Spectroscopy: A Key Part of the Astronomer's Toolbox

Key points: origin of emission and absorption lines; spectra as a cosmic barcode; Doppler effect

Spectrum: the distribution of intensities of light over wavelength

A continuous spectrum A continuous spectrum has at least some light at all the wavelengths.(From R. J. Lavery, http://www..phy.nau.edu/~lavery/Mypage/Astrostuff/A150WEB1998/main2.html#startnotes)

A blackbody spectrum, such as those just discussed, is a particular type of continuous spectrum.

In normal experience, solids and liquids tend to emit blackbody spectra or spectra close to blackbodies. Gases tend to have more complex emission- and absorption-line spectra, allowing us to learn a lot about their conditions.

Emission- and absorption-line spectra are produced by atoms (and molecules)

sketch of hydrogen and helium atoms Atoms consist of nuclei made of protons and neutrons, and electrons around them. Hydrogen (1 proton) and helium (2 protons) are the simplest. Most aspects of the behavior of an atom (e.g., in chemistry) depend just on the number of protons and electrons; there are atoms with up to about 100 protons, giving 100 elements, each with distinct behavior. (figure by G. Rieke).

The electrons in an atom are held by the electric force, which is proportional to 1/r2 just like gravity. This force attracts positive and negative electric charges, but repels like charges - two positives or two negatives.  The protons in the nucleus of the atom are held together by the "strong force", which is clearly much stronger than the electric one but works only over very small distances.

Permitted and forbidden electron orbits in a hydrogen atom Electrons can only be in certain energy levels in an atom because wave-particle duality means they interfere with themselves in the other levels. This behavior is described by the branch of physics called quantum mechanics. buttonbook.jpg (10323 bytes)(Figure by G. Rieke).

Electron transitions between energy levels lead to absorption or emission of photons of specific energy corresponding to the energy level difference.

If an electron moves from an outer, higher energy orbit to an inner, lower energy orbit, energy is released in the form of photon. The properties of this photon depend on the energy difference between the orbits:

Energy = Eorbit 1 - Eorbit 2 = hnu.jpg (6708 bytes) = hc/lambda.jpg (8443 bytes)

animation of excitation of an atom by absorbing a photon If a photon of exactly the right energy "hits" an atom, it can be absorbed and cause an electron to jump to an outer, higher energy orbit.(The Amazing World of Electrons and Photons - Thinkquest http://library.thinkquest.org/16468/gather/english.htm)

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

animation of exciting an atom by colliding with another atom  

Electrons can also be raised to outer orbits when atoms collide

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

animation of emission-line and absorption spectrum formation In astronomical situations, we may see either emission lines in a spectrum or absorption lines depending on the relationships of the the sources and gases involved (animation by G. Rieke)

An absorption line spectrum is produced when cool gas lies between a continuum source and us; the specific wavelengths absorbed by the atoms in the gas are removed from the light that comes to us.

An emission line spectrum is produced when photons are emitted by gas that is thin enough to be transparent in the continuum.

Absorption- and emission-line spectra:

absorption line spectrum (From R. J. Lavery, http://www..phy.nau.edu/~lavery/Mypage/Astrostuff/A150WEB1998/main2.html#startnotes)
emission line spectrum (From R. J. Lavery, http://www..phy.nau.edu/~lavery/Mypage/Astrostuff/A150WEB1998/main2.html#startnotes)

If even more energy is supplied to an electron, it can escape from the atom leaving the positively charged nucleus. Because the electron is no longer transitioning between two specific energy states, the atom can absorb a range of energies in this situation. Electrons over a range of energies can be captured by the positive nucleus, emitting photons over a range of energiesbuttonex.jpg (1228 bytes)

Diagram of probability states of electrons in quantum mechanical atom Although it is convenient to draw protons, neutrons, and electrons as little dots, quantum mechanics tells us that they cannot be located accurately and are in fact more like fuzzy little fog clouds. We cannot predict precisely what they will do, leading to a scientific confrontation with the philosophy of determinism: science shows that there is fundamental uncertainty in what will happen in the future ribbon.jpg (3557 bytes) (Figure from The Essential Cosmic Perspective by Bennett et al.)

Significance of Spectra

Spectroscopy of astronomical sources has been a key to our understanding of the Universe because spectra are:

1) Aids in determining temperatures (can be more reliable than looking at the wavelength peak)

The higher the temperature, the more electrons are in high energy orbits or have escaped altogether from their atoms because high-temperature atoms run into each other at high enough speeds to shift the electrons. As a result, in hot gas there is emission and absorption of specific lines associated only with the high energy orbits that are inaccessible at low temperature. In this example, the cold atom can only absorb photons with energy corresponding to the transition between its two lowest energy levels, whereas the hot atom can absorb photons of that energy, plus ones corresponding to the transition between its second and third levels.

2) Probes of composition. Because each element (and also each type of molecule) has its own set of permitted orbits for its electrons and hence its own pattern of spectral lines, spectra can be used to determine what an object is made of (here are some examples from A. Larson, http://www.astro.washington.edu/astro101v): 


spectrum of argon
 Helium helium spectrum
Mercury mercury spectrum
 Sodium spectrum of sodium
 Neon neon spectrum

 We can consider spectra to be a "cosmic barcode" that identifies the conditions in the object (Fraunhofer spectrum of the sun, from R. Fosbury, http://www.stecf.org/~rfosbury/home/photography/Eclipse99/csp_description.html)

Fraunhofer spectrum of the sun

3) Indicators of motions. By understanding the Doppler effect, we can use spectra as a means of measuring the speed at which a distant object is moving

animation of Doppler shift The frequency of a wave is modified by the motion of a source toward or away from the observer. In the case of electromagnetic radiation:

Toward produces "blueshift" ==> spectral lines are shifted towards shorter wavelengths

Away produces "redshift" ==> spectral lines are shifted towards longer wavelengths

This animation shows why these changes occur. As the source moves toward the right, it "catches up" with the waves it has emitted in that direction and shortens their wavelength, shifting the light to the blue. Similarly, it "leaves behind" the waves it has emitted to the left, shifting the light to the red. (From Univ. of Saskatchewan, http://physics.usask.ca/~hirose/ep225/animation/doppler/anim-doppler.htm)


dopplereff.gif (24715 bytes) See how the wavelength of the sound into the boy's left ear is shortened in wavelength because the ambulance is approaching him, while the wavelength of the sound into his right ear is lengthened because the ambulance is moving away. The Doppler effect with light is similar to that with sound. It is how the policeman's radar gun works - it sends out a radio wave of a fixed wavelength and then measures the Doppler change in wavelength when the wave comes back, reflected off your car. The change in wavelength goes as the speed you are driving toward the policeman.

(From Japanese Aerospace Exploration Egency, JAXA, http://spaceinfo.jaxa.jp/note/shikumi/e/shi10_e.html.)

animation of Doppler shifting in a double star Although the entire spectrum is shifted, it is easiest to notice the shifts when looking at spectral lines because their wavelengths are so specific.(From R. McCray, http://cosmos.colorado.edu/astr1120/lesson1.html)

Doppler Effect as a Speedometer

The amount of frequency (or wavelength) shift is proportional to an object's velocity

where c = speed of light and the Greek capital delta (the triangle) means the "the shift in" - that is the shift in  wavelength divided by the original wavelength is equal to the speed of the source divided by the speed of light.

Why are photons so important to astronomylink to a key question

Test your understanding before going onbuttongrad.jpg (11232 bytes)

closeenc.jpg (9278 bytes)



A brilliant beam of light from an alien spaceship, from Steven Spielberg's "Close Encounters of the Third Kind," http://www.caiusfilms.com

sirtflaunch.jpg (4413 bytes)

einstein1.jpg (8543 bytes)






Click to return to syllabus

Click to return to Light

hypertext copyright.jpg (1684 bytes) G. H. Rieke

Click to go to Modern Physics