Have you ever wondered how astronomers are able to tell the chemical composition, temperature, speed, motion and other attributes of stars which are thousands of light years away?  The answer lies in the science of spectroscopy.

Most everyone has heard of the Doppler effect.  If a moving object emits a sound, such as a train engine, the sound has the highest pitch to an observer in front of the object, while the pitch is much lower for an observer behind the object.  The sound waves in front of the object are compressed:

Light works in exactly the same way, except instead of pitch the difference is color.  Pitch is to sound as color is to light.  By splitting starlight with prisms we can see the constituent colors, how many reds and blues, and thus determine speed and motion from wavelength.

A brief refresher on wavelengths:

Remember ROYGBIV - Red, Orange Yellow, Green, Blue, Indigo and Violet

Infrared has a wavelength below that of red and thus is longer

Ultraviolet has a wavelength above violet and thus is shorter

Radio waves have an even longer wavelength and xrays, gamma rays and so forth much shorter.

When we see stars with a spectra shifted heavily toward the red we know therefore that they are very far away.  Because of the expansion of the universe, nearly all galaxies are red-shifted (moving away).  There are, however, some galaxies in our Local Group which are blue-shifted, because they are close and possess their own intrinsic motion relative to the Milky Way.

It was discovered over 200 years ago that the light from certain objects, when split with a prism through a small slit, presented dark lines in certain areas of the color bands.  It was discovered that objects with different elements presented a different pattern.  This is due to certain elements absorbing, or blocking that wavelength of light.  Every element has a precise spectrographic signature, thus enabling astronomers to tell the chemical composition of stars.

A spectrograph from a star showing absorption lines