Lesson - Colors of Stars

Hot objects glow. This can be seen in a stove burner, a light bulb, or our Sun. A hot object, like the sun or a stove burner, glows because the molecules and atoms within the object are vibrating and moving rapidly. These atoms and molecules are made up of charged particles, and the movement of charged particles causes the emission of radiation. This is called blackbody radiation. The wavelength associated with each object depends on that object's temperature. Basically, the warmer the object, the faster the molecules vibrate, and the shorter the wavelengths (the higher the energy) of the emitted radiation.

1. Describe what happens to the color of a stove burner or a coal as it gets hotter.

Find an image of Orion.

Consider three stars in the constellation Orion with three different colors. Betelgeuse appears as a bright red star, Rigel appears as a bright light blue star, and mu Ori appears as a dimmer deep blue star.

2. Based on the knowledge of the stars brightness and color, what would you assume about the stars relative temperature? List the stars from coolest to hottest.

The applet in this exercise allows you to look at a virtual light bulb capable of a wide range of temperatures. The applet screen is broken into four parts. The bottom right is your temperature control. You can use the slider bar, or type the temperature in Kelvin. The bottom left is the light bulb. The upper right is what you see when you split the light with a spectral grating or prism. The upper left is a plot of brightness versus the wavelength of the light. Notice that the visible part of the spectrum is the part of the plot between the red and blue lines. To the left of the red line is infrared and radio light. To the right of the blue line is ultraviolet light.

3. Using the applet, determine what range of temperatures would give you a red light, what range of temperatures would give you a white light, and what range of temperatures would give you a blue light.
4. Apply this to the results you have from question 1. Is you list correct? If not, put the stars in the correct order from coolest to hottest.

Many people think that white hot should be hotter than blue hot. Maybe this goes back to our ancestors sitting around a fire, and seeing the hottest parts of the flame being white. However, it actually takes a pretty hot source, hotter than we get with charcoal or wood, to create a blue light. Blue stars have a surface temperature of 10000 K or higher.

Red light is the low energy end of the electromagnetic spectrum. Blue light is the high energy end. So, hotter (higher energy) objects will produce bluer light. Light that is "in between" will get a mixture of all visible colors, and since when you combine all different colors of light you get white light, white stars are stars of an "in between" temperature.

5. With the three stars from Orion that were listed, the red and light blue stars were very bright, but the deep blue star was somewhat dim. If the deep blue star is hotter than the red star, what are some reasons the star might appear less bright?