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)

Photometric letters

Each letter designates a particular section of the electromagnetic spectrum; most of these sections fall within the region spanning the near-ultraviolet (NUV), the visible and the majority of the near-infrared (NIR).

Indigo and cyan are not standard colors.[1] Orange, yellow, and green fall under visual bands, while violet and purple are under the blue bands. The letters are not standards, but are recognized by common agreement among astronomers and astrophysicists.

Filter
Letter
Effective Wavelength Midpoint
λeff for Standard Filter[2]
Full Width Half Maximum[2]
(Bandwidth Δλ)
Variant(s) Description
Ultraviolet
U 365 nm 66 nm u, u', u* "U" stands for ultraviolet.
Visible
B 445 nm 94 nm b "B" stands for blue.
V 551 nm 88 nm v, v' "V" stands for visual.
G[3] 464 nm 128 nm g' "G" stands for green.
R 658 nm 138 nm r, r', R', Rc, Re, Rj "R" stands for red.
Near-Infrared
I 806 nm 149 nm i, i', Ic, Ie, Ij "I" stands for infrared.
Z 900 nm[4] z, z'
Y 1020 nm 120 nm y
J 1220 nm 213 nm J', Js
H 1630 nm 307 nm
K 2190 nm 390 nm K Continuum, K', Ks, Klong, K8, nbK
L 3450 nm 472 nm L', nbL'
Mid-Infrared
M 4750 nm 460 nm M', nbM
N 10500 nm 2500 nm
Q 21000 nm[5] 5800 nm[5] Q'

Combinations of these letters are frequently used; for example the combination JHK has been used more or less as a synonym of "near-infrared", and appears in the title of many papers.[6]

Filters used

The filters currently being used by other telescopes or organizations.

Units of measurements:

Name Filters Link
2.2 m telescope at La Silla, ESO J = 1.24 μm H = 1.63 μm K = 2.19 μm L' = 3.78 μm M = 4.66 μm N1 = 8.36 μm N2 = 9.67 μm N3 = 12.89 μm 2.2 m telescope at La Silla, ESO[7]
2MASS/PAIRITEL J = 1.25 μm H = 1.65 μm Ks = 2.15 μm Two Micron All-Sky Survey, Peters Automated InfraRed Imaging TELescope
CFHTLS (Megacam) u* = 374 nm g' = 487 nm r' = 625 nm i' = 770 nm z' = 890 nm Canada-France-Hawaii Telescope
Chandra X-ray Observatory LETG = 0.08-0.2 keV HETG = 0.4-10 keV Chandra X-ray Observatory
CTIO J = 1.20 μm H = 1.60 μm K = 2.20 μm L = 3.50 μm Cerro Tololo Inter-American Observatory, a division of NOAO
Cousins RI photometry Rc = 647 nm Ic = 786.5 nm Cousins RI photometry, 1976[8]
the Dark Energy Camera g = 472.0 nm r = 641.5 nm i = 783.5 nm z = 926.0 nm Y = 1009.5 nm Central wavelengths for bands in the Dark Energy Survey [9]
DENIS I = 0.79 μm J = 1.24 μm K = 2.16 μm Deep Near Infrared Survey
Eggen RI photometry Re = 635 nm Ie = 790 nm Eggen RI photometry, 1965[10]
FIS N60 = 65.00 μm WIDE-S = 90.00 μm WIDE-L = 145.00 μm N160 = 160.00 μm Far-Infrared Surveyor on board, AKARI space telescope
Gaia G = 673 nm GBP = 532 nm GRP = 797 nm GRVS = 860 nm Gaia (spacecraft)[11]
GALEX[12] NUV = 175–280 nm FUV = 135–175 nm GALaxy Evolution Explorer
GOODS (Hubble ACS) B = 435 nm V = 606 nm i = 775 nm z = 850 nm Advanced Camera for Surveys on the Hubble Space Telescope
HAWC+ Band 1 = 53 µm Band 2 = 89 µm Band 3 = 154 µm Band 4 = 214 µm High-resolution Airborne Wideband Camera+ for SOFIA[13]
HDF 450 nm 606 nm 814 nm Hubble Deep Field from the Hubble Space Telescope
IRTF NSFCAM J = 1.26 µm H = 1.62 µm K' = 2.12 µm Ks = 2.15 µm K = 2.21 µm L = 3.50 µm L' = 3.78 µm M' = 4.78 µm M = 4.85 µm NASA Infrared Telescope Facility NSFCAM[14]
ISAAC UTI/VLT[15] Js = 1.2 µm H = 1.6 µm Ks = 2.2 µm L = 3.78 µm Brα = 4.07 µm Infrared Spectrometer And Array Camera at Very Large Telescope
Johnson system (UBV) U = 364 nm B = 442 nm V = 540 nm UBV photometric system
LSST[16] u = 320.5–393.5 nm g = 401.5–551.9 nm r = 552.0–691.0 nm i = 691.0–818.0 nm z = 818.0–923.5 nm y = 923.8–1084.5 nm Large Synoptic Survey Telescope
OMC Johnson V-filter = 500-580 nm Optical Monitor Camera[17] on INTEGRAL
Pan-STARRS g = 481 nm r = 617 nm i = 752 nm z = 866 nm y = 962 nm Panoramic Survey Telescope And Rapid Response System[18]
ProNaOS/SPM Band 1 = 180-240 µm Band 2 = 240-340 µm Band 3 = 340-540 µm Band 4 = 540-1200 µm PROgramme NAtional d'Observations Submillerètrique/Systéme Photométrique Multibande, balloon-borne experiment[19]
Sloan, SDSS u' = 354 nm g' = 475 nm r' = 622 nm i' = 763 nm z' = 905 nm Sloan Digital Sky Survey
SPIRIT III Band B1 = 4.29 μm Band B2 = 4.35 μm Band A = 8.28 μm Band C = 12.13 μm Band D = 14.65 μm Band E = 21.34 μm Infrared camera on Midcourse Space Experiment[20]
Spitzer IRAC 3.6 μm 4.5 μm 5.8 μm 8.0 μm Infrared Array Camera on Spitzer Space Telescope
Spitzer MIPS 24 μm 70 μm 160 μm Multiband Imaging Photometer for Spitzer on Spitzer
Stromvil filters U = 345 nm P = 374 nm S = 405 nm Y = 466 nm Z = 516 nm V = 544 nm S = 656 nm Stromvil photometry
Strömgren filters u = 350 nm v = 411 nm b = 467 nm y = 547 nm β narrow = 485.8 nm β wide = 485 nm Strömgren photometric system
UKIDSS (WFCAM) Z = 882 nm Y = 1031 nm J = 1248 nm H = 1631 nm K = 2201 nm UKIRT Infrared Deep Sky Survey
Vilnius photometric system U = 345 nm P = 374 nm X = 405 nm Y = 466 nm Z = 516 nm V = 544 nm S = 656 nm Vilnius photometric system
VISTA IRC Z = 0.88 μm Y = 1.02 μm J = 1.25 μm H = 1.65 μm Ks = 2.20 μm NB1.18 = 1.18 μm Visible & Infrared Survey Telescope for Astronomy
WISE 3.4 μm 4.6 μm 12 μm 22 μm Wide-field Infrared Survey Explorer
XMM-Newton OM UVW2 = 212 nm UVM2 = 231 nm UVW1 = 291 nm U = 344 nm B = 450 nm V = 543 nm XMM-Newton Optical/UV Monitoring[21]
XEST Survey UVW2 = 212 nm UVM2 = 231 nm UVW1 = 291 nm U = 344 nm B = 450 nm V = 543 nm J = 1.25 μm H = 1.65 μm Ks = 2.15 μm Survey includes the point source of 2MASS with XMM-Newton OM[22]

See also

References

  1. ^ Spectral Colors
  2. ^ a b Binney, J.; Merrifield M. Galactic Astronomy, Princeton University Press, 1998, ch. 2.3.2, pp. 53
  3. ^ Bessell, Michael S. (September 2005). "Standard Photometric Systems" (PDF). Annual Review of Astronomy and Astrophysics. 43 (1): 293–336. Bibcode:2005ARA&A..43..293B. doi:10.1146/annurev.astro.41.082801.100251. ISSN 0066-4146.
  4. ^ Gouda, N.; Yano, T.; Kobayashi, Y.; Yamada, Y.; et al. (23 May 2005). "JASMINE: Japan Astrometry Satellite Mission for INfrared Exploration". Proceedings of the International Astronomical Union. 2004 (IAUC196): 455–468. Bibcode:2005tvnv.conf..455G. doi:10.1017/S1743921305001614. z-band: 0.9 μm
  5. ^ a b [1] Handbook of Geophysics and the Space Environment 1985, Air Force Geophysics Laboratory, 1985, ed. Adolph S. Jursa, Ch. 25, Table 25-1
  6. ^ Monson, Andrew J.; Pierce, Michael J. (2011). "Near-Infrared (Jhk) Photometry of 131 Northern Galactic Classical Cepheids". The Astrophysical Journal Supplement Series. 193: 12. Bibcode:2011ApJS..193...12M. doi:10.1088/0067-0049/193/1/12. Example of use of J for "near-infrared"
  7. ^ A study of the Chamaeleon I dark cloud and T-association. II – High-resolution IRAS maps around HD 97048 and 97300, Assendorp, R.; Wesselius, P. R.; Prusti, T.; Whittet, D. C. B., 1990
  8. ^ ADPS
  9. ^ DES
  10. ^ ADPS
  11. ^ . Bibcode:2010A&A...523A..48J. Missing or empty |title= (help)
  12. ^ "GALEX Instrument Summary". Goddard Space Flight Center. Retrieved 5 June 2019.
  13. ^ HAWC
  14. ^ NSFCAM
  15. ^ "ISAAC Overview". Paranal Instrumentation. ESO. Retrieved 13 October 2011.
  16. ^ LSST filter characteristics taken from https://github.com/lsst/throughputs/blob/master/baseline/ (see the filter_X.dat files) with the limits at half the peak transmission.
  17. ^ About INTEGRAL
  18. ^ The Pan-STARRS1 Photometric System, Tonry et al. 2012
  19. ^ Calibration of the PRONAOS/SPM submillimeter photometer, F.Pajot et al. 2006
  20. ^ MSXPSC – Midcourse Space Experiment (MSX) Point Source Catalog, V2.3
  21. ^ XMM-Newton SAS: Watchout Page
  22. ^ The XMM-Newton Optical Monitor Survey of the Taurus Molecular Cloud, M.Audard et al. 2006

External links

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):

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:

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.

Extinction (astronomy)

In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler. However, its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve, and its effect on the colors of stars had been observed by a number of individuals who did not connect it with the general presence of galactic dust. For stars that lie near the plane of the Milky Way and are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies (photometric system) is on the order of 1.8 magnitudes per kiloparsec.For Earth-bound observers, extinction arises both from the interstellar medium (ISM) and the Earth's atmosphere; it may also arise from circumstellar dust around an observed object. Strong extinction in earth's atmosphere of some wavelength regions (such as X-ray, ultraviolet, and infrared) is overcome by the use of space-based observatories. Since blue light is much more strongly attenuated than red light, extinction causes objects to appear redder than expected, a phenomenon referred to as interstellar reddening.

Mercator Telescope

The Mercator Telescope is a 1.2 m telescope at the Observatorio del Roque de Los Muchachos on La Palma. It is operated by the Katholieke Universiteit Leuven (Leuven University), Belgium, in collaboration with the Observatory of the University of Geneva and named after Gerard Mercator, famous cartographer.

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.

Photometric-standard star

Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.A current set of photometric-standard stars for UBVRI photometry was published by Arlo U. Landolt in 1992 in the Astronomical Journal.

Photometry (astronomy)

Photometry, from Greek photo- ("light") and -metry ("measure"), is a technique used in astronomy that is concerned with measuring the flux or intensity of light radiated by astronomical objects. This light is measured through a telescope using a photometer, often made using electronic devices such as a CCD photometer or a photoelectric photometer that converts light into an electric current by the photoelectric effect. When calibrated against standard stars (or other light sources) of known intensity and colour, photometers can measure the brightness or apparent magnitude of celestial objects.

The methods used to perform photometry depend on the wavelength regime under study. At its most basic, photometry is conducted by gathering light and passing it through specialized photometric optical bandpass filters, and then capturing and recording the light energy with a photosensitive instrument. Standard sets of passbands (called a photometric system) are defined to allow accurate comparison of observations. A more advanced technique is spectrophotometry that is measured with a spectrophotometer and observes both of the amount of radiation and its detailed spectral distribution.Photometry is also used in the observation of variable stars, by various techniques such as, differential photometry that simultaneously measuring the brightness of a target object and nearby stars in the starfield or relative photometry by comparing the brightness of the target object to stars with known fixed magnitudes. Using multiple bandpass filters with relative photometry is termed absolute photometry. A plot of magnitude against time produces a light curve, yielding considerable information about the physical process causing the brightness changes. Precision photoelectric photometers can measure starlight around 0.001 magnitude.The technique of surface photometry can also be used with extended objects like planets, comets, nebulae or galaxies that measures the apparent magnitude in terms of magnitudes per square arcsecond. Knowing the area of the object and the average intensity of light across the astronomical object determines the surface brightness in terms of magnitudes per square arcsecond, while integrating the total light of the extended object can then calculate brightness in terms of its total magnitude, energy output or luminosity per unit surface area.

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.

Strömgren photometric system

Strömgren photometric system, abbreviated as uvbyβ or simply uvby, and sometimes referred as Strömgren - Crawford photometric system, is a four-colour medium-passband photometric system plus Hβ (H-beta) filters for determining magnitudes and obtaining spectral classification of stars. Its use was pioneered by the Danish astronomer Bengt Strömgren in 1956 and was extended by his colleague the American astronomer David L. Crawford in 1958.It is often considered to be a powerful tool and successful investigating the brightness and effective temperature of stars. This photometric system also has a general advantage as it can be used to measure the effects of reddening and interstellar extinction. This system also allows calculation of parameters from the b and y filters (b − y) without the effects of reddening, termed m 1 and c 1.

Surface brightness

In astronomy, surface brightness quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on its surface luminosity density, i.e., its luminosity emitted per unit surface area. In visible and infrared astronomy, surface brightness is often quoted on a magnitude scale, in magnitudes per square arcsecond in a particular filter band or photometric system.

Measurement of the surface brightnesses of celestial objects is called surface photometry.

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.

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.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.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. 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. However, a large number of measurements have been made in this system, including many of the bright stars.

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.

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.

Vytautas Straižys

Vytautas Straižys (born August 20, 1936) is a Lithuanian astronomer. In 1963–65 he and his collaborators created and developed the Vilnius photometric system, a seven-color intermediate band system, optimized for photometric stellar classification. In 1996 he was elected a Corresponding Member of the Lithuanian Academy of Sciences. Straižys is an editor of the journal Baltic Astronomy. He is currently working at the Molėtai Astronomical Observatory. Asteroid 68730 Straizys in 2002 was named after him.

W Ursae Majoris variable

A W Ursae Majoris variable, also known as a low mass contact binary, is a type of eclipsing binary variable star. These stars are close binaries of spectral types F, G, or K that share a common envelope of material and are thus in contact with one another. They are termed contact binaries because the two stars touch and transfer mass and energy through the connecting neck, although astronomer R.E. Wilson argues that the term "overcontact" is more appropriate.

The class is divided into two subclasses: A-type and W-type A-type W UMa binaries are composed of two stars both hotter than the Sun, having spectral types A or F, and periods of 0.4 to 0.8 day. The W-types have cooler spectral types of G or K and shorter periods of 0.22 to 0.4 day. The difference between the surface temperatures of the components is less than several hundred kelvins. A new subclass was introduced in 1978: B-type. The B-types have larger surface temperature difference. In 2004 the H (high mass ratio) systems were discovered by Sz. Csizmadia and P. Klagyivik. The H-types have a higher mass ratio than ( = (secondary's mass)/(primary's mass)) and they have extra angular momentum.

These stars were first shown to follow a period-color relation (shorter period systems are redder) by Olin J. Eggen. In 2012, Terrell, Gross and Cooney published a color-survey of 606 W UMa systems in the Johnson-Cousins photometric system.

Their light curves differ from those of classical eclipsing binaries, undergoing a constant ellipsoidal variation rather than discrete eclipses. This is because the stars are gravitationally distorted by one another, and thus the projected area of the stars is constantly changing. The depths of the brightness minima are usually equal because both stars have nearly equal surface temperatures.

W Ursae Majoris is the prototype of this class.

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