Astronomy 101
Astronomers study objects in the sky using the light that comes from them.  This light is collected using telescopes.  Light can tell astronomers many things about the object it came from, like its temperature and composition.


Light is made up of particles called photons that travel in waves.   The distance between two wave crests or troughs,  i.e. how long or short these waves are,  is called the wavelength.  Light comes in many different wavelengths ranging from quite short to very long.    Different wavelengths have different colors.   Blue light has short wavelengths and red light has long wavelengths.  All of the wavelengths of light put together are known as the electromagnetic (EM) spectrum.    The categories of light in this EM spectrum are, in order of long to short wavelength: Radio Waves, Microwaves, Infrared (IR), Visible, Ultraviolet (UV), X Rays, and Gamma Rays.
EM Spectrum
Light can be spread out into separate wavelengths using a prism or grating.  The result is a rainbow, which is called a spectrum.

Prism
www.physics.mcmaster.ca/ phys3n03/

Each element (Hydrogen, Nitrogen, etc) has its own characteristic spectrum which acts as a fingerprint for each element, but all spectra fall into three categories:  continuous, emission, and absorption spectra.

Spectra Fingerprints
http://ganymede.nmsu.edu/tharriso/ast110/classnotes.html


Three Spectra Types
physics.syr.edu/.../ tutorial/measure/part7.html

To understand how each of these spectra are formed, one must begin with the basic building blocks of everything-atoms.  The physicist Nils Bohr came up with a model of the atom, hydrogen in particular, that we still use today.    The model looks much like a miniature solar system.  It consists of a nucleus made up of the subatomic particles protons and neutrons orbited by electrons.  The orbits of the electrons were called shells and each shell had a particular energy associated with it. 

Bohr Model of Atom
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/BohrModel/BohrModel.html


Electrons can gain energy (become excited) and move up to other shells if the atom absorbs photons that have the exact amount of energy required to move the electron to the new level.  Electrons will eventually loose that energy (de-excite), emitting it in the form of light at the same energy.
Excitation of electrons
http://astronomy.nmsu.edu/nicole/teaching/ASTR110/lectures/lecture18/home.html

Using Bohr’s model and these ideas of emission and absorption, Gustav Kirchoff developed three laws of radiation which explained the three types of spectra seen. 

 

Law 1 – a hot dense object will emit a continuous spectrum (the entire rainbow).

 

Law 2 – a hot thin gas will emit an emission spectrum (bright lines at particular wavelengths).

 

Law 3 – a hot dense object shining through a cool, thin gas cloud will emit an absorption spectrum (full rainbow with certain lines missing).

Kirchoff's Laws
http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
Astronomers use these concepts to study the light coming from objects in the sky and can really learn a lot about them.