Photographic magnitude ( or ) is a measure of the relative brightness of a star or other

astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...

as imaged on a photographic film emulsion with a

camera A camera is an instrument used to capture and store images and videos, either digitally via an electronic image sensor, or chemically via a light-sensitive material such as photographic film. As a pivotal technology in the fields of photograp ...

attached to a

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

. An object's apparent photographic magnitude depends on its intrinsic

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

, its

distance Distance is a numerical or occasionally qualitative measurement of how far apart objects, points, people, or ideas are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two co ...

and any

extinction Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

of light by interstellar matter existing along the line of sight to the observer. Photographic observations have now been superseded by electronic photometry such as CCD

charge-coupled device A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...

cameras that convert the incoming light into an electric current by the photoelectric effect. Determination of magnitude is made using a

photometer A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Most photometers convert light into an electric current using a photoresistor, ...

Method

Prior to photographic methods to determine magnitude, the brightness of celestial objects was determined by visual photometric methods. This was simply achieved with the human eye by compared the brightness of an astronomical object with other nearby objects of known or fixed magnitude: especially regarding

stars A star is a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of ...

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

s and other planetary objects in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

s and deep-sky objects. By the late 19th Century, an improved measure of the

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

of astronomical objects was obtained by photography, often attached as a dedicated plate camera at the prime focus of the telescope. Images were made on

orthochromatic In chemistry, orthochromasia is the property of a dye or stain to not change color on binding to a target, as opposed to ''metachromatic'' stains, which do change color. The word is derived from the Greek '' orthos'' (correct, upright), and chr ...

photoemulsive film or plates. These photographs were created by exposing the film over a short or long period of time, whose total exposure length accumulates

photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that ...

and reveals fainter stars or astronomical objects invisible to the

human eye The human eye is a sensory organ in the visual system that reacts to light, visible light allowing eyesight. Other functions include maintaining the circadian rhythm, and Balance (ability), keeping balance. The eye can be considered as a living ...

. Although stars viewed in the sky are approximate point sources, the process in collecting their light cause each star to appear as small round disk, whose brightness is approximately proportional to the disk's diameter or its area. Simple measurement of the disk size can be optically judged by either a

microscope A microscope () is a laboratory equipment, laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic ...

or by an specially designed astronomical microdensitometer. Early black and white photographic plates used silver halide emulsions that were more sensitive to the blue end of the visual spectrum. This caused bluer stars to have a brighter photographic magnitude against the equivalent

visual magnitude Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...

: appearing brighter on the photograph than the

or modern electronic photometers. Conversely, redder stars appear dimmer, and have a fainter photographic magnitude than its visual magnitude. For example, the red supergiant star KW Sagittarii has the photographic magnitude range of 11.0p to 13.2p but in the visual magnitude of about 8.5p to 11.0p. It is also common for variable star charts to feature several blue magnitude (B) comparison stars. e.g. S Doradus and WZ Sagittae. Photographic photometric methods define magnitudes and colours of astronomical objects using astronomical photographic images as viewed through selected or standard coloured bandpass filters. This differs from other expressions of apparent visual magnitude observed by the human eye or obtained by photography: that usually appear in older astronomical texts and catalogues. Early photographic images initially employed inconsistent quality or unstable yellow coloured filters, though later filter systems adopted more standardised bandpass filters which are still used with today's CCD photometers.

Magnitudes and colour indices

Apparent photographic magnitude is usually given as ''m_pg'' or ''m_p'', or photovisual magnitudes m_p or m_pv. Absolute photographic magnitude is ''M_pg''. These are different from the commonplace photometric systems (UBV, UBVRI or JHK) that are expressed with a capital letter. e.g. 'V" (m_V), "B" (m_B), etc. Other visual magnitudes estimated by the human eye are expressed using lower case letters. e.g. "v" or "b", etc. e.g. Visual magnitudes as m_v. Hence, a 6th magnitude star might be stated as 6.0V, 6.0B, 6.0v or 6.0p. Because starlight is measured over a different range of wavelengths across the electromagnetic spectrum and are affected by different instrumental photometric sensitivities to light, they are not necessarily equivalent in numerical value.

References

{{Portal bar, Astronomy, Stars, Outer space Astrophysics

Method

Magnitudes and colour indices

See also

References