Photographic magnitude

Photographic magnitude (mph or mp ) is a measure of the relative brightness of a star or other astronomical object as imaged on a photographic film emulsion with a camera attached to a telescope. An object's apparent photographic magnitude depends on its intrinsic luminosity, its distance and any extinction 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Photographic observations have now been superseded by electronic photometry such as CCD charge-couple device cameras that convert the incoming light into an electric current by the photoelectric effect. Determination of magnitude is made using a photometer.


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, planets and other planetary objects in the Solar System, variable stars[1] and deep-sky objects.

By the late 19th Century, an improved measure of the apparent magnitude 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 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 and reveals fainter stars or astronomical objects invisible to the human eye. 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 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: appearing brighter on the photograph than the human eye 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[2] observed by the human eye or obtained by photography[1]: 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 mpg or mp, or photovisual magnitudes mp or mpv.[3][1] Absolute photographic magnitude is Mpg.[3] These are different from the commonplace photometric systems (UBV, UBVRI or JHK) that are expressed with a capital letter. e.g. 'V" (mV), "B" (mB), etc. Other visual magnitudes estimated by the human eye are expressed using lower case letters. e.g. "v" or "b", etc.[4] e.g. Visual magnitudes as mv[3]. 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.[4]


  1. ^ a b c Miles, R. (2007). "A light history of photometry: from Hipparchus to the Hubble Space Telescope". Journal of the British Astronomical Association. 117: 178–186. Bibcode:2007JBAA..117..172M.
  2. ^ North, G.; James, N. (21 August 2014). Observing Variable Stars, Novae and Supernovae. Cambridge University Press. ISBN 978-1-107-63612-5.
  3. ^ a b c Norton, A.P. (1989). Norton's 2000.0 : Star Atlas and Reference Handbook. Longmore Scientific. p. 133. ISBN 0-582-03163-X.
  4. ^ a b MacRobert, A. (1 August 2006). "The Stellar Magnitude System". Sky and Telescope. Retrieved 21 May 2019.

See also

9 Boötis

9 Boötis is a single, variable star in the northern constellation of Boötes, located around 630 light years away from the Sun. It is visible to the naked eye as a faint, orange-hued star with a baseline apparent visual magnitude of 5.02. This object is moving closer to the Earth with a heliocentric radial velocity of −41 km/s.This is an aging giant star with a stellar classification of K3 III, which indicates it has exhausted the hydrogen at its core and evolved of the main sequence. As a consequence, its outer atmosphere has swollen to 55 times the radius of the Sun. It is a suspected irregular variable that ranges in photographic magnitude from 6.1 down to 6.6. 9 Boötis is considered mildly lithium-rich with a moderate level of chromospheric activity. It is radiating 716 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 4,197 K.

AE Andromedae

AE Andromedae (AE And) is a luminous blue variable (LBV), a type of variable star. The star is one of the most luminous variables in M31, the Andromeda Galaxy.


Antares , designated α Scorpii (Latinised to Alpha Scorpii, abbreviated Alpha Sco, α Sco), is on average the fifteenth-brightest star in the night sky, and the brightest object in the constellation of Scorpius. Distinctly reddish when viewed with the naked eye, Antares is a slow irregular variable star that ranges in brightness from apparent magnitude +0.6 to +1.6. Often referred to as "the heart of the scorpion", Antares is flanked by σ Scorpii and τ Scorpii near the center of the constellation.

Classified as a red supergiant of spectral type M1.5Iab-Ib, Antares is a red supergiant, a large evolved massive star. Its exact size remains uncertain, but if placed at the center of the Solar System it would reach to somewhere between the orbits of Mars and Jupiter. Its mass is calculated to be around 12 times that of the Sun.

Antares is the brightest, most massive, and most evolved stellar member of the nearest OB association, the Scorpius–Centaurus Association. Antares is a member of the Upper Scorpius subgroup of the association, which contains thousands of stars with a mean age of 11 million years, about 170 parsecs (550 ly) from Earth.

Antares appears as a single star when viewed with the naked eye, but it is actually a binary star, with its two components called α Scorpii A and α Scorpii B. The brighter of the pair is the red supergiant, while the fainter is a hot main sequence star of magnitude 5.5.

HV 2112

HV 2112 is a cool luminous variable star in the Small Magellanic Cloud. Until 2018, it was considered to be the most likely candidate for a Thorne–Żytkow object, but it is now thought to be an asymptotic giant branch star.

Henry Draper Catalogue

The Henry Draper Catalogue (HD) is an astronomical star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars; it was later expanded by the Henry Draper Extension (HDE), published between 1925 and 1936, which gave classifications for 46,850 more stars, and by the Henry Draper Extension Charts (HDEC), published from 1937 to 1949 in the form of charts, which gave classifications for 86,933 more stars. In all, 359,083 stars were classified as of August 2017.The HD catalogue is named after Henry Draper, an amateur astronomer, and covers the entire sky almost completely down to an apparent photographic magnitude of about 9; the extensions added fainter stars in certain areas of the sky. The construction of the Henry Draper Catalogue was part of a pioneering effort to classify stellar spectra, and its catalogue numbers are commonly used as a way of identifying stars.

KY Cygni

KY Cygni is a red supergiant of spectral class M3.5Ia located in the constellation Cygnus. It is one of the largest and most luminous stars, with a luminosity about 300,000 or more times that of the Sun and a radius of over 1,000 times that of the Sun. If it was placed at the center of the Solar System, it would extend past the orbit of Jupiter (or Saturn). It is approximately 5,000 light-years away.

List of galaxies

The following is a list of notable galaxies.

There are about 51 galaxies in the Local Group (see list of nearest galaxies for a complete list), on the order of 100,000 in our Local Supercluster and an estimated number of about one to two trillion in all of the observable universe.

The discovery of the nature of galaxies as distinct from other nebulae (interstellar clouds) was made in the 1920s. The first attempts at systematic catalogues of galaxies were made in the 1960s, with the Catalogue of Galaxies and Clusters of Galaxies listing 29,418 galaxies and galaxy clusters, and with the Morphological Catalogue of Galaxies, a putatively complete list of galaxies with photographic magnitude above 15, listing 30,642. In the 1980s, the Lyons Groups of Galaxies listed 485 galaxy groups with 3,933 member galaxies. Galaxy Zoo is a project aiming at a more comprehensive list: launched in July 2007, it has classified over one million galaxy images from The Sloan Digital Sky Survey, The Hubble Space Telescope and the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey.There is no universal naming convention for galaxies, as they are mostly catalogued before it is established whether the object is or isn't a galaxy. Mostly they are identified by their celestial coordinates together with the name of the observing project (HUDF, SDSS, 3C, CFHQS, NGC/IC, etc.)

Magellanic Catalogue of Stars

The Magellanic Catalogue of Stars is a catalogue of positions for 243,561 stars covering large areas around the Large and Small Magellanic Cloud (LMC and SMC). The catalogue was compiled by H.-J. Tucholke, K.S. de Boer and W.C. Seitter, who measured the positions on ESO Schmidt plates taken in 1988/91 and refer to the FK5 system via the PPM Star Catalogue. Stars to a photographic magnitude of 15 have been included, but the catalogue is incomplete as only those stars which are undisturbed by close neighbours have been catalogued. The positional accuracy is claimed to be better than 0.5" for 99% of the stars.

Magnitude (astronomy)

In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.

The scale is logarithmic and defined such that each step of one magnitude changes the brightness by a factor of the fifth root of 100, or approximately 2.512. For example, a magnitude 1 star is exactly 100 times brighter than a magnitude 6 star. The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values.

Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. The apparent magnitude (m) is the brightness of an object as it appears in the night sky from Earth. Apparent magnitude depends on an object's intrinsic luminosity, its distance, and the extinction reducing its brightness. The absolute magnitude (M) describes the intrinsic luminosity emitted by an object and is defined to be equal to the apparent magnitude that the object would have if it were placed at a certain distance from Earth, 10 parsecs for stars. A more complex definition of absolute magnitude is used for planets and small Solar System bodies, based on its brightness at one astronomical unit from the observer and the Sun.

The Sun has an apparent magnitude of −27 and Sirius, the brightest visible star in the night sky, −1.46. Apparent magnitudes can also be assigned to artificial objects in Earth orbit with the International Space Station (ISS) sometimes reaching a magnitude of −6.

Morphological Catalogue of Galaxies

The Morphological Catalogue of Galaxies (MCG) or Morfologiceskij Katalog Galaktik, is a Russian catalogue of 30,642 galaxies compiled by Boris Vorontsov-Velyaminov and V. P. Arkhipova. It is based on scrutiny of prints of the Palomar Sky Survey plates, and putatively complete to a photographic magnitude of 15. Including galaxies to magnitude 16 would have resulted in an unmanageably large dataset. The catalog was published in five parts (chapters) between 1962 and 1974, the final chapter including a certain number of galaxies with a photographic magnitude above 15.

NGC 7314

NGC 7314 is a spiral galaxy located in the constellation Piscis Austrinus. It is a Seyfert (active) galaxy. Since it appears to have detached spiral arm segments (either from dust lanes or bright star clusters), it was listed in Halton Arp's Atlas of Peculiar Galaxies.Walter Scott Houston describes its appearance in small telescopes:

Do not let its photographic magnitude of 11.6 scare you off, for it can be seen in a 6-inch telescope as a curiously fuzzy object. But it is small, appearing only 4' by 2'.


R145 (HD 269928) is a spectroscopic binary star in the Tarantula Nebula in the Large Magellanic Cloud located in the constellation Dorado. Both components are amongst the most luminous known.

RR Telescopii

RR Telescopii is a symbiotic nova in the southern constellation Telescopium. It was recorded on photographic survey plates as a faint variable star between photographic magnitude (mpg) 9 to 16.6 from 1889 to 1944. In late 1944 the star began to brighten, increasing by about 7 magnitudes, from mpg ≈ 14 to brighter than 8. Brightening continued with a diminished rate of increase after early 1945, but the overall outburst was not noted until the star was seen at about 6.0, the threshold of naked eye brightness, in July 1948. At that time it was given the designation Nova Telescopii 1948. Since mid-1949 it has declined in brightness slowly, albeit accompanied by some remarkable changes in its spectrum, and as of August 2013 it had faded to visual magnitude around 12.

Ring Nebula

The Ring Nebula (also catalogued as Messier 57, M57 or NGC 6720) is a planetary nebula in the northern constellation of Lyra. Such objects are formed when a shell of ionized gas is expelled into the surrounding interstellar medium by a star at the end of its asymptotic giant branch phase, in the last stages of its evolution before becoming a white dwarf.

Romano's Star

Romano's Star (GR 290) is a luminous blue variable star located in the Messier 33 galaxy in the constellation of Triangulum.

V404 Cygni

V404 Cygni is a microquasar and a binary system in the constellation of Cygnus. It contains a black hole with a mass of about 9 M☉ and an early K giant star companion with a mass slightly smaller than the Sun. The star and the black hole orbit each other every 6.47129 days at fairly close range. Due to their proximity and the intense gravity of the black hole, the companion star loses mass to an accretion disk around the black hole and ultimately to the black hole itself.

The "V" in the name indicates that it is a variable star, which repeatedly gets brighter and fainter over time. It is also considered a nova, because at least three times in the 20th century it produced a bright outburst of energy. Finally, it is a soft X-ray transient because it periodically emits short bursts of X-rays.

In 2009, the black hole in the V404 Cygni system became the first black hole to have an accurate parallax measurement for its distance from the Solar System. Measured by very-long-baseline interferometry using the High Sensitivity Array, the distance is 2.39±0.14 kiloparsecs, or 7800±460 light-years.

In April 2019, astronomers announced that jets of particles shooting from the black hole were wobbling back and forth on the order of a few minutes, something that had never before been seen in the particle jets streaming from a black hole. Astronomers believe that the wobble is caused by the warping of space/time by the huge gravitational field in the vicinity of the black hole. .

WZ Sagittae

WZ Sagittae (WZ Sge) is a cataclysmic dwarf nova star system in the constellation Sagitta. It consists of a white dwarf primary being orbited by a low mass companion. The white dwarf is about 0.85 solar masses while the companion is only 0.08 solar masses. This implies that the companion is a spectral class L2 star, although this has yet to be confirmed. The distance to this system has been determined by parallax, yielding a distance of 45.1 parsecs.WZ Sagittae is an ultrashort period cataclysmic nova, with outbursts observed in 1913, 1946, 1978 and 2001. During the well-observed 2001 outburst, the nova reached a peak visual magnitude of 8.21. The 1913 event was the brightest of the observed outbursts, reaching a photographic magnitude of 7.0.This nova is classified as a SU Ursae Majoris class star, which is a subclass of dwarf nova that produces what are termed superoutbursts spaced several months apart, interspaced with normal outbursts every few weeks. The normal outbursts typically last 2−3 days, while a superoutburst lasts a few weeks. However, WZ Sagittae is unusual in that it is only observed to emit superbursts.The outbursts of a dwarf nova are caused when matter from a ring-like accretion disk becomes unstable. In this system, the companion star is sufficiently close to the white dwarf that the tidal bulge of the former overlaps the Roche limit, allowing matter to pass across and accumulate onto the disk. When the disk reaches a critical temperature, the gas collapses onto the white dwarf resulting in the release of gravitational potential energy.A superoutburst may be caused by a tidal interaction of the accretion disk with the donor star, resulting in a greater deposition of matter on the white dwarf. In the case of WZ Sagittae, however, magnetic effects may act to enhance the deposition, resulting in the lack of normal outbursts. Because of the unique timing differences in the rate of superoutbursts of this nova, it been designated the prototype star for a WZ Sagittae subclass.

The orbital period of this system is 1.361 hours. Based upon observations of eclipses of the hot spot on the white dwarf (created by infalling material), the orbital plane of this system is inclined by 76° ± 6° to the line of sight from the Earth.

Y Centauri

Y Centauri or Y Cen (HD 127233, HIP 70969) is a semiregular variable star in the constellation of Centaurus.

The variability in the star was discovered by Williamina Fleming in 1895 and published in the Third Catalogue of Variable Stars. The photographic magnitude range was given as 7.7 - 8.8, but the variability was described as "somewhat doubtful". It was later given the designation HV 52 in the Harvard Catalogue of Variable Stars. The General Catalogue of Variable Stars lists it as a possible semiregular variable star with a period of 180 days and a photographic magnitude range of 8.9 - 10.0. A study of Hipparcos satellite photometry found a small amplitude range of 0.2 magnitudes at a visual magnitude of 8.53.The distance of the star is poorly known. The revised Hipparcos annual parallax of 3.50 mas gives a distance of 900 light years. A study taking into account the variability of the star found a parallax of 5.57 mas, corresponding to a distance of 585 light years. It is an asymptotic giant branch star 330 times as luminous as the sun. Its spectral type varies between M4 and M7 as it pulsates.The star has been observed to produce 22 GHz water maser emission, although later searches did not find any maser emission.

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