A small, relatively cool star with low luminosity: A red dwarf is an old and cool star that is rather small compared to other stars, but it is still bigger than Jupiter, for example.

A large red colored star with a relatively low surface temperature, a few thousand degrees at most, and a radius between 10 and 100 times that of the Sun: Such red giants are representative of stars at the end of their evolutionary life.

The observed shift of the characteristic spectral lines of a galaxy, for example, toward the red, longer-wavelength, end of the spectrum as a result of the galaxy's recession from the earth: The red shift is the lengthening of the wavelengths of light from an object as a result of the object's motion away from the Earth.

It is an example of the Doppler effect. The red shift in light from galaxies is evidence that the universe is expanding.