Let us now turn our attention to the brightness of stars. Recall that Hipparchus created the magnitude system for labeling the brightness of stars. Astronomers still use this system today. However the system has become more refined. Instead of just looking at the star and determining magnitude one or magnitude two, an astronomer measures the brightness of the star using a device called a photometer. The photometer counts the number of photons coming from the star. This photon count is then compared to the photon count from a star whose magnitude is known. An accurate magnitude can then be calculated.

When measuring magnitude using the method above, an astronomer is measuring the magnitude of the star as viewed from the Earth. This magnitude is referred to as apparent magnitude. It is a valid measure of the stars brightness as seen in our skies. But what about a measure of true brightness? For this astronomers use a value referred to as absolute magnitude. It is based on the same system that is used for apparent magnitude. Keep in mind, the smaller the number the brighter the star.

In order to make a valid comparison of stellar brightness, the stars must all be the same distance from the observer. The value of that distance is irrelevant, therefore astronomers have chosen the distance of ten parsecs. If the star's distance is known, it is mathematically moved to a distance of ten parsecs and the magnitude is recalculated. This magnitude is the absolute magnitude. In general, the apparent magnitude is designated by the letter m, the absolute magnitude is designated by M. Other designated letters may also be used (see below).

If a star's distance is not known than the absolute magnitude must be determined in some other fashion. If that is possible than the distance to the star can be determined. The mathematical relationship between absolute magnitude, apparent magnitude, and distance is very important to astronomers. If any two of these values are known the third can be calculated.

Closely related to absolute magnitude is the parameter called luminosity. It is also a measure of the star's true brightness. However, it is more precisely a measure of the power per unit area coming from the star. As we have seen in our discussion of blackbody radiation, this power is dependent upon the temperature of the star and the surface area of the star. Luminosity is generally expressed in terms of the luminosity of the sun. For example, a star may have a luminosity of four solar luminosities. That is to say, it is four times brighter than the sun.

Let us now turn our attention to stellar temperatures. There are two main methods for determining the temperature of the star. The first of these is color index. To determine the color index of the star, an astronomer places a blue filter over a photometer and measures the magnitude of the star in the blue portion of the spectrum. Then the astronomer will place a yellow filter over the photometer and measure the magnitude of the star in the yellow portion of the spectrum. These two magnitude are then subtracted to calculate the color index. The blue magnitude is designated B (sometimes m_B) and the yellow magnitude is designated V (m_V) (which stands for visual). The color index is therefore C = B - V. In general, an ultraviolet filter is also used (the magnitude is designated U, or m_U). However we shall leave that part of our discussion to the text.

To understand the concept of color index we must recall the properties the black body radiation. A hot star will be brighter in the blue portion of the spectrum than in the yellow portion of the spectrum. Therefore the blue magnitude will be a smaller number than the yellow magnitude (remember small magnitudes are brighter than large magnitudes). This will yield a color index which is generally negative. The opposite holds true for a cooler star. Therefore, a large color index indicates a cool star while a small color index indicates a hot star.

The second method used to determine stellar temperatures is spectral classification. In this case, the astronomer places a prism or diffraction grating between the star and a photographic plate (or some other measuring device). The light from the star is then spread out into its spectra. In general these spectra will show absorption lines. The pattern of these absorption lines will vary depending upon the temperature of the star. Astronomers have labeled the various spectral classifications with the lettering scheme O B A F G K M, where O is the hottest classification and M is the coolest classification.

In addition to determining the temperature of the star, the spectra of the star can be used to determined which elements are within the atmosphere of the star. This is because spectral lines for one element are never the same as the spectral lines for any other elements. The strength of the lines (how easy to see) can be used to determine the abundance of these elements. Astronomers can also measure any Doppler shift of the spectral lines to determine the speed of approach or recession of a star from Earth.

Stellar Properties

Size and the H-R Diagram

This page was last updated on 06/13/01.