Red Shift-Blue Shift

This short, suggestive, and excited plea from the Eagle’s Life in the Fast Lane is also an apt description of how space and everything in it acts.  The farther out we look, the faster things are speeding away from us, red-shifted.  In this article I’ll look at what it means to be red-shifted and blue-shifted.  These terms sound so…esoteric.

Red and blue shifts are also called doppler effects so let’s take a look at what doppler effects are before diving into the red and blue.

In 1842 an Austrian physicist named Christian Doppler first hypothesized how light wavelength (color) appears to change for stars moving toward or away from us.  A first test of this hypothesis, done by Buys Ballot in 1845, used sound waves as a surrogate.   He confirmed the sound from an object such as a high-speed train appears to increase in pitch as it approaches you. Then, as the train passes and moves away its sound appears to go down in pitch.  This phenomenon was named Doppler shift or effect.

This effect occurs with anything emitting waves and in motion, or even if it is still and the observer is moving.  The faster the motion (of the object or observer) the more pronounced the effect.  If the observer is on/in the object or both are stationary or moving in parallel paths, there is no effect. Motion toward or away causes the waves emitted to get compressed or stretched out for the observer.  Compressed sound waves produce a higher pitch and stretched out sound waves produce a lower pitch.

The same goes for light.  Remember, Doppler’s hypothesis was about light waves being compressed or stretched out for stars moving toward or away from us.  Daylight (the Sun’s light) is composed of numerous wavelengths that can be separated or spread out into a rainbow of wavelengths (colors).  The visible wavelengths go from RED – long, to BLUE – short.

On to the fast lane.  In the early 20th century several astronomers (Keeler, Slipher, Campbell, Hubble, and Humason) observed that light wavelengths for some stars and “spiral nebulae” appeared stretched out.  They discovered the redshift.

They discovered the redshift, meaning these galaxies were moving away from us – their light wavelengths (colors) were doppler shifted to longer (redder) appearing wavelengths. Like that train moving away.  It seems nearly all galaxies are moving away from us, but a few, like the Andromeda galaxy (M31) are heading toward us. Galaxies and stars moving toward us are blue shifted (their light is compressed into shorter wavelengths).

The farther out we look, the faster galaxies are moving away (larger redshift). Some are moving at an appreciable fraction of the speed of light!  Faster, faster.

What’s in the Sky?

July 11 & 12; 45 minutes after sunset; west:  Watch a waxing crescent Moon sidle up as Venus and Mars make a pretty pair above the horizon. Use binoculars if you have them.