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 kg2. 2 tons3. 2,000 tons4. 2 million tons5. 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. Infrared2. Red3. Blue/Green4. Violet5. Ultraviolet

Spectral Classification of Stars (I)

Tem

pera

ture

Spectral Classification of Stars (II)

Mnemonics to remember the spectral sequence:

Oh Oh OnlyBe Boy, BadA An AstronomersFine F ForgetGirl/Guy Grade GenerallyKiss Kills KnownMe 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 BroadeningHigher 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: