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Light and Atoms
Light and MatterSpectra of stars are
more complicated than pure blackbody spectra.
→ characteristic lines, called Fraunhofer (absorption) lines.
→ atomic structure and the interactions between light and
atoms.
Atomic Structure
• An atom consists of an atomic nucleus (protons and neutrons) and a cloud of electrons surrounding it.
• Almost all of the mass is contained in the nucleus, while almost all of the space is occupied by the electron cloud.
If you could fill a teaspoon just with material as dense as the matter in an atomic nucleus, how much would
you guess this would weigh?
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1. 2 kg
2. 2 tons
3. 2,000 tons
4. 2 million tons
5. 2 billion tons
Different Kinds of Atoms• The kind of atom
depends on the number of protons in the nucleus.
Helium 4
Different numbers of neutrons ↔ different isotopes
• Most abundant: Hydrogen (H), with one proton (+ 1 electron).
• Next: Helium (He), with 2 protons (and 2 neutrons + 2 el.).
Electron Orbits• Electron orbits in the electron cloud are
restricted to very specific radii and energies.
r1, E1
r2, E2
r3, E3
• These characteristic electron energies are different for each individual element.
Larger orbital radus r
Higher electron energy
=> E3 > E2 > E1
Orbit 1:
“Ground State”
Atomic Transitions
• An electron can be kicked into a higher orbit when it absorbs a photon with exactly the right energy.
• All other photons pass by the atom unabsorbed.
Eph = E4 – E1
Eph = E3 – E1
(Remember that Eph = h*f)
Wrong energy• The photon is absorbed,
and the electron is in an excited state.
Which one of the three photons has the highest frequency (i.e., the highest energy)?
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A BC
D: They all have the same frequency.
=> Photoionization
For very high photon energy ( high frequency; short wavelength), an electron can be kicked out of the atom completely.
Absorption spectra
• Only light at very specific frequencies (energies) is absorbed.
• Light at all other frequencies passes through.
Animation
This is causing the typical absorption spectra of stars.
Analyzing absorption spectra• Each element produces a specific set of
absorption (and emission) lines.
By far the most abundant elements in the Universe
• Comparing the relative strengths of these sets of lines, we can study the composition of gases.
animation
The Balmer Lines
n = 1
n = 2
n = 4
n = 5n = 3
H H H
The only hydrogen lines in the visible wavelength range.
Transitions from 2nd to higher levels of hydrogen
2nd to 3rd level = H (Balmer alpha line)2nd to 4th level = H (Balmer beta line)
…
The Cocoon Nebula (H emission)
Knowing that the H line is red, what color would you expect the H line to have?
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1. Infrared
2. Red
3. Blue/Green
4. Violet
5. Ultraviolet
Spectral Classification of Stars (I)
Tem
pera
ture
Spectral Classification of Stars (II)
Mnemonics to remember the spectral sequence:
Oh Oh Only
Be Boy, Bad
A An Astronomers
Fine F Forget
Girl/Guy Grade Generally
Kiss Kills Known
Me Me Mnemonics
If a star is moving towards us with a velocity of 30,000 km/s, we will see its light approaching
us with a velocity of … and its color …
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1. 330,000 km/s; unchanged.
2. 300,000 km/s; unchanged.
3. 330,000 km/s; shifted towards the blue end of the spectrum.
4. 300,000 km/s; shifted towards the blue end of the spectrum.
5. 300,000 km/s; shifted towards the red end of the spectrum.
The Doppler Effect
Blue Shift (to higher frequencies)
Red Shift (to lower frequencies)
The light of a moving source is blue/red shifted by
/0 = vr/c
0 = actual wavelength emitted by the source
Wavelength change due to Doppler effect
vr = radial velocity
vr
animation
The Doppler effect allows us to measure the source’s radial velocity.
vr
Example:Take of the H (Balmer alpha) line:
0 = 658 nmAssume, we observe a star’s spectrum
with the H line at = 660 nm. Then, = 2 nm.
We find = 0.003 = 3*10-3
Thus,
vr/c = 0.003, or
vr = 0.003*300,000 km/s = 900 km/s.The line is red shifted, so the star is receding
from us with a radial velocity of 900 km/s.
Doppler BroadeningIn principle, line absorption
should only affect a very unique wavelength.
Observer
Atoms in random thermal motion
vr
vr
Red shifted abs.Blue shifted abs.
In reality, also slightly different wavelengths are
absorbed.
↔ Lines have a finite width; we say:
they are broadened.
One reason for broadening: The Doppler effect!
Line Broadening
Higher Temperatures Higher thermal velocities
broader lines
Doppler Broadening is usually the most important broadening mechanism.
• Pressure Broadening (density diagnostic)
• Natural Broadening
Other line broadening mechanisms: