Cheryl Hoagland, age 13, of Salt Lake City, Utah, for her question:

How do they judge the brightness of the stars?

The brightness of stars is called stellar magnitude and it comes in two types  ¬apparent and absolute. The brightness of the stars is judged by comparing them with each other. But things Out There can fool the eye and what we see from the earth is not necessarily so. This is why we need two scales of stellar magnitude. Apparent magnitude charts the brilliance that a star appears to have when we see it in our skies. Absolute magnitude computes its true brilliance, compared with other stars,

Any scale of measurement requires a basic unit. It was decided to make the basic unit of star brightness roughly equal to the luminosity of one candle seen from a distance of 1,300 feet. This is the brilliance of a first magnitude star. On a summer night you can judge its value from Antares. This first magnitude star in the constellation Scorpius has a rating of 1.2. It is 2 1/2 times brighter than Polaris, the North Star, which is a second magnitude star. The difference between one magnitude and the next is 2 1/2, so that a star of first magnitude is 100 times brighter than a dim, barely visible star of sixth magnitude.

Bright Vega is about 2 1/2 times brighter than Antares and it rates as 0.1    a zero magnitude star. Minus signs are added to chart still more brilliant heavenly bodies. Sirius, the dazzling Dog Star, has a rating of minus 1.6 and the sun, as seen from merely 93 million miles, has a magnitude of minus 27. On the magnitude scale, the dimmer stars have larger plus numbers, the brighter ones have larger minus numbers.

Apparent magnitude evaluates the brilliance of stars as seen from the earth. But objects diminish with distance, especially in the sky where the dazzling stars are separated by spaces that stagger the mind. They are at different distances from us and each other. What's more, they come in assorted sizes and degrees of brilliance. Absolute magnitude gives a truer picture because it allows for distances. It estimates the brilliance each star would have if all of them were placed at about 33 light years, or about 220 trillion miles, from the earth.

In this evaluation, Sirius is moved back about 24 1/2 light years and Antares is moved 217 light years closer. Now we can make a fair comparison of their actual brilliance. Sirius is demoted from a minus 1.6 rating on the apparent magnitude scale to a plus 1.3 rating of absolute magnitude. Giant Antares is promoted from an apparent rating of 1.2 to an absolute rating of minus 3.2. Many of our medium bright stars are actually dazzling giants at enormous distances. Their absolute magnitude far surpasses some of the stars that appear brighter because they are closer to us.

The same scale of figuring is used for judging both types of brilliance. However, absolute magnitude is far more difficult to determine because it must estimate the distance of each star. This may be figured from the tricky parallax angle. In some cases a star's distance and apparent magnitude can be computed to give its true absolute magnitude.