Nathan Melnick, age 13, of Longview, Wash., for his question:
HOW IS A STAR'S BRIGHTNESS MEASURED?
An instrument called a photometer is used by astronomers to measure the brightness of stars. The instrument is attached to a telescope.
Light from a star enters the telescope and then the photometer. The star's light produces an electric current in the photometer. An electric meter then indicates the star's brightness in terms of the strength of the current.
Astronomers compare the brightness of stars by using a number called its magnitude. A star with a magnitude of 1.00 is brighter than one with a magnitude of 2.00.
The brightest stars in the heavens have magnitudes so low they are less than zero. Because of this, the magnitude is 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 the 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.
Absolute magnitude is the brightness the star would appear to have if it were a standard distance of 126 light years away from the Earth.
Starlight is a combination of colors. The hottest stars give off more blue light than red light. Stars with lower surface temperatures give off more red light than blue light.
Astronomers measure the intensity of a star's blue and red light by passing the light through color filters and then into a photometer. They use a blue filter to separate the blue light from the other colors. A red filter separates the red light.
Astronomers also may measure a star's color with a spectrograph. A spectrograph takes a photograph of the star's spectrum. The photograph indicates how much light of each color is in the starlight.
Astronomers indicate the main color of a star with one of the following letters: O, B, A, F, G, K or M. Each letter represents a color group called a "spectral class."
Blue stars fall into the "O" spectral class, yellow stars are "G11 stars and red stars are classified "M."
The colors and temperatures associated with the spectral classes appear on the Hertzsprung Russell diagram. This chart illustrates the relationship between star luminosity and spectral class.
The diagram was named for the Danish astronomer Eijnar Hertzsprung and the American astronomer Morris Russell. These scientists independently developed the idea of the diagram in the early 10s.
The Hertzsprung Russell diagram is a graph with the absolute magnitude shown vertically and spectral class horizontally. Absolute magnitude ranges from minus 7.00 to plus 15.00.