The Color of Stars

Twinkle twinkle little star, the children's nursery rhyme begins. How I wonder what you are! Up above the sky so bright, like a diamond in the night. Have you ever wondered what stars are, and what makes them twinkle? I know I did! I also wondered why some stars looked white and some stars looked red or blue. The main reason why stars are differently colored is that some are hotter than others. Deep in their interior all stars are hot. Their temperatures are measured in tens and hundreds of millions of degrees. Their temperature lessens toward their outer layers, and the coolest stars put most of their radiation in the red part of the spectrum. These coolest stars appear red. Hotter stars, stars like our Sun, appear yellow. Still hotter stars appear white and the hottest stars of all appear blue.

The most familiar form of electromagnetic radiation is visible light. 'White' light, such as sunlight, is a mixture of light at different wavelengths. On passing through a prism, white light is split into the colors of the rainbow. This is a result of refraction, the change in direction of light as it passes from one medium, such as air, into another, such as glass.

We can see the spectrum of a star easily. Light from a star is collected in a telescope, passed through a slit, diffracted through a prism or across a diffraction grating, and focused in an arrangement called a spectrograph. Perhaps you have used a diffraction grating, mounted on a slide, to show students the spectrum of various light sources in the classroom. There are several terrific classroom activities about the size of stars, color of stars and types of stars in the Astronomical Society of the Pacific's activity and resource notebook, "The Universe at your Fingertips". Information about ordering the notebook is at the end of the article. I want to take you out under the stars, however. At your class star party, your students can see with very own their eyes the colors of stars! They don't need a telescope or anything! The constellation of Orion and its neighbors contain what is called the Winter Circle of Stars. Look on any star chart and find Orion, Canis Major, Canis Minor, Gemini, Auriga and Taurus, circling westward as winter ends. These six constellations contain a visible "circle" of very bright and colorful stars. If you can imagine the circle as a clock, we'll begin with Capella, yellow like our sun in the one o'clock position. Capella is the bright yellow star in the constellation Auriga, found above the shoulders of Orion. Red Aldebaran, the eye of the bull Taurus is at the three o'clock position. Red stars are the oldest and coolest. At five o'clock, stands Rigel, the brilliant blue knee of Orion, the hunter. Rigel is young and very hot! Diamond white Sirius, the brightest star in the sky, is below Orion in the constellation Canis Major, the great dog. It fills the seven o'clock position. At nine o'clock is Procyon, another yellow star like our sun, in the constellation Canis Minor. The Gemini Twins, Castor and Pollux complete the circle at eleven o'clock. Castor is white and Pollux (the brighter of the twins) is red. Within the circle are red Betelgeuse, the shoulder of Orion, and blue/purple stars Alnitak and Mintaka, the pretty belt stars of the constellation Orion.

Through a telescope these stars give away many secrets. I have a nifty little star spectroscope that I can attach to an eyepiece. When I bring my star spectroscope to a school star party, and a student looks at one of these brilliant stars, she sees the color spread out in a spectrum. The bar she sees is broken into the colors of the rainbow, purple at one end and moving through the spectrum of blue, green yellow, orange and red. She can take the temperature of the star, and tell you its age from the information on the spectrogram she sees! Bars of shadow called absorption lines and bars of concentrated illumination called emission lines are the main indicators of age and temperature.

A simple exercise I often demonstrate is to show one red old cold star like Betelgeuse and one white hot young star like Sirius. Thick black molecular absorption bands of titanium oxide - the evidence of a star that is cool and that has lost its ability to ionize the elements show vividly on old red stars. It is clear in the eyepiece.

A hot young white star like Sirius does not show this thick absorption band. Instead, delicate shadows, which are hair like in thickness show instead. This is evidence of some of the four Balmer hydrogen emission lines. They show in and near the blue area of the spectrum. I could go on and on. Old cool stars have big thick molecular bands and hot young stars do not. That's a pretty interesting observation to share with your students!

Universe at your Fingertips:

Contact ASP at

390 Aston Avenue

San Francisco, CA 94112

(415) 337-1100

or email the coordinator at astro@aspsky.org

Rainbow Optics Star Spectroscope:

Contact Jim Badura at

1593 E Street

Hayward, CA 94541

(510) 581-8266

Jane Houston is an amateur astronomer and telescope mirror maker. She volunteers with several Northern California schools as part of The Astronomical Society of the Pacific's (ASP's) Project Astro program. She is also the president of the 25 astronomy club strong Astronomical Association of Northern California. http://aanc-astronomy.org. Her email is jane@whiteoaks.com.