1. Red Shift & Blue Shift

2. The Doppler Affect
The apparent shortening or lengthening of the wavelength & frequency of waves as they approach and move away from a certain point.
This is the reason why sirens, horns etc. from vehicles seem to get higher and higher in pitch as they approach you and then lower in pitch after they speed past.

3. Stars in the Universe
The stars in the universe are also constantly in motion! We can use this same principle to determine whether celestial objects are moving towards us or away from us.
We call this phenomenon Red Shift & Blue Shift

4. Red Shift Objects in space that are moving farther away from us:
Their wavelengths increase, (appear to get longer), as they move away.
Longer wavelengths = towards the red end of the spectrum.
Objects moving away from us = appear more red.
Red Shift!

5. Blue Shift Objects in space that are moving closer to us:
Their wavelengths decrease, (appear to get shorter), as they move closer.
Shorter wavelengths = towards the blue end of the spectrum.
Objects moving closer to us = appear more blue.
Blue Shift!

6. An Expanding Universe
Approximately 90% of the stars we can see in space, (as well as other objects), are in red shift, (moving away from us).
This is strong evidence for the theory that the universe is still in the process of expanding.
Why?