Spectral Lines

Celestial Fingerprinting

http://jersey.uoregon.edu/elements/Elements.html

What we need to know?1. How are spectral lines formed?2. Define the three types of spectra3. Compare and contrast an absorption and emission spectra4. What is Kirchhoff law?5. What is the connection between emission lines and

elements?6. What does an emission spectra of a star tell us?7. What is the Doppler effect?8. What does the Doppler effect tell us about stars?

Spectral Line formation?

• Electron has different energy levels: Floors in a building.

• Lowest is called the Ground State.

• Higher states are Excited States.

Changing Levels• If you add the RIGHT amount of energy to an

atom, the electron will jump up energy floors.• If the electron drops down energy floors, the

atom gives up the ….same amount of energy.

• From before, LIGHT IS ENERGY: E = hc/

Types of Spectra– Continuous spectrum

• Spectra of a blackbody• Typical objects are solids and dense gases

– Emission-line spectrum• Produced by hot, tenuous gases• Fluorescent tubes, aurora, and many interstellar clouds

are typical examples– Dark-line or absorption-line spectrum

• Light from blackbody passes through cooler gas leaving dark absorption lines

• Fraunhofer lines of Sun are an example• Periodic Table according to Spectra

Continuum Spectra

•A Continuum Spectrum:

Light emitted across a continuous range of wavelengths.

•A thermal spectrum is a continuum spectrum.

But what are these?

The Sun

A Spectrum

• A spectrum = the amount of light given off by an object at a range of wavelengths.

Emission lines Absorption linesContinuum

Continuous and Absorption Spectra

Absorption Lines• Pass light at all

wavelengths through low density gas.

• Pass this light through our spectrometer.

• We see the continuum spectrum that is MISSING certain wavelengths.

Absorption

• Dark hydrogen absorption lines appear against a continuous visual spectrum, the light in the spectrum absorbed by intervening hydrogen atoms

• Compare with the emission spectrum of hydrogen.

From "Astronomy! A Brief Edition," J. B. Kaler, Addison-Wesley, 1997.

Emission Spectrum

Emission Spectrum

Continuum, Absorption, Emission

Kirchhoff’s Laws

Light of all wavelengths shines on an atom.Only light of an energy equal to the difference

between “floors” will be absorbed and cause electrons to jump up in floors.

The rest of the light passes on by to our detector.We see an absorption spectrum: light at all

wavelengths minus those specific wavelengths.

Kirchhoff’s Laws Cont…• Excited electrons, don’t stay excited forever.• Drop back down to their ground floors.• Only light of the precise energy difference

between floors is given off.• This light goes off in all directions.• From a second detector, we see these specific

energy wavelengths: an emission spectrum.

Emission Lines• Every element has a DIFFERENT finger print.

Multiple elements• Gases, stars, planets made up of MANY

elements have spectra which include ALL of the component spectral lines.

• It’s the scientist’s job to figure out which lines belong to which element.

Different stars, different spectra

• Different stars have different types of spectra.

• Different types of spectra mean different stars are made of different elements.

Hot

Cool

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Annals of the Harvard College Observatory, vol. 23, 1901.

Astronomical Spectra

The Sun

Courtesy of NOAO/AURA

HOT YouCoolerLow Density

Helium• The element Helium (He) was first discovered

on the Sun by its spectral lines.

Continuum Concept Test• The sun shines on a cold airless asteroid made

of black coal. What light from the asteroid do we detect?

a. No light at all.b. Some reflected visible light.c. Some reflected visible, plus emitted visible light.d. Some reflected visible, plus emitted infrared light.e. Some reflected visible, plus emitted visible and

emitted infrared light.

Concept Test• The sunlight we see is thermal radiation caused by the extreme

heat of the sun’s surface. However, the very top thin layer of the sun’s surface is relatively cooler than the part below it. What type of spectrum would you expect to see from the sun?

a. A continuous spectrum.b. A continuous spectrum plus a second, slightly redder

continuous spectrum.c. A continuous spectrum plus a second slightly bluer

continuous spectrum.d. A continuous spectrum plus an emission spectrum.e. A continuous spectrum plus an absorption spectrum.

To Sum Up…To Sum Up…

• EVERY element has a SPECIAL set of lines.– Atom’s fingerprint.

• Observe the lines and you identify the component elements.

• Identify:– Absorption spectrum– Emission spectrumLearn about the environment of the element

Doppler Shift

The Doppler Effect and Sonic Booms

Larry Sessions:

Image from: http://imagine.gsfc.nasa.gov/YBA/M31-velocity/doppler-shift-derive-2.html

Larry Sessions:

Image from: http://imagine.gsfc.nasa.gov/YBA/M31-velocity/doppler-shift-derive-2.html

Doppler Shift in Sound

• If the source of sound is moving, the pitch changes!• The Doppler Shift Song! Just the Lyrics here!)

Doppler Shift

• The greater the velocity the greater the shift.

Doppler Shift

• The greater the velocity the greater the shift.

NOTE that all lines are shifting

Redshift and Blueshift• An observed increase in

wavelength is called a redshift, and a decrease in observed wavelength is called a blueshift (regardless of whether or not the waves are visible)

• Doppler shift is used to determine an object’s velocity

Concept Test

• A car passes by blaring its horn. What do you hear?a. A constant tone.b. A tone that goes back and forth between high

and low frequency.c. A constant tone of lower intensity.d. Two constant tones, one of higher frequency

and one of lower frequency.e. One tone going from smoothly from low to

high back to low tone.

Concept Test• I spin an object emitting a constant tone

over my head. What do I hear?a. A constant tone.b. A tone that goes back and forth between high

and low frequency.c. A constant tone of lower intensity.d. Two constant tones, one of higher frequency

and one of lower frequency.e. One tone going from smoothly from low to

high back to low tone.

Absorption in the Atmosphere

• Gases in the Earth’s atmosphere absorb electromagnetic radiation to the extent that most wavelengths from space do not reach the ground

• Visible light, most radio waves, and some infrared penetrate the atmosphere through atmospheric windows, wavelength regions of high transparency

• Lack of atmospheric windows at other wavelengths is the reason for astronomers placing telescopes in space

So Now…So Now…

• From the presence and position of Spectral Lines we can know:– Composition (H, He, H2O, etc.)

– Movement through space (towards or away)– How fast it is moving away or towards us

Vc

• Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each traveling in a wave-like pattern and moving at the speed of light.

• Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons.

• The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and ... the most energetic of all ... gamma-rays.

• Actually, the electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency.

• Each way of thinking about the EM spectrum is related to the others in a precise mathematical way.

• So why do we have three ways of describing things, each with a different set of physical units? After all, frequency is measured in cycles per second (which is called a Hertz), wavelength is measured in meters, and energy is measured in electron volts.

Summary• Light can be a wave or a particle. Each has characteristics• White light is a mix of all colors• Frequency is determined by speed of light divided by

wavelength, or Wavelength = Speed of light/frequency c = f , f = c/ , = c/f• Light color is determined by wavelength• Atoms emit light when electrons shift orbits• Each atom emits light with unique wavelengths (colors)• Color and intensity of light determines temperature (for most

objects) – hotter-bluer, hotter-more light emitted. (Wien’s Law)• Wavelengths of radiation are shifted when objects move

(Doppler effect)