In the last section, on stellar magnitudes, we discussed the fact that the magnitude most often quoted is the star's visual magnitude - that is, its magnitude at visual wavelengths. Stars emit many different kinds of electromagnetic radiation - in radio waves, exactly like those used by your stereo, in infrared, which is essentially what we would call 'body heat', at microwave wavelengths, the same as is used by your microwave at home.
Stars emit radiation in what is known as a "blackbody" curve - a perfect emitter. If we graph the intensity - how much radiation there is - versus the wavelength - essentially a measure of the energy of the radiation - we see a shape that looks like this:
The easiest way to visualize light (or any other kind of radiation; microwave, radio, etc.) is to think of it like a wave on a string. Suppose you stretch a rope out on the floor, so that it lies straight, and tie one end of it to a fixed point - a table leg, or a pole. Then take the other end and move it back and forth. You'll get patterns on the rope that look like an S, or a snake moving:
The wavelength (l) of the wave is the distance between the beginning of one wave and the beginning of the next. The frequency (n) measures how many beginnings pass a specific point in a specific period of time, usually one second. If you multiply the frequency and the wavelength - the number of waves that pass times the distance each wave occupies - you get the velocity (speed in the forward direction) of the wave. When we're working with electromagnetic radiation, all of the waves move at the same speed, known as c, the speed of light (300,000,000 meters per second!!!).
The energy carried by a wave is related to its frequency by the simple equation E=hn, where h is a constant known as Planck's Constant. A wave with a shorter wavelength has a faster frequency, meaning it has more energy. So if we look at the blackbody curve again, we see that the higher energy waves are found to the left of the graph, the lower energies to the right.
A look at the graphic above demonstrates how curves at different temperatures, though they're all the same shape, have different values for the same wavelength of light. Although the values in the graphic are approximate (and exaggerated!), we can clearly see that the intensity of the 8000 K source in blue light is much greater than that of the 4000 K source.
For each temperature possible, the graph of wavelength versus intensity is shifted left or right - higher temperatures shift left, because they have more energy than lower temperatures, which are found to the right. By comparing the difference between the intensity of one band and another band, we can determine the temperature of the star giving off the light. In the information given for each star, we have the entry (B-V)=(some number):
The (B-V) reading that astronomers use is normalized - that is, they declared that one specific star, Vega, had a (B-V) reading of 0.00, and set everything at a relative scale from there. However, since they are all relative to the same thing, that means they're all relative to each other, as well. (B-V) readings can range from about -0.5 for very hot sources to 4 or 5 in the positive direction for very cool sources. A negative (B-V) value means the source is hotter than Vega - which is about 10,000 K - and a positive (B-V) value means the source is cooler than Vega.
The information given by the "Spectra" entry, next to (B-V) in the information box, tells us stellar temperatures and much, much more. It's a whole code worked out by astronomers to convey complex information about stars - and it can get really complex! In the tutorial section on Stellar Spectra, however, we'll go through some of the basic designations and what they mean.
