Wayne Ferrell, age 14, of Annapolis, Md., for his question:
IN ASTRONOMY, WHAT IS MAGNITUDE?
Magnitude is a term used in astronomy to designate the apparent brightness of a star as viewed from the earth.
The ancient Alexandrian astronomer Ptolemy originally divided all visible stars into six magnitudes. The brightest stars were called first magnitude while those barely visible to the naked eye were called sixth magnitude. The other visible stars were assigned intermediate positions.
After the introduction of the telescope in the 17th century, this system of magnitudes was used and extended to the fainter stars in different ways by different astronomers.
In the 19th century a standard system was finally adopted under which a star of any given magnitude is 2.512 times as bright as a star of the next higher magnitude. Thus, for example, a star of the second magnitude is 2.412 times as bright as a star of the third magnitude.
The advantage of this particular magnitude rating, 2.512, is that it coincides closely with the Ptolemaic system; and because 2.512 is the fifth root of 100, a star of the first magnitude is exactly 100 times as bright as a star of the sixth magnitude, a star of the sixth magnitude is exactly 100 times as bright as a star of the 11th magnitude, and so on.
The mean of the magnitudes of several hundred stars in the Bonn Durchmusterung catalog that was prepared by the German astronomer Friedrick Argelander about 1860, was taken as the standard of the scale for calibration purposes.
With accurate instruments, such as bolometers and radiometers, astronomers today can measure differences as small as one hundredth of a magnitude.
Stars with magnitudes between 1.5 and 2.5 are called second magnitude stars. Stars brighter than magnitude 1.5, of which there are 20, are called first magnitude stars.
There are about 60 second magnitude stars and about 180 third magnitude stars.
The first magnitude star Aldebaran has an actual magnitude of 1.1 while the slightly brighter first magnitude star Altair has a magnitude of 0.9.
The brightest stars are brighter than magnitude zero. Sirius, the brightest star outside the solar system, has a magnitude of minus 1.6. The sun has a magnitude of minus 26.7 inasmuch as it is about 10 billion times as bright as Sirius.
Because the eye is more sensitive to yellow light than to blue light, whereas ordinary photographic film is more sensitive to blue than to yellow, the visual magnitude of a star may differ from its photographic magnitude. A star of visual magnitude 2 may have photographic magnitude 1 if blue, or it may have photographic magnitude 3 if yellow or red.
The faintest star that can be observed by long photographic exposure with the largest telescope is of the 23rd magnitude.
Absolute magnitude, as opposed to apparent magnitude, indicates the brightness that a star would have if it were placed at a distance from the earth of 10 parsecs, or 32.6 light years. By rating stars in this way, astronomers are able to compare them with respect to intrinsic brightness. The sun, for example, has an absolute magnitude of plus 4.7.