effect
We can also use the light from a star to measure its motion
You have no doubt heard the Doppler Effect with sound as a Police car
approaches and passes you.
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Doppler Effect
• Same thing happens with light (and is the basis for Radar Speed Traps!)
• If a source of radiation moves relative to us, the wavelength we observe
will be shifted from what it would have been if it were not moving.
• The amount of shift is given by
∆λ
vr
=
λ
c
where c is the speed of light
• We say it is moving away (red shifted) if ∆λ > 0 and heading towards us
(blue shifted) if ∆λ < 0
This only gets us the motion away or towards us. The motion perpendicular
to that is harder to get.
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