Starlight and Atoms, Starlight and Atoms, or How to Make Light or How to Make Light

Origin of lightOrigin of light(actually, electromagnetic radiation)(actually, electromagnetic radiation)

Light is Electromagnetic Radiation (EMR) that we see Radiant heat is EMR that we feel (like heat) EMR is a changing electric and magnetic field -

electromagnetic radiation. Better yet: EMR is energy being transported in space

by making the E. and M. field vary It is similar to waves, I.e. energy being transported by

making velocity and density of stuff (water, rubber) vary.

Changing electric field, e.g. by accelerating electrons We discuss two major mechanisms of EMR production:

Thermal blackbody Radiation Spectral Line Emission

Heat and TemperatureHeat and Temperature• The hotness or coldness of an object is characterized by its temperature

•Temperature ONLY refers to the degree of agitation, or the speed with which the particles move.

•Heat is the total amount of thermal energy. So, it increases if you have more temperature. But it also increases if you have more mass

•Heat is energy. It takes much more energy to make a 10 gallon pot of water boil than to make a pint-size pot of water boil (yet, same temperature: 100 C).

TemperatureTemperature

•In science, temperature is measured in K (degree Kelvin)

•All atoms are vibrating unless at absolute zero temperature (T = 0 K = -459.7 F). •Water freezes at 273 K and boils at 373 K.

•Heat is amount of energy stored in a body. It is measured in Joule (J)

•Yet, heat can only go from bodies with higher temperature to bodies with lower temperature

Blackbody SpectrumBlackbody Spectrum•Remember that EMR is characterized by wavelength (frequency)

•Spectrum: distribution of wavelength (or frequency) of some EMR

•Blackbody: the distribution of EMR at equilibrium with matter

•Equilibrium means that matter and EMR exchange the same amount of energy

•Blackbody emission is a continuum: all wavelengths are present, although with different intensity

EMR and MatterEMR and Matter EMR and matter interact all the timeEMR and matter interact all the time This means that matter absorbs and emits EMRThis means that matter absorbs and emits EMR Often, the means of interaction is the acceleration of an Often, the means of interaction is the acceleration of an

electron. But this is not the only way, and other ways electron. But this is not the only way, and other ways are possible. The details are not crucial noware possible. The details are not crucial now

As long as there is a strong interaction, matter and EMR As long as there is a strong interaction, matter and EMR can reach an equilibrium can reach an equilibrium

When matter and EMR are at equilibrium in a given When matter and EMR are at equilibrium in a given body, absorption and emission balance each other, I.e. body, absorption and emission balance each other, I.e. matter does not gain or loose energy, and keep its matter does not gain or loose energy, and keep its temperature the same. temperature the same.

At equilibrium, the EMR has the At equilibrium, the EMR has the black-body spectrumblack-body spectrum Black-body spectrum: it is when interacting matter and Black-body spectrum: it is when interacting matter and

radiation are at equilibrium.radiation are at equilibrium. Matter keeps it Matter keeps it temperature; radiation keeps it spectrumtemperature; radiation keeps it spectrum

Hotter objects emit photons with a Hotter objects emit photons with a higher average energy.higher average energy.

Wien’s LawWien’s Law

• The peak of the blackbody emission The peak of the blackbody emission spectrum is given by spectrum is given by

nmT(Kelvin)

109.2 6

max

The higher the temperature, the shorter the wavelength, i.e. the bluer

The graph below shows the blackbody spectra of The graph below shows the blackbody spectra of three different stars. Which of the stars is at three different stars. Which of the stars is at the highest temperature?the highest temperature?

1) Star A1) Star A

2) Star B2) Star B

3) Star C3) Star CA

B

C

EnergyperSecond

Wavelength

Hotter objects emit more total Hotter objects emit more total radiation per unit surface area.radiation per unit surface area.

Stefan-Boltzmann Law: Emitted power per unit area = σ T4

σ = 5.7 x 10-8 W/(m2K4)

You are gradually heating up a rock in an oven You are gradually heating up a rock in an oven to an extremely high temperature. As it to an extremely high temperature. As it heats up, the rock emits nearly perfect heats up, the rock emits nearly perfect theoretical blackbody radiation – meaning theoretical blackbody radiation – meaning that itthat it

1) is brightest when hottest.1) is brightest when hottest.2) is bluer when hotter.2) is bluer when hotter.3) is both3) is both4) is neither4) is neither

For star: L=A For star: L=A TT44 , , A is its surface area A is its surface area

ThermalThermal Radiation Radiation

• Thermal radiation is basically Black-Thermal radiation is basically Black-body radiation, or nearly sobody radiation, or nearly so

• Every object with a temperature Every object with a temperature greater than absolute zero emits greater than absolute zero emits radiation.radiation.

• Hotter objects emit more total radiation Hotter objects emit more total radiation per unit area.per unit area.

• Hotter objects emit photons with a Hotter objects emit photons with a higher average energy.higher average energy.

• Thermal Radiation (BB) is an example Thermal Radiation (BB) is an example of “continuum emission”.of “continuum emission”.

How to Make Light (Part 2)How to Make Light (Part 2)

Structure of Structure of atomsatoms

Energy levels Energy levels and transitionsand transitions

Emission and Emission and absorption linesabsorption lines

Light scatteringLight scattering

AtomsAtoms

A Planetary Model of the Atom

Atoms are made of electrons, protons, and neutrons.

The bounding force: the attractive Coulomb(electrical) force between the positively charged nucleus and the negatively charged electrons.

Electrons can be in different orbits of certain energies, called energy levels.

Different atoms have different energy levels, set by quantum physics.

Quantum means discrete!

Energy Levels Energy Levels

To change its energy levels, an electron must either absorb or emit a photon that has the same amount of energy as the difference between the energy levelsE = h = h c/

Larger energy difference means higher frequency.

Different jumps in energy levels means different frequencies of light absorbed.

Excitation of AtomsExcitation of Atoms

Spectral Line EmissionSpectral Line Emission

If a photon of exactly the right energy is absorbed by an electron in an atom, the electron will gain the energy of the photon and jump to an outer, higher energy orbit.

A photon of the same energy is emitted when the electron falls back down to its original orbit.

Spectral Line EmissionSpectral Line Emission

Collisions (like in a hot gas) can also provide electrons with enough energy to change energy levels.

A photon of the same energy is emitted when the electron falls back down to its original orbit.

Energy Levels of a Hydrogen AtomEnergy Levels of a Hydrogen Atom

Different allowed “orbits” or energylevels in a hydrogenatom.

Emission line spectrum

Absorption line spectrum

Spectral Lines of Some ElementsSpectral Lines of Some Elements

 Argon 

                                                                                

 Helium

                                                                                 

 Mercury

                                                                                 

 Sodium

                                                                                 

 Neon

                                                                                 

Spectral lines are like a cosmic barcode system for elements.

Atoms of different elements have unique Atoms of different elements have unique spectral lines because each elementspectral lines because each element

has atoms of a unique colorhas atoms of a unique color has a unique set of neutronshas a unique set of neutrons has a unique set of electron orbitshas a unique set of electron orbits has unique photonshas unique photons

Blackbody radiation

The Solar SpectrumThe Solar Spectrum

There are similar absorption lines in the other regions of the electromagnetic spectrum. Each line exactly corresponds to chemical elements in the stars.

Reflection nebulaReflection nebula

Emission nebulaEmission nebula

An Object’s SpectrumAn Object’s Spectrum

Encoded in an object’s spectrumis information about the emitter/absorber. This is how we learn what the Universeis made of!

Intensity(spatial distribution of the light) Spectra

(composition of the objectand the object’s velocity)

There are three basic aspects ofthe light from an object that we can study from the Earth.

Variability(change with time)

The Doppler EffectThe Doppler Effect::other information contained in spectrumother information contained in spectrum

A moving light or sound source A moving light or sound source emits a different frequency in the emits a different frequency in the forward direction than in the reverse forward direction than in the reverse direction. direction.

Take a look at the police car to see Take a look at the police car to see how this works.how this works.

In general …In general … The “native” frequency at which an object is The “native” frequency at which an object is

emitting is called the emitting is called the restrest frequency. frequency.

You will see/hear frequencies You will see/hear frequencies higherhigher than the than the rest frequency from objects moving rest frequency from objects moving towardstowards you.you.

You will see/hear frequencies You will see/hear frequencies lowerlower than the than the rest frequency from objects moving rest frequency from objects moving awayaway from from you.you.

V/c =V/c = //obsobsrestrestrestrest

Doppler EffectDoppler Effect

The first crest travels out in circle from the original position of the plane

At a later time, a second crest is emitted from the planes new position,but the old crest keeps moving out in a circle from the planes original position

The same thing happens again at a later time

Longer wavelength (more red)

Shorter wavelength (more blue)

What we actually see in AstronomyWhat we actually see in Astronomy

Emission spectrum of hot gas as seen in lab

Emission spectrum of hot gas as seen in rapidly moving object

Is this object moving towards or away from us?

Two identical stars are moving towards the Earth. Two identical stars are moving towards the Earth. Star A’s emission lines are observed to be at Star A’s emission lines are observed to be at visible wavelengths. The same emission lines for visible wavelengths. The same emission lines for Star B are observed to be at ultraviolet Star B are observed to be at ultraviolet wavelengths. From these observations you wavelengths. From these observations you conclude that: conclude that:

Both stars are moving away from the EarthBoth stars are moving away from the Earth Star A is moving towards the Earth faster than Star A is moving towards the Earth faster than

Star BStar B Star B is moving towards the Earth faster than Star B is moving towards the Earth faster than

Star AStar A Star B is moving away from the Earth while Star Star B is moving away from the Earth while Star

A is moving towards the Earth.A is moving towards the Earth.

Two otherwise identical stars are rotating at Two otherwise identical stars are rotating at different rates. Star A is rotating slower different rates. Star A is rotating slower than Star B. How do Star A’s spectral than Star B. How do Star A’s spectral lines appear with respect to Star B’s lines? lines appear with respect to Star B’s lines?

Star A’s lines are narrower than Star B’s Star A’s lines are narrower than Star B’s lines.lines.

Star B’s lines are narrower than Star A’s Star B’s lines are narrower than Star A’s lines.lines.There is no difference in the lines of the There is no difference in the lines of the two stars.two stars.Star A’s lines are stronger than Star B’s Star A’s lines are stronger than Star B’s lines.lines.

Most kinds of e-m radiation cannot Most kinds of e-m radiation cannot penetrate the Earth's atmospherepenetrate the Earth's atmosphere