Bob Sachdera, age 16, of Chattanooga, Tenn., for his question:

HOW IS A STAR'S BRIGHTNESS MEASURED?

Astronomers use an instrument called a photometer to measure the brightness of stars. They attach the instrument to a telescope. Light from a star enters the photometer and produces an electric current in it. An electrical meter indicates the star's brightness in terms of the strength of the current.

Astronomers compare the brightness of stars by using a number called the star's magnitude. The brighter a star is, the lower is its magnitude.

A star with magnitude of 1.00 is brighter than one with a magnitude of 2.00. The brightest stars have magnitudes so low they are less than zero    that is, they are expressed in negative numbers.

A star with a magnitude of minus 1.00 is brighter than one of magnitude 1.00, but not so bright as one with a magnitude of minus 2.00

A photometer measures the brightness of a star as it appears from earth. Astronomers call this brightness the star's apparent magnitude. If astronomers know how far away a star is, they can calculate its absolute magnitude, which indicates the star's luminosity or actual brightness.