Among stars, blue-hot is for bragging rights

Greetings, stargazers. In a previous article about the magnitude scale, I mentioned the bright stars Vega and Arcturus. They still are both overhead, but now shifted significantly to the west. Although these two are almost identical in apparent brightness, if you compare the colors, you should be able to tell that Arcturus is orange-yellow and Vega, definitely bluer.

For some odd reason, we tend to think of red as a warm color and blue as a cool color, but for both flames and stars, blue is hotter than red. Actually, all stars radiate at all colors of the visible spectrum; it just happens that sometimes one end of the spectrum will be radiating a bit more than the other.

The color of a star is directly related to the temperature of the star. Therefore, the color makes a great stellar thermometer. According to Wienís law, if you can measure the wavelength of light of maximum emission from any warm, dense object, such as a star, you can calculate the temperature of that object.

Many of you are familiar with this effect from shopping for light bulbs. It is not uncommon to describe the color of a light by its temperature. A lower temperature means an overall redder color for the light.

For stars, that peak wavelength might even be in the infrared for a very cool star, or the ultraviolet for a very hot star. If the peak happens to be right in the middle of the visible spectrum, all visible colors radiate with approximately equal intensity and you get white light, such as what comes from the sun.

So why donít we see lots of colors when we look up at the night sky? If you look at any of the numerous photographs of star clusters or wide field shots of entire constellations, most show a full range of star colors from red to yellow to white to blue. But to your naked eye, most of the stars look like pale blue dots. If you ever have wondered why, there are two reasons that may vary in importance, but they both lead to the same result.

The first reason is that your eyes just are not sensitive enough to colors of any kind at very low light levels to detect the differences unless the star is exceptionally bright, like Vega or Arcturus. Your eyes detect this dim light as gray, or maybe pale blue.

The second reason has to do with star luminosity, or how much energy it is radiating. Generally speaking, the bluer, hotter stars are much more luminous than the red ones, sometimes thousands of times more so. Thus they can be seen at much greater distances. Although the smaller, red stars are far more numerous than blue ones, the blue ones are the ones that are bright enough to be seen more easily.

hakes_c@fortlewis.edu. Charles Hakes is an assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory.