Apparent Magnitude

Want to stay on top of all the space news? Follow @universetoday on Twitter

Apparent magnitude is the factor used to measure the brightness of any celestial body as seen from Earth. That brightness is then normalized to eliminate the atmosphere. The brighter the object the lower its apparent magnitude. Absolute magnitude, on the other hand, is the true measurement of a star’s brightness seen from a set distance.

The current values for apparent magnitude are based on ancient traditions of dividing star that could be seen by the unaided eye into six different magnitudes. The brightest were called the magnitude one stars while those that stretched the human eye were said to be of a magnitude six. Each grade of magnitude was considered to be twice the brightness of the following grade. This somewhat crude method of indicating the brightness of stars was popularized by Ptolemy. This original system did not measure the magnitude of the Sun.

Norman Pogson formalized the apparent magnitude system by defining a typical first magnitude star as a star that is 100 times as bright as a typical sixth magnitude star; thus, a first magnitude star is about 2.512 times as bright as a second magnitude star. Pogson’s scale was originally fixed by assigning Polaris a magnitude of 2. Astronomers later discovered that Polaris is slightly variable, so they first switched to Vega, and then switched to using tabulated zero points for the measured fluxes. The magnitude depends on the wavelength band.

The modern apparent magnitude system is no longer limited to 6 magnitudes or only to visible light. Very bright objects have negative magnitudes. For example, Sirius, the brightest star in the celestial host, has an apparent magnitude of –1.4. The modern scale includes the Moon and the Sun; the full Moon has an apparent magnitude of –12.6 and the Sun has an apparent magnitude of –26.73. Stars have been located with magnitudes of 30 at visible wavelengths and the Keck telescopes have located similarly faint stars in the infrared.

There is an article here that tells you the apparent magnitude of many stars and the formula used to figure them. Here on Universe Today we have a great article on how to derive absolute magnitude from apparent magnitude values. Since we are talking about stars, take a look at this episode on Astronomy Cast about the life of stars.

Sources:
Wikipedia
UT-Knoxville
Cornell Astronomy

Tagged as: absolute magnitude, apparent magnitude