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Starlight and AtomsChapter 6

The Amazing Power of StarlightJust by analyzing the light received from a star,

astronomers can retrieve information about a star’s

1. Total energy output

2. Surface temperature

3. Radius

4. Chemical composition

5. Velocity relative to Earth

6. Rotation period

Black Body Radiation (I)

The light from a star is usuallyconcentrated in a rather narrow

range of wavelengths.

The spectrum of a star’s light isapproximately a thermal

spectrum called black bodyspectrum.

A perfect black body emitterwould not reflect any radiation.Thus the name ‘black body’.

Two Laws of Black Body Radiation

2. The peak of the black body spectrum shifts towardsshorter wavelengths when the temperature increases.

→ Wien’s displacement law:

λmax ≈ 3,000,000 nm / TK

(where TK is the temperature in Kelvin).

1. The hotter an object is, the more luminous it is.

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Examples: Wien’s Law

1. Calculate the peak wavelength of a starwhose surface temperature is 400000 K.

2. Calculate the temperature of a starwhose peak wavelength is 30000 K.

Color and Temperature

OrionBetelgeuse

Rigel

Stars appear indifferent colors,

from blue (like Rigel)

via green / yellow (likeour sun)

to red (like Betelgeuse).

These colors tell usabout the star’s

temperature.

Light and MatterSpectra of stars are

more complicated thanpure black body spectra.

→ characteristic lines,called absorption lines.

To understandthose lines, we

need tounderstand atomicstructure and the

interactionsbetween light and

atoms.

Atomic Structure• An atom consists of

an atomic nucleus(protons andneutrons) and acloud of electronssurrounding it.

• Almost all of themass is containedin the nucleus,while almost all ofthe space isoccupied by theelectron cloud.

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If you could fill ateaspoon just with

material as dense asthe matter in an atomicnucleus, it would weigh

~ 2 billion tons!!

Different Kinds of Atoms• The kind of atom

depends on thenumber of protonsin the nucleus.

Helium 4

Differentnumbers ofneutrons ↔differentisotopes

• Most abundant:Hydrogen (H),with one proton(+ 1 electron).

• Next: Helium (He),with 2 protons (and2 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 differentfor each individual element.

IncreasingEnergy

Atomic Transitions

• An electron canbe kicked into ahigher orbitwhen it absorbsa photon withexactly the rightenergy.

• All other photons pass by the atom unabsorbed.

Eph = E4 – E1

Eph = E3 – E1

(Remember that Eph = h*f)

Wrong energy• The photon isabsorbed, and theelectron is in anexcited state.

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Emission line spectra• Record the spectrum of each emission tube.

Kirchhoff’s Laws of Radiation (I)1. A solid, liquid, or dense gas excited to emit

light will radiate at all wavelengths and thusproduce a continuous spectrum.

Kirchhoff’s Laws of Radiation (II)2. If light comprising a continuous spectrum

passes through a cool, low-density gas, theresult will be an absorption spectrum.

Light excites electrons inatoms to higher energy states

Wavelengths corresponding to thetransition energies are absorbedfrom the continuous spectrum.

Gas re-emits in all directions, so thereis a net loss along line of sight to star

Kirchhoff’s Laws of Radiation (III)3. A low-density gas excited to emit light will do

so at specific wavelengths and thus producean emission spectrum.

Light excites electrons inatoms to higher energy states

Transition back to lower statesemits light at specific frequencies

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The Spectra of StarsInner, dense layers of a

star produce a continuous(black body) spectrum.

Cooler surface layers absorb light at specific wavelengths.

=> Spectra of stars are absorption spectra ontop of blackbody spectrum!!!

Most prominent linesin many astronomicalobjects: Balmer lines

of hydrogen

Lines of Hydrogen Hydrogen is most abundant element

Absorption spectrum dominated by Balmer lines

Modern spectra are usually recordeddigitally and represented as plots of

intensity vs. wavelength

Emission nebula, dominatedby the red Hα line.

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Spectral Classification of Stars (I)

Tem

pera

ture

Different types of stars show differentcharacteristic sets of absorption lines.

Spectral Classification of Stars (II)

MnemonicsMeMe

KnownKillsKiss

GenerallyGradeGirl/Guy

ForgetFFine

AstronomersAnA

BadBoy,Be

OnlyOhOh

Mnemonics to remember thespectral sequence: