Most of us know the old nursery rhyme that begins “Star light, star bright, first star I see tonight.” Have you ever asked yourself “How do we measure star bright?” The history of measuring brightness goes back to the Greek astronomer, Hipparchus, who lived in the second century B.C.
Hipparchus classified the stars visible to the naked eye into six different brightness classes called magnitudes. The brightest stars were said to be of first magnitude, while the faintest were of sixth magnitude, defined as the limit of human visual perception without the aid of a telescope. Each step was roughly 2 times dimmer or brighter. The magnitude system of Hipparchus is still in use today in a slightly revised form. Note that, because of Hipparchus’ original concept, higher magnitudes mean dimmer stars.
With the invention of the telescope in the early 17th century, stars were discovered that were much fainter than 6th magnitude. Over the subsequent years, the need increased for a more mathematical definition of brightness. Norman Pogson, an English astronomer, proposed that a change in magnitude of five steps meant a change in brightness of 100. This meant that a magnitude change of 1 step was 2.5. This preserved the original Hipparchus system reasonably well, while allowing for magnitudes much dimmer than 6.
The reference star is Vega, defined as magnitude zero. Vega is visible now, high in the northwest evening sky, from about an hour after sunset until about 10 pm. Several stars, including our Sun, are brighter than Vega. Therefore, astronomers agreed that negative magnitudes were allowed. Sirius, the brightest star in the night sky is -1.4. Sirius is visible in the southeast now after about 10 pm.
Today the magnitude range goes from -26, the Sun, the brightest object in the sky to +32, the dimmest objects that the Hubble Space Telescope can image.
Magnitude can become a complex topic, if we start considering the specific wavelengths emitted by an object. For more information, see this Sky and Telescope article. Wikipedia also has a good discussion.
Western Slope Skies is produced by members of the Black Canyon Astronomical Society. This episode was written and recorded by Bryan Cashion.