Every star in the night sky is moving – orbiting the center of the galaxy. Some are moving toward us, while others are moving away. We can’t see that motion because the stars are so remote. But we can measure it with special instruments – one of the most important techniques in astronomy.

The instruments break the light of a star or other object into its individual wavelengths or colors. Each chemical element imprints its own “barcode” in that array of wavelengths. A star’s motion toward or away from us causes the barcodes to shift position.

If it’s moving away from us, the shift is toward longer, redder wavelengths: a redshift. And if it’s moving toward us, the shift is toward shorter, bluer wavelengths: a blueshift. The size of the shift reveals the speed.

The technique also can reveal how fast a star is spinning; the side that’s rotating toward us is blueshifted, while the opposite side is redshifted. And it can reveal orbiting companions; their gravity pushes and pulls the star, slightly changing its motion.

Two bright stars with well-measured shifts are in view as the sky darkens this evening. Regulus is in the west, well to the upper left of the brilliant planets Venus and Jupiter. Its light is redshifted; it’s moving away from us at about 9300 miles per hour. Antares, quite low in the southeast, is sliding toward us at about 8600 miles per hour – giving its light a definite blueshift.

Script by Damond Benningfield