# Stellar Magnitude

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## stellar magnitude

[′stel·ər ′mag·nə‚tüd]
(astronomy)
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Stellar Magnitude

(apparent), a measure of the illumination produced by a celestial body (star, planet, the sun) on the earth in a plane perpendicular to the incident rays; a measure of the brightness of a celestial body. It is generally assumed that corrections have been introduced into the values of stellar magnitudes that take into account the attenuation of the light in the earth’s atmosphere, and thus stellar magnitudes are extraatmospheric.

The concept of stellar magnitude was first introduced in the second century B.C. by Hipparchus, who divided all stars visible to the naked eye into six magnitudes. The brightest stars were relegated to the first magnitude and the weakest stars (of those visible to the naked eye), to the sixth. Stellar magnitudes m are related to the illuminations E that correspond to them by the formula

m = k log E + C0

The value of the coefficient k, according to the suggestion of the British astronomer N. Pogson (mid-19th century), is taken to be equal to -2.5. It determines the steps of the stellar magnitude scale, whereas the constant C0 determines its reference, or zero, point. A change in illumination by a factor of 100 corresponds to a change of 5 units in stellar magnitude. Moreover, the brighter the celestial object, the smaller the number expressing its magnitude; magnitudes may have both positive and negative values. The constant C0 is determined from results of measurements of certain aggregates of stars selected as standards. In practice, it is rather difficult to carry out measurements of brightness with rigorous adherence to the generally accepted reference point and steps of the scale. In this connection, the parameters A: and C0 may vary somewhat in different photometric catalogs of celestial bodies, as is evident from comparison.

Depending on the method of measurement, magnitudes are differentiated into visual (determined by the naked eye with the help of a visual photometer), photographic (from photographs), photoelectric (with the aid of a photoelectric photometer), and bolometric (with the aid of a bolometer) magnitudes. Stellar magnitudes obtained by photographing a celestial body on a photographic plate with an orthochromatic or panchromatic emulsion through a yellow filter are called photovisual (such magnitudes are close to visual). The use of different radiation detectors and light filters makes it possible to measure the brightness of celestial bodies in different regions of their spectra and thereby to determine the magnitudes belonging to different photometric systems. In the international photographic and photovisual systems (in the blue and yellow regions of the spectrum), the standard consists of 96 stars in the region of the north celestial pole, the so-called North Polar Sequence; selected areas in which secondary standards have been established are located throughout the entire sky. More convenient is the UBV system, in which stellar magnitudes are given in ultraviolet U (3500 Å), blue B (4350 Å), and yellow V (5550 Å) regions of the spectrum. The B magnitudes are close to the photographic magnitudes, and the V magnitudes coincide with the photovisual magnitudes of the international system. In addition to the UBV system, magnitudes in the red and infrared regions of the spectrum are used: R (0.7 μm), I (0.90 μm), J (1.25 μm), K(2.2 μm), L(3.7μm), and so on. It is accepted that, in the establishment of any new systems of stellar magnitudes, all types of magnitudes will coincide for selected stars of spectral type AO from the main sequence of the Hertzsprung-Russell diagram. Several dozen stars, situated throughout the entire sky, serve as the magnitude standards in the UBVRIJKL … system. Differences between magnitudes obtained in different photometric systems characterize the energy distribution in the spectra of stars. These are called color indexes, for example, B—V, U—B.

The stellar magnitudes and color indexes of more than 20,000 stars have been measured photoelectrically. The precision of the measurements is about 0.01–0.02 magnitude. The precision of photographic and visual measurements is about 0.05–0.1 magnitude. The brightest star in the sky, Sirius, has a magnitude V = -1.46; the weakest belong to the 23rd stellar magnitude. The magnitude of the sun is V = -26.78, while that of the full moon is V = -12.71. The magnitude of a light source producing an illumination of 1 lux is V = -13.78.

The magnitude that a celestial body would possess if it were located at a standard distance of 10 parsecs is termed its absolute stellar magnitude. Absolute magnitudes (in contrast to apparent magnitudes) characterize the physical properties of the bodies themselves, their luminosity. Absolute stellar magnitude M is related to apparent magnitude m by the formula

M = m + 5 - 5logr

where r is the distance to the body expressed in parsecs.

### REFERENCES

Parenago, P. P. “Shkaly i katalogi zvezdnykh velichin.” Uspekhi astronomicheskikh nauk, 1948, vol. 4.
Sharov, A. S. “Sovremennoe sostoianie problemy fotometricheskikh si stem i standartov zvezdnykh velichin i pokazatelei tsveta.” Biul. Abastumanskoi astrofizicheskoi observatorii, 1962, vol. 27.

A. S. SHAROV

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
The standard error is abbreviated as SE and is in stellar magnitudes; the range of solar phase angles is given in the second column.
Theoretically both planets when centrally transiting the Sun's disk will reflect no sunlight towards Earth at all (Venus with its atmosphere will scatter some light earthwards) and so may be considered to be black bodies with an infinitely high stellar magnitude.
A 10-inch scope has roughly 1,500 times the light grasp of the unaided eye, which translates to a gain of eight stellar magnitudes. From my backyard I'm lucky to see 4th-magnitude stars at the zenith on a moonless night, so I was expecting to reach 12th magnitude with the XT10--maybe more with higher magnification.
Observing at around 4:00 UT under superb conditions with a limiting stellar magnitude of 6.4, he noted typical Perseid activity, with a ZHR of 70 and actual observed rates about two-thirds as much.
Moreover, he plotted his diagram with stellar magnitudes on the horizontal axis and a measurement of color on the vertical axis, reversed from Russell's form and the form in use today.
On the Question of Extinction of Light in Space and the Relations between Stellar Magnitudes, Distances and Proper Motions.
Based on a modern astronomy course he has given to university students, the book concisely describes astronomy and cosmology, stellar magnitudes and calculations, the sun and planets of our solar system, extra-solar planets, methods of observing the universe, the life and death of stars, galaxies and the large-scale structure of the universe, and other topics.
The Albertson Robotic Telescope (ART) was configured primarily for use in determining stellar magnitudes; however, the equipment may also be used for color astrophotography.
Ferneyhough's On Stellar Magnitudes, composed in 1994, lends further support to this assessment.
In 1935, Richter dressed up the Japanese method to create an earthquake index -- a simple numerical scale much like the stellar magnitudes used by his astronomical colleagues at the California Institute of Technology in Pasadena.
All estimated errors are given in units of stellar magnitudes. Comparison Object Error Symbol Jupiter [alpha]-CMa [alpha]-Ari Description and [alpha]-Boo Comparison [U.sub.c] 0.03 0.01 0.01 object brightness Scattered light [U.sub.s] 0.01 0.02 0.03 from Moon Color [U.sub.ct] 0.01 0.04 0.03 transformation Mask [U.sub.m] 0.02 0.02 0.02 correction Atmospheric [U.sub.a] * * * extinction Random [U.sub.t] 0.02 0.02 0.02 measurement error * This varies with the altitude of the Moon.
These photographs, totaling about 525,000, were part of a long-term, full-sky survey of stellar magnitudes and positions that took place over more than a century, from 1885 to 1989.

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