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Surface brightness is a concept used in astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
 when describing extended astronomical object
Astronomical object

s are significant entity, associations or structures which current science has confirmed to exist in outer space. This does not necessarily mean that more current science will not disprove their existence....
s such as galaxies
Galaxy

A galaxy is a massive, gravitation system that consists of stars and stellar remnants, an interstellar medium of gas and cosmic dust, and an important but poorly-understood component tentatively dubbed dark matter....
 and nebula
Nebula

A nebula is an interstellar cloud of cosmic dust, hydrogen gas and Plasma . Originally nebula was a general name for any extended astronomy astronomical object, including galaxy beyond the Milky Way ....
e.

General description
The apparent magnitude
Apparent magnitude

The apparent magnitude of a celestial body is a measurement of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the Earth's atmosphere....
 of an astronomical object is generally given as an integrated value—if a galaxy
Galaxy

A galaxy is a massive, gravitation system that consists of stars and stellar remnants, an interstellar medium of gas and cosmic dust, and an important but poorly-understood component tentatively dubbed dark matter....
 is quoted as having a magnitude of 12.5, it means we see the same total amount of light from the galaxy as we would from a star with magnitude 12.5. However, while a star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth....
 is so small it is effectively a point source in most observations, the galaxy may extend over several arcseconds or arcminutes.






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Encyclopedia


Surface brightness is a concept used in astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
 when describing extended astronomical object
Astronomical object

s are significant entity, associations or structures which current science has confirmed to exist in outer space. This does not necessarily mean that more current science will not disprove their existence....
s such as galaxies
Galaxy

A galaxy is a massive, gravitation system that consists of stars and stellar remnants, an interstellar medium of gas and cosmic dust, and an important but poorly-understood component tentatively dubbed dark matter....
 and nebula
Nebula

A nebula is an interstellar cloud of cosmic dust, hydrogen gas and Plasma . Originally nebula was a general name for any extended astronomy astronomical object, including galaxy beyond the Milky Way ....
e.

General description


The apparent magnitude
Apparent magnitude

The apparent magnitude of a celestial body is a measurement of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the Earth's atmosphere....
 of an astronomical object is generally given as an integrated value—if a galaxy
Galaxy

A galaxy is a massive, gravitation system that consists of stars and stellar remnants, an interstellar medium of gas and cosmic dust, and an important but poorly-understood component tentatively dubbed dark matter....
 is quoted as having a magnitude of 12.5, it means we see the same total amount of light from the galaxy as we would from a star with magnitude 12.5. However, while a star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth....
 is so small it is effectively a point source in most observations, the galaxy may extend over several arcseconds or arcminutes. Therefore, the galaxy will be harder to see than the star against the airglow
Airglow

The airglow is the very weak emission of light by the Earth's atmosphere; as a result, the night sky is never completely dark. It was first noticed in 1868 by Anders ?ngstr?m....
 background light. Quoting an object's surface brightness gives an indication of how easily observable it is.

Calculating surface brightness


Surface brightnesses are usually quoted in magnitudes per square arcsecond. Because the magnitude is logarithmic, calculating surface brightness cannot be done by simple division of magnitude by area. Instead, for a source with magnitude m extending over an area of A square arcseconds, the surface brightness S is given by

Surface brightness is constant with luminosity distance
Luminosity distance

Luminosity distance DL is defined in terms of the relationship between the absolute magnitude M and apparent magnitude m of an astronomical object....
. For nearby objects, the luminosity distance
Luminosity distance

Luminosity distance DL is defined in terms of the relationship between the absolute magnitude M and apparent magnitude m of an astronomical object....
 is equal to the physical distance of the object. For a nearby object emitting a given amount of light, radiative flux
Flux

In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.*In the study of transport phenomena , flux is defined as the amount that flows through a unit area per unit time....
 decreases with the square of the distance to the object, but the physical area corresponding to a given solid angle
Solid angle

The solid angle, O, is the angle in three-dimensional space that an object subtends at a point. It is a measure of how big that object appears to an observer looking from that point....
 (e. g. 1 square arcsecond) increases in the same fashion, resulting in the same surface brightness.

Relationship to physical units


The surface brightness in magnitude units is related to the surface brightness in physical units of solar luminosities per square parsec
Parsec

The parsec is a units of measurement of astronomical units of length, equal to just under 31 orders_of_magnitude_#1012 kilometres , or about 3.26 light-years....
 by

where and are the absolute magnitude
Absolute magnitude

In astronomy, absolute magnitude measures a celestial object's intrinsic brightness. To derive the absolute magnitude from the observed apparent magnitude of a celestial object its value is corrected for distance to the observer....
 and the luminosity of the Sun in chosen color-band
Photometric system

In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used....
 respectively.

See also

  • Low surface brightness galaxy
    Low surface brightness galaxy

    A low surface brightness galaxy, or LSB galaxy, is a diffuse galaxy with a surface brightness that, when viewed from Earth, is at least one magnitude lower than the ambient night sky....


General references