June 22 , 2004 — As astronomers observe celestial bodies, there are a number of concepts and laws of physics they rely upon to interpret what they see. The terms below are important stepping stones to some of the discoveries described in "The Dark Side of the Universe."
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Spectrum
Celestial bodies emit energy at all different wavelengths along what's called the electromagnetic spectrum. Light that has wavelengths sort of in the middle of the spectrum - in the range of a millionth of a meter - is visible to our eyes. There's also energy we can't see, like infrared and microwave radiation that have longer wavelengths, and x-ray radiation, that has a shorter wavelength.

The electromagnetic spectrum illustrates wavelengths of energy ranging from gamma rays as small as atomic nuclei to radio waves as big as a house.

Even light that to our eyes looks like it's one color is really made up of lots of different wavelengths. Astronomers shine light from a star through a device called a diffraction grating to separate it into its constituent parts. It's like shining white light into a prism and seeing the rainbow come out the other side. Each star has a distinct spectrum, made up of varying amounts of light at the different wavelengths. Its spectrum can be like a unique 'fingerprint' scientists can use to figure out information about the star.

Astronomers can use the predominant color of a star's light to figure out its age based on whether it is burning hotter or colder. Hotter stars emit light that is more blue, cooler stars are more red. They can also use spectra to figure out a star's molecular ingredients. If a star's spectrum has a dark line at a particular spot, it means that the outer gases of the star contain a particular molecule that absorbs light at that wavelength. Scientists can also deduce the age of the star based on its spectrum as well.

This distinct pattern of absorbtion lines makes up the "fingerprint" of our sun.

Type Ia Supernova
A supernova is the highly energetic explosion that occurs at the end of a particular type of star's lifetime. If a compact dense star -- known as a white dwarf -- pulls in material from a neighboring star, it will start to collapse under the extra weight. The nuclear chemistry of the star is upset and a massive shock wave explodes the star.

	Supernova 2001el was observed in September of 2001 in galaxy NGC 1448. The study of this particular Type 1a supernova has led to a greater understanding of our expanding universe.

Scientists categorize the different kinds of supernovae based on what their spectra look like. An important tool for astronomers measuring the expansion of the universe is the group known as type Ia supernovae. This category is readily identified by its spectrum: a silicon emission line at the precise wavelength of 6150 Å and no hydrogen or helium lines. Astronomers love type Ia supernovae because they are "standard candles." The burst of light that every type Ia emits has the exact same intrinsic brightness. Type Ia's are bright enough to be seen over vast distances. Since we know how bright all type Ia supernovae burn, scientists are able to gage how far away a particular supernova is by how bright it appears from earth. They're like mile markers in the universe.


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