The University Record, January 10, 1994

Orion and stars’ colors features of January skies

Michigan sky watchers will have no trouble picking out the brilliant stars that make up Orion, the constellation of the Great Hunter, in January’s evening skies, according to astronomer Richard G. Teske.

“Orion shows us colorful examples of stars that astronomers designate red, white and blue,” Teske says. “Decide for yourself if each star’s color lives up to its advertised hue.”

Betelgeuse, marking Orion’s left (eastern) shoulder, is extremely red according to the way astronomers determine star color, Teske explains. Bellatrix, in Orion’s right (western) shoulder, is one of the bluest stars to have its color measured. Rigel, in Orion’s right knee, has a neutral color and is considered to be white.

“To astronomers, a star’s color is an important property that can be used to indicate the temperature of its outermost layers,” Teske says. “We are all used to the idea that color and temperature are related. Something can be red-hot or white-hot. The trick is to be sure that everyone agrees on what colors are what, and then to relate a temperature to each agreed-upon color.”

Astronomers have developed a way to actually measure the quantity they refer to as “color.” A sensitive television camera attached to a telescope is used to compare a star’s brightness at two different wavelengths or colors of light. One brightness measurement is made with the star’s blue light; the other with light at yellow-green wavelengths.

“Astronomers describe a star’s relative brightness with a single number called its ‘color,’ Teske says. “This number is really just a comparison of the star’s brightness as seen in what human eyes perceive as two colors. Over many years of observations, astronomers have carefully built up detailed knowledge of the relationship between their measured ‘colors’ and the temperatures of stars’ outer layers. Nowadays all that is needed to determine a star’s temperature is to measure its corresponding astronomical ‘color.’”

In order to get results that are compatible with those obtained by other scientists, all astronomical observers try to use the same color measurement system with their telescopes, Teske explains. Observing methods and television detectors are carefully compared among observatories to maintain high standards of accuracy.

“It’s inevitable, though, that astronomers have taken to using color words. Stars that are coolest and therefore emit a lot of reddish light are called ‘red,’ while others that are extremely hot and emit a lot of blue light are called ‘blue,’” Teske says. “By examining the stars of Orion, an observer can get a good notion of how ‘blue’ or how ‘white’ or ‘red’ stars can really be.”

Red Betelgeuse has one of the lowest temperatures recorded for a naked-eye star—about 5,000 F. This is the temperature at which pure liquid iron boils and vaporizes.

The outer layers of “white” Rigel have a temperature of 22,000 F. Rigel is among the brightest stars of the galaxy in which our sun is located. It shines with the brilliance of 50,000 suns.

In blue Bellatrix, the temperature soars to 45,000 F. “Bellatrix is not the hottest of known normal stars, however,” Teske notes. “That distinction belongs to a star in the southern constellation of Carina—one too faint to be seen with the eye alone. Labelled only as HD93250, it has a temperature approaching 95,000 F.”