# UBV photometric system

The UBV photometric system (Ultraviolet, Blue, Visual), also called the Johnson system (or Johnson-Morgan system), is a wide band photometric system for classifying stars according to their colors. It is the first known standardized photoelectric photometric system. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are measured for a star then two subtractions are performed in a specific order to classify it in the system.[1]

The choice of colors on the blue end of the spectrum is because of the bias that photographic film has for those colors. It was introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan. A 13 in (330 mm) telescope and the 82 in (2,100 mm) telescope at McDonald Observatory were used to define the system.[1][2]

The filters are selected so that the mean wavelengths of response functions (at which magnitudes are measured to mean precision) are 364 nm for U, 442 nm for B, 540 nm for V. Zero points were calibrated in the B−V (B minus V) and U−B (U minus B) color indices selecting such A0 main sequence stars which are not affected by interstellar reddening.[1] These stars correspond with a mean effective temperature (Teff (K)) of between 9727 and 9790 Kelvin, the latter being stars with class A0V.

The UBV system has some disadvantages. The short wavelength cutoff that is the U filter is defined mainly by the terrestrial atmosphere rather than the filter itself; thus, it (and observed magnitudes) can vary with altitude and atmospheric conditions.[3] However, a large number of measurements have been made in this system, including many of the bright stars.[4]

## Extensions

The Johnson-Cousins UBVRI photometric system is a common extension of Johnson's original system that provides redder passbands.[5]

## References

1. ^ a b c Johnson, H. L.; Morgan, W. W. (1953). "Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas". The Astrophysical Journal. 117 (3): 313–352. Bibcode:1953ApJ...117..313J. doi:10.1086/145697.
2. ^ Hearnshaw, J. B. (1996). The Measurement of Starlight: Two Centuries of Astronomical Photometry. Cambridge University Press. p. 421. ISBN 9780521403931. In the 1950–51 winter, Johnson had commenced photometry in three passbands (designated U, V, and Y) on the McDonald 13- and 82-inch reflectors [54].
3. ^ Hearnshaw, J. B. (1996). The Measurement of Starlight: Two Centuries of Astronomical Photometry. Cambridge University Press. p. 425. ISBN 9780521403931.
4. ^ Iriarte, Braulio, Johnson, Harold L., Mitchell, Richard I., and Wisniewski, Wieslaw K. (1965), Five-Color Photometry of Bright Stars, Sky & Telescope, vol. 30, p. 21
5. ^ Landolt, Arlo U (2009). "Ubvriphotometric Standard Stars Around the Celestial Equator: Updates and Additions". The Astronomical Journal. 137 (5): 4186. arXiv:0904.0638. Bibcode:2009AJ....137.4186L. doi:10.1088/0004-6256/137/5/4186.
Absolute magnitude

Absolute magnitude (M) is a measure of the luminosity of a celestial object, on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10.0 parsecs (32.6 light-years), without extinction (or dimming) of its light due to absorption by interstellar matter and cosmic dust. By hypothetically placing all objects at a standard reference distance from the observer, their luminosities can be directly compared on a magnitude scale.

As with all astronomical magnitudes, the absolute magnitude can be specified for different wavelength ranges corresponding to specified filter bands or passbands; for stars a commonly quoted absolute magnitude is the absolute visual magnitude, which uses the visual (V) band of the spectrum (in the UBV photometric system). Absolute magnitudes are denoted by a capital M, with a subscript representing the filter band used for measurement, such as MV for absolute magnitude in the V band.

The more luminous an object, the smaller the numerical value of its absolute magnitude. A difference of 5 magnitudes between the absolute magnitudes of two objects corresponds to a ratio of 100 in their luminosities, and a difference of n magnitudes in absolute magnitude corresponds to a luminosity ratio of 100(n/5). For example, a star of absolute magnitude MV=3.0 would be 100 times more luminous than a star of absolute magnitude MV=8.0 as measured in the V filter band. The Sun has absolute magnitude MV=+4.83. Highly luminous objects can have negative absolute magnitudes: for example, the Milky Way galaxy has an absolute B magnitude of about −20.8.An object's absolute bolometric magnitude (Mbol) represents its total luminosity over all wavelengths, rather than in a single filter band, as expressed on a logarithmic magnitude scale. To convert from an absolute magnitude in a specific filter band to absolute bolometric magnitude, a bolometric correction (BC) is applied.For Solar System bodies that shine in reflected light, a different definition of absolute magnitude (H) is used, based on a standard reference distance of one astronomical unit.

Asteroid spectral types

An asteroid spectral type is assigned to asteroids based on their emission spectrum, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition. For small bodies that are not internally differentiated, the surface and internal compositions are presumably similar, while large bodies such as Ceres and Vesta are known to have internal structure. Over the years, there has been a number of surveys that resulted in a set of different taxonomic systems such as the Tholen, SMASS and Bus–DeMeo classification.

The Astronomical Observatory of Belogradchik or Belogradchik Observatory is an astronomical observatory owned and operated by the Institute of Astronomy of the Bulgarian Academy of Sciences. It is located near the town of Belogradchik in northwestern Bulgaria, at the foot of the Western Balkan Mountains. The other observatory operated by the same institute is the Rozhen Observatory.

Color index

In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The smaller the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones. For comparison, the yellowish Sun has a B−V index of 0.656 ± 0.005, whereas the bluish Rigel has a B−V of −0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B−V = −0.03). Traditionally, the color index uses Vega as a zero point.

To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also: UBV system). The set of passbands or filters is called a photometric system. The difference in magnitudes found with these filters is called the U−B or B−V color index respectively.

In principle, the temperature of a star can be calculated directly from the B−V index, and there are several formulae to make this connection. A good approximation can be obtained by considering stars as black bodies, using Ballesteros' formula (also implemented in the PyAstronomy package for Python):

${\displaystyle T=4600\,\mathrm {K} \left({\frac {1}{0.92(B-V)+1.7}}+{\frac {1}{0.92(B-V)+0.62}}\right).}$

Color indices of distant objects are usually affected by interstellar extinction, that is, they are redder than those of closer stars. The amount of reddening is characterized by color excess, defined as the difference between the observed color index and the normal color index (or intrinsic color index), the hypothetical true color index of the star, unaffected by extinction. For example, in the UBV photometric system we can write it for the B−V color:

${\displaystyle E_{B-V}=(B-V)_{\text{observed}}-(B-V)_{\text{intrinsic}}.}$

The passbands most optical astronomers use are the UBVRI filters, where the U, B, and V filters are as mentioned above, the R filter passes red light, and the I filter passes infrared light. This system of filters is sometimes called the Johnson–Cousins filter system, named after the originators of the system (see references). These filters were specified as particular combinations of glass filters and photomultiplier tubes. M. S. Bessell specified a set of filter transmissions for a flat response detector, thus quantifying the calculation of the color indices. For precision, appropriate pairs of filters are chosen depending on the object's color temperature: B−V are for mid-range objects, U−V for hotter objects, and R−I for cool ones.

Harold Johnson (astronomer)

Harold Lester Johnson (April 17, 1921 – April 2, 1980) was an American astronomer.

Harold Johnson was born in Denver, Colorado, on April 17, 1921. He received his early education in Denver public schools and went to the University of Denver, graduating with a degree in mathematics in 1942. Johnson was recruited by

the MIT Radiation Laboratory to work on World War II related radar research. After the war Johnson began graduate studies in astronomy at University of California, Berkeley where he completed his thesis under Harold Weaver in 1948.

In the following years working at Lowell Observatory, University of Wisconsin–Madison, Yerkes Observatory

(where he met William Wilson Morgan), McDonald Observatory, University of Texas–Austin, the

Lunar and Planetary Laboratory in Tucson, Arizona, and the National Autonomous University of Mexico he applied his instrumental and electronic talents to developing and calibrating astronomical photoelectric detectors.

He died of a heart attack in Mexico City in 1980.

He and his wife, Mary Elizabeth Jones, had two children.

Johnson was awarded the Helen B. Warner Prize by the American Astronomical Society in 1956. He was elected to the National Academy of Sciences in 1969.

He is remembered for introducing the UBV photometric system (also called the Johnson or Johnson-Morgan system), along with William Wilson Morgan in 1953.

Hipparcos

Hipparcos was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky. This permitted the accurate determination of proper motions and parallaxes of stars, allowing a determination of their distance and tangential velocity. When combined with radial velocity measurements from spectroscopy, this pinpointed all six quantities needed to determine the motion of stars. The resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, while the enhanced Tycho-2 Catalogue of 2.5 million stars was published in 2000. Hipparcos' follow-up mission, Gaia, was launched in 2013.

The word "Hipparcos" is an acronym for HIgh Precision PARallax COllecting Satellite and also a reference to the ancient Greek astronomer Hipparchus of Nicaea, who is noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.

NGC 5286

NGC 5286 (also known as Caldwell 84) is a globular cluster of stars located some 35,900 light years away in the constellation Centaurus. At this distance, the light from the cluster has undergone reddening from interstellar gas and dust equal to E(B – V) = 0.24 magnitude in the UBV photometric system. The cluster lies 4 arc-minutes north of the naked-eye star M Centauri. It was discovered by Scottish astronomer James Dunlop, active in Australia, and listed in his 1827 catalog.This cluster is about 29 kly (8.9 kpc) from the Galactic Center and is currently orbiting in the Milky Way halo. It may be associated with the Monoceros Ring—a long tidal stream of stars that could have been formed from a disrupted dwarf galaxy. NGC 5286 may be one of the oldest globular clusters in the galaxy, with an estimated age of 12.54 billion years. It is not perfectly spherical, but has a projected ellipticity of 0.12.The velocity dispersion of stars at the center of the cluster is (8.1 ± 1.0) km/s. Based upon the motions of stars at the core of this cluster, it may host an intermediate mass black hole with less than 1% of the cluster's mass. The upper limit for the mass estimate of this object is 6,000 times the mass of the Sun.NGC 5286 is part of the Gaia Sausage, the hypothesised remains of a merged dwarf galaxy.

NGC 7510

NGC 7510 is an open cluster of stars located around 11,400 light years away in the constellation Cepheus, near the border with Cassiopeia. At this distance, the light from the cluster has undergone extinction from interstellar gas and dust equal to E(B – V) = 0.90 ± 0.02 magnitude in the UBV photometric system. Its brightest member is a giant star with a stellar classification of B1.5 III. This cluster forms part of the Perseus Spiral Arm. It has a Trumpler class rating of II 2 m and is around 10 million years old.

NGC 7790

NGC 7790 is a young open cluster of stars located some 10,800 light years away from Earth in the northern constellation of Cassiopeia. At this distance, the light from the cluster has undergone extinction from interstellar gas and dust equal to E(B – V ) = 0.51 magnitude in the UBV photometric system. NGC 7790 has a Trumpler class rating of II2m and the estimated age is 60–80 million years. It contains three cepheid variables: CEa Cas, CEb Cas, and CF Cas.This cluster is on an orbit through the Milky Way galaxy that has an eccentricity of 0.22 ± 0.07 and a period of (225.0 ± 27.1) million years. It will come as close as 20.2 ± 3.9 kly (6.2 ± 1.2 kpc) to, and as distant as 31.6 ± 2.9 kly (9.7 ± 0.9 kpc) from, the Galactic Center. The maximum distance reached above (or below) the galactic plane is 0.78 ± 1.30 kly (0.24 ± 0.40 kpc). On average, it will cross the galactic plane every (35.7 ± 13.0) million years.

North polar sequence

The North polar sequence is a group of 96 stars that was used to define stellar magnitudes and colors. The cluster of stars lies within two degrees of the Northern Celestial pole. That fact makes them visible to everyone in the northern hemisphere.Originally proposed by Edward Charles Pickering, the system was used between 1900 and 1950. Today it has been replaced by the UBV photometric system.

Otto Struve Telescope

The Otto Struve Telescope was the first major telescope to be built at McDonald Observatory. Located in the Davis Mountains in West Texas, the Otto Struve Telescope was designed by Warner & Swasey Company and constructed between 1933 and 1939 by the Paterson-Leitch Company. Its 82-inch (2.1 m) mirror was the second largest in the world at the time. It was named after the Russian-American astronomer Otto Struve in 1966, three years after his death.

The Davis Mountains is an excellent location for astronomical research because of the clear dry air and moderately high elevation. The remote nature of the facility proved to be a significant challenge in transporting such a large mirror. It was a very precarious journey for the Otto Struve Telescope's mirror to this remote part of Texas and up to the top of Mount Locke. The mirror was transported from the local town of Fort Davis up the mountain by Carleton D. Wilson, owner of a local trucking company, while locals cheered as they looked on.

The Otto Struve telescope is still in use today. It is updated with modern imaging detectors allowing astronomers to conduct many types of research.

Photometric system

In astronomy, a photometric system is a set of well-defined passbands (or filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.

A commonly adopted standardized photometric system is the Johnson-Morgan or UBV photometric system (1953). At present, there are more than 200 photometric systems.

Photometric systems are usually characterized according to the widths of their passbands:

broadband (passbands wider than 30 nm, of which the most widely used is Johnson-Morgan UBV system)

intermediate band (passbands between 10 and 30 nm wide)

narrow band (passbands less than 10 nm wide)

Stewart Sharpless

Stewart Sharpless (March 29, 1926 – January 19, 2013) was an American astronomer who carried out fundamental work on the structure of the Milky Way galaxy.

As a graduate student at Yerkes Observatory he worked under William Morgan with fellow graduate student Don Osterbrock. He helped Johnson and Morgan with calculations used to help define the UBV photometric system. In 1952, Sharpless and Osterbrock published their observations that demonstrated the spiral structure of the Milky Way by estimating the distances to H II regions and young hot stars. For a while Sharpless was at Mount Wilson Observatory where he worked on galaxy photography with Walter Baade and Edwin Hubble.In 1953 Sharpless joined the staff of the United States Naval Observatory Flagstaff Station. Here he surveyed and cataloged H II regions of the Milky Way Galaxy using the images from the Palomar Sky Survey. From this work Sharpless published his catalog of H II regions in two editions, the first in 1953 with 142 nebula. The second and final edition was published in 1959 with 313 nebulae (see Sharpless catalog).

Stewart Sharpless was before his death a retired Professor Emeritus in the Department of Physics and Astronomy at the University of Rochester.

UBV

UBV may mean:

UBV photometric system, (or Johnson photometric system) in astronomy

Universidad Bolivariana de Venezuela, Bolivarian University of Venezuela

UBV Photoelectric Photometry Catalogue

The UBV Photoelectric Photometry Catalogue, or UBV M, is the star brightness catalogue that complies to the UBV photometric system developed by astronomer Harold Johnson.

Ultraviolet/Optical Telescope

In astronomical photometry, the Ultraviolet and Optical Telescope (UVOT) on the Neil Gehrels Swift Observatory observes astronomical objects in its 17-by-17 arc minute field of view through one of several filters or grisms. The seven filters, which are similar to those on the XMM-Newton-OM (Optical Monitor) instrument, cover the near-ultraviolet and optical range. The brightness of an object observed in the three optical filters, called u, b, and v, can be converted into the more common Morgan-Johnson (see the UBV photometric system) magnitudes.

The three ultraviolet filters probe a spectral region that is not observable from the ground.

Although the main mission is to chase gamma-ray bursts as soon as they occur, many other transient celestial sources and other objects in the field of view are being measured.

The filters, not being like any other photometric system in use from the ground or in space, give unique photometric measurements. Their response has been defined as the UVOT photometric system, as outlined by.

Vega

Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only 25 light-years from the Sun, and, together with Arcturus and Sirius, one of the most luminous stars in the Sun's neighborhood. It is the fifth-brightest star in the night sky, and the second-brightest star in the northern celestial hemisphere, after Arcturus.

Vega has been extensively studied by astronomers, leading it to be termed “arguably the next most important star in the sky after the Sun”. Vega was the northern pole star around 12,000 BC and will be so again around the year 13,727, when the declination will be +86°14'. Vega was the first star other than the Sun to be photographed and the first to have its spectrum recorded. It was one of the first stars whose distance was estimated through parallax measurements. Vega has functioned as the baseline for calibrating the photometric brightness scale and was one of the stars used to define the zero point for the UBV photometric system.

Vega is only about a tenth of the age of the Sun, but since it is 2.1 times as massive, its expected lifetime is also one tenth of that of the Sun; both stars are at present approaching the midpoint of their life expectancies. Vega has an unusually low abundance of the elements with a higher atomic number than that of helium. Vega is also a variable star that varies slightly in brightness. It is rotating rapidly with a velocity of 236 km/s at the equator. This causes the equator to bulge outward due to centrifugal effects, and, as a result, there is a variation of temperature across the star's photosphere that reaches a maximum at the poles. From Earth, Vega is observed from the direction of one of these poles.Based on an observed excess emission of infrared radiation, Vega appears to have a circumstellar disk of dust. This dust is likely to be the result of collisions between objects in an orbiting debris disk, which is analogous to the Kuiper belt in the Solar System. Stars that display an infrared excess due to dust emission are termed Vega-like stars.

Vilnius photometric system

The Vilnius photometric system is a medium-band seven-colour photometric system (UPXYZVS), created in 1963 by Vytautas Straižys and his coworkers. This system was highly optimized for classification of stars from ground-based observations. The system was chosen to be medium-band, to ensure the possibility to measure faint stars.

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