Light and Atoms

Light and Matter Spectra 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? 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. r 1, E 1 r 2, E 2 r 3, E 3 These characteristic electron energies are different for each individual element. Larger orbital radus r  Higher electron energy => E 3 > E 2 > E 1 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. E ph = E 4 – E 1 E ph = E 3 – E 1 (Remember that E ph = 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)? AB C 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 = 5 n = 3 HH HH HH The only hydrogen lines in the visible wavelength range. Transitions from 2 nd to higher levels of hydrogen 2 nd to 3 rd level = H  (Balmer alpha line) 2 nd to 4 th 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? 1.Infrared 2.Red 3.Blue/Green 4.Violet 5.Ultraviolet

Spectral Classification of Stars (I) Temperature

Spectral Classification of Stars (II) Mnemonics to remember the spectral sequence: OhOhOhOhOnly BeBeBoy,Bad AAnAnAstronomers FineFForget Girl/GuyGradeGenerally KissKillsKnown MeMeMeMeMnemonics

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 … 1.330,000 km/s; unchanged ,000 km/s; unchanged ,000 km/s; shifted towards the blue end of the spectrum ,000 km/s; shifted towards the blue end of the spectrum ,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 = v r /c 0 = actual wavelength emitted by the source  Wavelength change due to Doppler effect v r = radial velocity vrvr animation

The Doppler effect allows us to measure the source’s radial velocity. vrvr

Example: Take  of the H  (Balmer alpha) line: 0 = 658 nm Assume, we observe a star’s spectrum with the H  line at = 660 nm. Then,  = 2 nm. We find   = = 3*10 -3 Thus, v r /c = 0.003, or v r = 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 Broadening In principle, line absorption should only affect a very unique wavelength. Observer Atoms in random thermal motion vrvr vrvr 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: