Disclaimer: links to web sites are ever-changing.  It turns out to be a Sisyphus task to keep them updated all the time.  Therefore, either try a different "spelling" of the hyperlink, look for it on google.com and/or let me know about an outdated link by writing an e-mail to aveh@wncc.net .
see especially: Chaisson's Astronomy Today ch.3 and ch.4 Image Archive, and Snow's Universe ch.6 Figures (Snow's Universe is temporarily offline).
Light and Matter

The electromagnetic (EM-) spectrum
(link doesn't work)

g rays   -                   -                       -                        -                       -                    -   radio

(check your astronomy book and/or look below)

All of the above are of the same nature: they are part of the electromagnetic spectrum (Snow's Universe is temporarily offline).  Differences occur only because they have different frequencies, wavelengths and energies. Differences for us occur, because we use or perceive them for/as different purposes/hazards.  Stars radiate all kinds of EM waves, how much of each kind depends on the star's surface temperature (Planck's radiation law).

A very pretty diagram can be found at Microsoft's Encarta.

Frequency:     ___ creases from left to right
Wavelength:   ___ creases from left to right;    Speed of light = Frequency times Wavelength
Energy:          ___ creases from left to right;    Energy = Planck's constant times Frequency

This means that frequency, wavelength and energy are interrelated. Whenever the talk is about radiation, the terms frequency, wavelength and energy could be used interchangeably.  It's customary though to designate low energy EM waves with frequency (e.g. the MHz for FM radio), medium energy EM waves with wavelength (e.g. nanometers or Angstrom for visible light), and high energy EM waves with energy (electronvolts for x-rays and g-rays).

An interesting feature of EM radiation is that it can appear as a wave or as a particle, named the photon.  We speak of EM radiation as photons when they're send out during nuclear fusion of atomic nuclei and when they're absorbed or emitted by atoms. EM radiation is eventually radiated from a star.  After traveling through space finally reaches our telescopes, it is refracted by lenses or reflected by mirrors, both of which are indicative of the wave properties, so we speak of waves.

Due to its composition of gases, the Earth's atmosphere is opaque to some frequencies, and transparent to others.  Of course much visible and infrared reaches the Earth's surface and, luckily, most UV and x-rays are absorbed by the atmosphere (e.g. ozone).

During the lectures, on websites and in textbooks, you'll notice that different words are used to describe the same thing.  Don't be too confused by that.  Instead of EM (Electromagnetic) the term "light" is often used, meaning all radiation from radio to gamma, what we perceive as light then, white, red to purple, is then referred to as "visible light".  EM (or "light") can be described as a photon or a wave, as radiation or spectrum.  Frequency, wavelength and energy are used exchangeable courtesy the above mentioned preferences.  Makes sense?
 
 

Radioactive sources producing a, b, g rays (alpha are high-energy Helium nuclei, beta are high-energy electrons, gamma are high-energy EM radiation).
An x-ray tube: electrons are burnt off the cathode on the left, slam into a tilted metal target in the middle, whereupon they lose their energy and produce x-rays which leave the tube at a right angle.
A "black light" sends out ultraviolet as well as visible light from the adjacent purple.  The high energy UV raises an electron to a quite higher orbit, from where it cascades down, thereby emitting photons of lesser energy, i.e. visible light of various colors.  These are the fluorescent minerals in the photos).


Clinohedrite, Hardystonite, Willemite, Franklin, NJ, 3 x 4" (SW); w/o permission from Ken's Fluorsecent Minerals.

The familiar visible light froma a halogen light bulb.  It's spectrum is produced by a diffraction grating (see my lab F5 Spectra).  The human eye recognizes those frequencies as colors.

The "1985 Astronomy Song"
The radio is assembled from a Radio Shack kit.  Its antenna received radiowaves in the AM short wave band, its circuit filters out the (still electronic) sound frequencies and the loudspeaker converts them to audible sound.  (You don't listen to the EM radiowaves themeselves, you listen to the sound waves carried along by the EM radioaves.) 
The klystron on the left (the black cylinder on top) contains an antenna producing microwaves.  The receiver on the right shows the intensity.
This experiment also shows an important property of EM waves: they can be polarized, so that a grid parallel to the electric field absorbs them (top) while a perpendicular lets them through. 
This candle produces heat.  Actually, most of the apparatus shown here produces heat and would show up brightly in an infrared picture: the radio's amd loudspeaker's batteries, the microwave's klystron, the halogen lamp, the black light, the x-ray tube, the radioactive sources.


 

Temperature scales
(at King's University)

 Kelvin (British) - Celsius (Swedish) - Fahrenheit (German)

 Kelvin is used in science because there are no negative temperatures in Kelvin (which makes calculations easier).  The Kelvin defines Absolute Zero (nothing can be colder than that).

Temperature is a measure of the motion of atoms, molecules, and free electrons.


Low Temperature.                                High Temperature.

Motion ceases at Absolute Zero.  That's why the Kelvin is so handy:  0 K means no motion of molecules.

Temperature, and therefore the amount of motion, is insofar important as atoms collide more often at higher temperatures (how many collisions depends on density as well) and thereby transfer energy from one atom to another.  Collisions can ionize atoms (remove an electron) or merely "excite" atoms (make an electron jump up) which results in the emission of an EM photon when the electron jumps back down.  And as we know, EM radiation is the most important stuff for astronomers.
 
 
Commonly used temperatures
Absolute Zero     0 K - 460 F
Cosmic background radiation      3 K - 455 F
Room temperature 295 K     80 F
Interstellar space 100 to 10,000 K - 250 to 20,000 F
Surface temperatures of stars 3,000 to 50,000 K 5,000 to 100,000 F
Core temperatures of stars 10 to 500 Million K  20 to 1,000 Million F

 
 
 
 
 
 
 

Black Body

A hot body (e.g. a star's surface) emits all frequencies of the EM-spectrum, each with an intensity according to a black body / Planck curve.

Wien's law tells us where the peak frequency (Snow's Universe is temporarily offline) is.
The Stefan-Boltzmann law tells us how much energy is radiated.

From this follows that both laws depend on temperature, i.e. if the peak of a star's black body curve is known, its surface temperature can be determined.
 
 
 
 
 
 
 

The Atom

Nucleus: PROTONS (positive charge) - number of   p   is responsible for which element is at hand
                NEUTRONS (neutral), different number of   n  makes different "isotopes" of the same element (same number of  p )
Shell: ELECTRON (negative), same number of  e  as  p  if the atom is neutral.

Check a table of the periodic system (see Johnson Memorial Elementary School) or one in your textbook.  The following diagram shows only the nuclei of some elements but omits their shells (electrons).  It is therefore more useful for the discussion about nuclear fusion in my Sun lecture.


 

An electron "jumps" from a higher to a lower shell: a PHOTON (i.e. a light particle of certain frequency of EM-radiation) is emitted.
A photon is absorbed by an electron: the electron "jumps" from a _________ to a _________ shell.
Most atoms in stellar atmospheres are either neutral (#e = #p) or singly ionized (#e = #p - 1), some atoms have more than one electron removed (#e < #p - 1).  Differently ionized atoms of the same element make for different spectral lines, e.g. HeI (2 electrons) and HeII (1 electron).
 


 Spectral lines

 Kirchhoff's laws: (at ZEBU)

(and at Strobel's lightspeed)
 
 

-> Lab F5 Spectra of gases

 Observing spectral lines reveals:

(Snow's Universe is temporarily offline) Chemical Composition of star's atmosphere, temperature of star's surface, rotation of star (Doppler effect), radial motion (Doppler) and binary stars, electric/magnetic fields, ...
 
 

As an example:

Doppler Effect
(and at Snow's Universe) (Snow's Universe is temporarily offline

When a train/police car approaches, the pitch of its whistle/horn becomes ___________, when it recedes, its pitch becomes ___________.

 Light is a wave just as sound is, so the same effect happens to light:
It's a ______ shift, when approaching, and a _______ shift, when receding. The light doesn't become blue/red, but it's shifted towards the blue/red side of the spectrum. (Example from CLEA's Stellar Spectra Lab)

Doppler shift

Wavelengths l are in Angstrom.  This CLEA software produces only the purple part of the spectrum since that information is sufficient for the analysis we're doing.  Deep purple is at left (hardly noticeable at all to the human eye), deep blue on the right.
 
 

This intensity graph shows clearly the spectral lines of HD 35215 are shifted towards the red when comparing them to a standard B0 V Main Sequence star.

Is this star receding or approaching?
 
 
Doppler shift of spectral standard versus HD 35215.
standard l measured l   Dl  v = c (Dl) / (l)
Ca II (H-line) 3970.4 3969.0
H I (H d) 4102.5 4101.2
H I (H g) 4341.3 4340.4

Determine how fast this star moves by using the formula for the Doppler shift:  v = c (Dl) / (l) with c = 186,000 miles/sec.