Driftless Dark Skies: Stellar Colors

December 2nd, 2016 by John Heasley

“In a dark time, the eye begins to see.”  –Theodore Roethke

The stars seem brighter as we move into winter.  Fourteen of the 25 brightest stars are visible on a December evening.  The Summer Triangle of Vega, Altair, and Deneb is setting in the west.  Fomalhaut is all solitary in the south.  The Winter Hexagon of Sirius, Procyon, Pollux, Capella, Aldebaran, and Rigel along with Betelgeuse, Castor, Adhara, and Regulus is rising in the east.  It’s darker earlier.  There’s less humidity to blur the starlight.  And the night side of our planet is facing away from the Milky Way and the glow of billions of stars.

Photo is from APOD

Photo is from APOD

As you get dark adapted, you’ll see that the stars have colors. Binoculars will help to gather in more photons to trigger those cones in your retina.  Try softening the focus to bring out more of the colors.  Look for the red of Betelgeuse, the orange of Aldebaran, the yellow of Capella, the white of Sirius, and the blue of Rigel.  Even though we cannot travel to the stars, their colors tell us about them.  From red to blue, the temperature of the star is increasing.  That’s a little different from everyday life where we use blue to mean cold and red to mean hot, but it makes sense.  The hottest part of a flame from a match or candle or campfire is blue not red.

We can thank astronomer Annie Jump Cannon for the classification scheme we use to connect color and temperature.  At the beginning of the last century at the Harvard Observatory, she classified over 350,000 stars (at 25 cents an hour).  If you were lucky enough to see Silent Sky at Forward Theater last fall, you met Annie Jump Cannon portrayed by Colleen Madden.

As the stars rise higher in the sky, notice how they get brighter and twinkle less.  Starlight close to the horizon passes through more of the atmosphere that absorbs the light and through more convection layers that bend and distort the light.  Twinkle, twinkle, little star….

The brightest “star” in the December sky is no star at all.  Venus is brilliant and bright in the southwest.  You will begin to see it right at sunset, and it will be visible for another two hours.  As it sinks lower to the horizon, watch how it dims as its light is absorbed and how it becomes redder as its blue light is scattered.  Don’t miss the evenings of December 2 and 3 when the waxing crescent moon passes by Venus for a beautiful pairing.  The moon continues to wax the evenings of December 4 and 5 as it passes by Mars a little higher in the southwest.   Have a look while you are waiting for the sky to darken and the stellar colors to emerge.

John Heasley is an astronomy educator and stargazer who enjoys connecting people with the cosmos. He volunteers with NASA/JPL as a Solar System Ambassador. For more information about stargazing in southwest WI, like Driftless Stargazing LLC on Facebook and find out whenever there’s something awesome happening in the skies. Driftless Dark Skies appears monthly in the Voice of the River Valley.