Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface; the rotation of a star produces an equatorial bulge due to centrifugal force. As stars are not solid bodies, they can undergo differential rotation, thus the equator of the star can rotate at a different angular velocity than the higher latitudes. These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field; the magnetic field of a star interacts with the stellar wind. As the wind moves away from the star its rate of angular velocity slows; the magnetic field of the star interacts with the wind, which applies a drag to the stellar rotation. As a result, angular momentum is transferred from the star to the wind, over time this slows the star's rate of rotation. Unless a star is being observed from the direction of its pole, sections of the surface have some amount of movement toward or away from the observer.
The component of movement, in the direction of the observer is called the radial velocity. For the portion of the surface with a radial velocity component toward the observer, the radiation is shifted to a higher frequency because of Doppler shift; the region that has a component moving away from the observer is shifted to a lower frequency. When the absorption lines of a star are observed, this shift at each end of the spectrum causes the line to broaden. However, this broadening must be separated from other effects that can increase the line width; the component of the radial velocity observed through line broadening depends on the inclination of the star's pole to the line of sight. The derived value is given as v e ⋅ sin i, where ve is the rotational velocity at the equator and i is the inclination. However, i is not always known, so the result gives a minimum value for the star's rotational velocity; that is, if i is not a right angle the actual velocity is greater than v e ⋅ sin i. This is sometimes referred to as the projected rotational velocity.
In fast rotating stars polarimetry offers a method of recovering the actual velocity rather than just the rotational velocity. For giant stars, the atmospheric microturbulence can result in line broadening, much larger than effects of rotational drowning out the signal. However, an alternate approach can be employed; these occur when a massive object passes in front of the more distant star and functions like a lens magnifying the image. The more detailed information gathered by this means allows the effects of microturbulence to be distinguished from rotation. If a star displays magnetic surface activity such as starspots these features can be tracked to estimate the rotation rate. However, such features can form at locations other than equator and can migrate across latitudes over the course of their life span, so differential rotation of a star can produce varying measurements. Stellar magnetic activity is associated with rapid rotation, so this technique can be used for measurement of such stars.
Observation of starspots has shown that these features can vary the rotation rate of a star, as the magnetic fields modify the flow of gases in the star. Gravity tends to contract celestial bodies into a perfect sphere, the shape where all the mass is as close to the center of gravity as possible, but a rotating star is not spherical in shape, it has an equatorial bulge. As a rotating proto-stellar disk contracts to form a star its shape becomes more and more spherical, but the contraction doesn't proceed all the way to a perfect sphere. At the poles all of the gravity acts to increase the contraction, but at the equator the effective gravity is diminished by the centrifugal force; the final shape of the star after star formation is an equilibrium shape, in the sense that the effective gravity in the equatorial region cannot pull the star to a more spherical shape. The rotation gives rise to gravity darkening at the equator, as described by the von Zeipel theorem. An extreme example of an equatorial bulge is found on the star Regulus A.
The equator of this star has a measured rotational velocity of 317 ± 3 km/s. This corresponds to a rotation period of 15.9 hours, 86% of the velocity at which the star would break apart. The equatorial radius of this star is 32% larger than polar radius. Other rotating stars include Alpha Arae, Pleione and Achernar; the break-up velocity of a star is an expression, used to describe the case where the centrifugal force at the equator is equal to the gravitational force. For a star to be stable the rotational velocity must be below this value. Surface differential rotation is observed on stars such as the Sun when the angular velocity varies with latitude; the angular velocity decreases with increasing latitude. However the reverse has been observed, such as on the star designated HD 31993; the first such star, other than the Sun, to have its differential rotation mapped in detail is AB Doradus. The underlying mechanism that causes differential rotation is turbulent convection inside a star. Convective motion carries energy toward the surface through the mass movement of plasma.
This mass of plasma carries a portion of the angular velocity of the star. When turbulence occurs through shear and rotation, the angular momentum can become redistributed to different latitudes thro
Sirius is a binary star and the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is twice as bright as Canopus, the next brightest star. The system has the Bayer designation α Canis Majoris; the binary system consists of a main-sequence star of spectral type A0 or A1, termed Sirius A, a faint white dwarf companion of spectral type DA2, designated Sirius B. The distance between the two varies between 8.2 and 31.5 astronomical units as they orbit every 50 years. Sirius appears bright because of its proximity to Earth. At a distance of 2.6 parsecs, as determined by the Hipparcos astrometry satellite, the Sirius system is one of Earth's near neighbours. Sirius is moving closer to the Solar System, so it will increase in brightness over the next 60,000 years. After that time, its distance will begin to increase, it will become fainter, but it will continue to be the brightest star in the Earth's night sky for the next 210,000 years. Sirius A is about twice as massive as the Sun and has an absolute visual magnitude of +1.42.
It is 25 times more luminous than the Sun but has a lower luminosity than other bright stars such as Canopus or Rigel. The system is between 300 million years old, it was composed of two bright bluish stars. The more massive of these, Sirius B, consumed its resources and became a red giant before shedding its outer layers and collapsing into its current state as a white dwarf around 120 million years ago. Sirius is known colloquially as the "Dog Star", reflecting its prominence in its constellation, Canis Major; the heliacal rising of Sirius marked the flooding of the Nile in Ancient Egypt and the "dog days" of summer for the ancient Greeks, while to the Polynesians in the Southern Hemisphere, the star marked winter and was an important reference for their navigation around the Pacific Ocean. The brightest star in the night sky, Sirius is recorded in some of the earliest astronomical records, its displacement from the ecliptic causes this heliacal rising to be remarkably regular compared to other stars, with a period of exactly 365.25 days holding it constant relative to the solar year.
This occurs at Cairo on 19 July, placing it just prior to the summer solstice and the onset of the annual flooding of the Nile during antiquity. Owing to the flood's own irregularity, the extreme precision of the star's return made it important to the ancient Egyptians, who worshipped it as the goddess Sopdet, guarantor of the fertility of their land; the Egyptian civil calendar was initiated to have its New Year "Mesori" coincide with the appearance of Sirius, although its lack of leap years meant that this congruence only held for four years until its date began to wander backwards through the months. The Egyptians continued to note the times of Sirius's annual return, which may have led them to the discovery of the 1460-year Sothic cycle and influenced the development of the Julian and Alexandrian calendars; the ancient Greeks observed that the appearance of Sirius heralded the hot and dry summer and feared that it caused plants to wilt, men to weaken, women to become aroused. Due to its brightness, Sirius would have been noted to twinkle more in the unsettled weather conditions of early summer.
To Greek observers, this signified certain emanations. Anyone suffering its effects was said to be "star-struck", it was described as "burning" or "flaming" in literature. The season following the star's reappearance came to be known as the "dog days"; the inhabitants of the island of Ceos in the Aegean Sea would offer sacrifices to Sirius and Zeus to bring cooling breezes, would await the reappearance of the star in summer. If it rose clear, it would portend good fortune. Coins retrieved from the island from the 3rd century BC feature dogs or stars with emanating rays, highlighting Sirius's importance; the Romans celebrated the heliacal setting of Sirius around April 25, sacrificing a dog, along with incense, a sheep, to the goddess Robigo so that the star's emanations would not cause wheat rust on wheat crops that year. Ptolemy of Alexandria mapped the stars in Books VII and VIII of his Almagest, in which he used Sirius as the location for the globe's central meridian, he depicted it as one of six red-coloured stars.
The other five are class M and K stars, such as Betelgeuse. Bright stars were important to the ancient Polynesians for navigation between the many islands and atolls of the Pacific Ocean. Low on the horizon, they acted as stellar compasses, they served as latitude markers. Sirius served as the body of a "Great Bird" constellation called Manu, with Canopus as the southern wingtip and Procyon the northern wingtip, which divided the Polynesian night sky into two hemispheres. Just as the appearance of Sirius in the morning sky marked summer in Greece, it marked the onset of winter for the Māori, whose name Takurua described both the star and the season, its culmination at the winter solstice was marked by celebration in Hawaii, where it was known as Ka'ulua, "Queen of Heaven". Many other Polynesian names have been recorded, including Tau-ua in the Marquesas Islands, Rehua in New Zealand, Ta'urua-fau-papa "Festivity of original high chiefs" and Ta'urua-e-hiti-i-te-tara-te-feiai "Festivity who rises with prayers and
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines; each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary due to the temperature of the photosphere, although in some cases there are true abundance differences; the spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photosphere's temperature. Most stars are classified under the Morgan-Keenan system using the letters O, B, A, F, G, K, M, a sequence from the hottest to the coolest; each letter class is subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.
In the MK system, a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd for sub-dwarfs, class D for white dwarfs; the full spectral class for the Sun is G2V, indicating a main-sequence star with a temperature around 5,800 K. The conventional color description takes into account only the peak of the stellar spectrum. In actuality, stars radiate in all parts of the spectrum; because all spectral colors combined appear white, the actual apparent colors the human eye would observe are far lighter than the conventional color descriptions would suggest. This characteristic of'lightness' indicates that the simplified assignment of colors within the spectrum can be misleading.
Excluding color-contrast illusions in dim light, there are indigo, or violet stars. Red dwarfs are a deep shade of orange, brown dwarfs do not appear brown, but hypothetically would appear dim grey to a nearby observer; the modern classification system is known as the Morgan–Keenan classification. Each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the star's spectral type. Other modern stellar classification systems, such as the UBV system, are based on color indexes—the measured differences in three or more color magnitudes; those numbers are given labels such as "U-V" or "B-V", which represent the colors passed by two standard filters. The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified a prior alphabetical system. Stars are grouped according to their spectral characteristics by single letters of the alphabet, optionally with numeric subdivisions.
Main-sequence stars vary in surface temperature from 2,000 to 50,000 K, whereas more-evolved stars can have temperatures above 100,000 K. Physically, the classes indicate the temperature of the star's atmosphere and are listed from hottest to coldest; the spectral classes O through M, as well as other more specialized classes discussed are subdivided by Arabic numerals, where 0 denotes the hottest stars of a given class. For example, A0 denotes A9 denotes the coolest ones. Fractional numbers are allowed; the Sun is classified as G2. Conventional color descriptions are traditional in astronomy, represent colors relative to the mean color of an A class star, considered to be white; the apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, appear white or bluish white to the unaided eye because they are too dim for color vision to work. Red supergiants are cooler and redder than dwarfs of the same spectral type, stars with particular spectral features such as carbon stars may be far redder than any black body.
The fact that the Harvard classification of a star indicated its surface or photospheric temperature was not understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated, this was suspected to be true. In the 1920s, the Indian physicist Meghnad Saha derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms. First he applied it to the solar chromosphere to stellar spectra. Harvard astronomer Cecilia Payne demonstrated that the O-B-A-F-G-K-M spectral sequence is a sequence in temperature; because the classification sequence predates our understanding that it is a temperature sequence, the placement of a spectrum into a given subtype, such as B3 or A7, depends upon estimates of the strengths of absorption features in stellar spectra. As a result, these subtypes are not evenly divided into any sort of mathematically representable intervals; the Yerkes spectral classification called the MKK system from the authors' initial
Hipparcos was a scientific satellite of the European Space Agency, 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, was launched in 2013; the word "Hipparcos" is an acronym for HIgh Precision PARallax COllecting Satellite and a reference to the ancient Greek astronomer Hipparchus of Nicaea, noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.
By the second half of the 20th century, the accurate measurement of star positions from the ground was running into insurmountable barriers to improvements in accuracy for large-angle measurements and systematic terms. Problems were dominated by the effects of the Earth's atmosphere, but were compounded by complex optical terms and gravitational instrument flexures, the absence of all-sky visibility. A formal proposal to make these exacting observations from space was first put forward in 1967. Although proposed to the French space agency CNES, it was considered too complex and expensive for a single national programme, its acceptance within the European Space Agency's scientific programme, in 1980, was the result of a lengthy process of study and lobbying. The underlying scientific motivation was to determine the physical properties of the stars through the measurement of their distances and space motions, thus to place theoretical studies of stellar structure and evolution, studies of galactic structure and kinematics, on a more secure empirical basis.
Observationally, the objective was to provide the positions and annual proper motions for some 100,000 stars with an unprecedented accuracy of 0.002 arcseconds, a target in practice surpassed by a factor of two. The name of the space telescope, "Hipparcos" was an acronym for High Precision Parallax Collecting Satellite, it reflected the name of the ancient Greek astronomer Hipparchus, considered the founder of trigonometry and the discoverer of the precession of the equinoxes; the spacecraft carried a single all-reflective, eccentric Schmidt telescope, with an aperture of 29 cm. A special beam-combining mirror superimposed two fields of view, 58 degrees apart, into the common focal plane; this complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the rectangular entrance pupil, providing an unvignetted field of view of about 1°×1°. The telescope used a system of grids, at the focal surface, composed of 2688 alternate opaque and transparent bands, with a period of 1.208 arc-sec.
Behind this grid system, an image dissector tube with a sensitive field of view of about 38-arc-sec diameter converted the modulated light into a sequence of photon counts from which the phase of the entire pulse train from a star could be derived. The apparent angle between two stars in the combined fields of view, modulo the grid period, was obtained from the phase difference of the two star pulse trains. Targeting the observation of some 100,000 stars, with an astrometric accuracy of about 0.002 arc-sec, the final Hipparcos Catalogue comprised nearly 120,000 stars with a median accuracy of better than 0.001 arc-sec. An additional photomultiplier system viewed a beam splitter in the optical path and was used as a star mapper, its purpose was to monitor and determine the satellite attitude, in the process, to gather photometric and astrometric data of all stars down to about 11th magnitude. These measurements were made in two broad bands corresponding to B and V in the UBV photometric system.
The positions of these latter stars were to be determined to a precision of 0.03 arc-sec, a factor of 25 less than the main mission stars. Targeting the observation of around 400,000 stars, the resulting Tycho Catalogue comprised just over 1 million stars, with a subsequent analysis extending this to the Tycho-2 Catalogue of about 2.5 million stars. The attitude of the spacecraft about its center of gravity was controlled to scan the celestial sphere in a regular precessional motion maintaining a constant inclination between the spin axis and the direction to the Sun; the spacecraft spun around its Z-axis at the rate of 11.25 revolutions/day at an angle of 43° to the Sun. The Z-axis rotated about the sun-satellite line at 6.4 revolutions/year. The spacecraft consisted of two platforms and six vertical panels, all made of aluminum honeycomb; the solar array consisted of three deployable sections. Two S-band antennas were located on the top and bottom of the spacecraft, providing an omni-directional downlink data rate of 24 kbit/s.
An attitude and orbit-control subsystem ensured correct dynamic attitude control and determination during the operational lifetim
USS Muliphen (AKA-61)
USS Muliphen was an Andromeda-class attack cargo ship named after Muliphen, a star in the constellation Canis Major. Muliphen was laid down under Maritime Commission contract on 13 May 1944 by Federal Shipbuilding and Drydock Co. Kearny, N. J. launched on 26 August 1944, sponsored by Mrs. John Hascock, acquired by the Navy on 21 October 1944, commissioned on 23 October 1944, Lt. Comdr. Walter W. Williamson in command. Following shakedown in Chesapeake Bay, Muliphen sailed on 1 December 1944 to operate with the Fleet Sonar School, Key West, Florida. On 14 December, she steamed for the Pacific where she joined Transport Division 43 off Pearl Harbor, sailed to prepare for the invasion of Iwo Jima at Eniwetok, arriving on 5 February 1945. Muliphen arrived off Iwo Jima on 19 February, unloaded until 4 March retired to Saipan, she departed on 27 March for the invasion of Okinawa, took part in a feint landing on 1 April, repeated the feint the following day. Held in reserve off Okinawa until 10 April, she sailed for Saipan and cargo duty between the Marianas and Solomons.
She arrived Manila on 18 September with a cargo of underwater demolition gear, spent the next three months carrying occupation troops to Japan from the Philippines, until sailing for Seattle on 24 November. Serving with the Naval Transportation Service, for the next four years she carried men and supplies to Asiatic and Pacific ports, supplied Point Barrow, Alaska in 1946 and 1947. In 1950 Muliphen transferred based at Norfolk; the following decade she rotated in a steady schedule of Caribbean and Mediterranean deployments. In 1958 she participated in the amphibious landings at Beirut, when a prompt response by the 6th Fleet prevented Communist subversion of Lebanon's government. Continuing similar duty in the 1960s, she took part in NATO exercises and the training of Naval Academy midshipmen. On 1 January 1969, Muliphen was redesignated LKA-61. Muliphen was decommissioned on 28 August 1970, transferred to the Maritime Administration for lay up in the National Defense Reserve Fleet; the ship was struck from the Naval Vessel Register on 1 January 1977, on 21 January 1989 was sunk as an artificial reef in a depth of 175 ft of water off Fort Pierce, Florida at 27°24.331′N 80°00.337′W.
Muliphen received two battle stars for World War II service. This article incorporates text from the public domain Dictionary of American Naval Fighting Ships; the entry can be found here. Photo gallery of USS Muliphen at NavSource Naval History USS Muliphen web site Military.com: USS Muliphen 51 Years of AKAs
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, many more are thought to exist, they are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring. Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an H II region.
Over time, radiation pressure from the cluster will disperse the molecular cloud. About 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away. Open clusters are key objects in the study of stellar evolution; because the cluster members are of similar age and chemical composition, their properties are more determined than they are for isolated stars. A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are visible with the naked eye; some others, such as the Double Cluster, are perceptible without instruments, while many more can be seen using binoculars or telescopes. The Wild Duck Cluster, M11, is an example; the prominent open cluster the Pleiades has been recognized as a group of stars since antiquity, while the Hyades forms part of Taurus, one of the oldest constellations. Other open clusters were noted by early astronomers as unresolved fuzzy patches of light; the Roman astronomer Ptolemy mentions the Praesepe, the Double Cluster in Perseus, the Ptolemy Cluster, while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster.
However, it would require the invention of the telescope to resolve these nebulae into their constituent stars. Indeed, in 1603 Johann Bayer gave three of these clusters designations; the first person to use a telescope to observe the night sky and record his observations was the Italian scientist Galileo Galilei in 1609. When he turned the telescope toward some of the nebulous patches recorded by Ptolemy, he found they were not a single star, but groupings of many stars. For Praesepe, he found more than 40 stars. Where observers had noted only 6-7 stars in the Pleiades, he found 50. In his 1610 treatise Sidereus Nuncius, Galileo Galilei wrote, "the galaxy is nothing else but a mass of innumerable stars planted together in clusters." Influenced by Galileo's work, the Sicilian astronomer Giovanni Hodierna became the first astronomer to use a telescope to find undiscovered open clusters. In 1654, he identified the objects now designated Messier 41, Messier 47, NGC 2362 and NGC 2451, it was realised as early as 1767 that the stars in a cluster were physically related, when the English naturalist Reverend John Michell calculated that the probability of just one group of stars like the Pleiades being the result of a chance alignment as seen from Earth was just 1 in 496,000.
Between 1774–1781, French astronomer Charles Messier published a catalogue of celestial objects that had a nebulous appearance similar to comets. This catalogue included 26 open clusters. In the 1790s, English astronomer William Herschel began an extensive study of nebulous celestial objects, he discovered. Herschel conceived the idea that stars were scattered across space, but became clustered together as star systems because of gravitational attraction, he divided the nebulae into eight classes, with classes VI through VIII being used to classify clusters of stars. The number of clusters known continued to increase under the efforts of astronomers. Hundreds of open clusters were listed in the New General Catalogue, first published in 1888 by the Danish-Irish astronomer J. L. E. Dreyer, the two supplemental Index Catalogues, published in 1896 and 1905. Telescopic observations revealed two distinct types of clusters, one of which contained thousands of stars in a regular spherical distribution and was found all across the sky but preferentially towards the centre of the Milky Way.
The other type consisted of a sparser population of stars in a more irregular shape. These were found in or near the galactic plane of the Milky Way. Astronomers dubbed the former globular clusters, the latter open clusters; because of their location, open clusters are referred to as galactic clusters, a term, introduced in 1925 by the Swiss-American astronomer Robert Julius Trumpler. Micrometer measurements of the positions of stars in clusters were made as early as 1877 by the German astronomer E. Schönfeld and further pursued by the American astronomer E. E. Barnard prior to his death in 1923. No indication of stellar motion was detected by these efforts. However, in 1918 the Dutch-American astronomer Adriaan van Maanen was able to measure the proper motion of stars in part of the Pleiades cluster by comparing photographic plates taken at different times; as astrometry became more accurate, cluster stars were found to share a common proper motion through space. By comparing the photographic plates of the Pleiades cluster taken in 1918 with images taken in 1943, van
The Kelvin scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The kelvin is the base unit of temperature in the International System of Units; until 2018, the kelvin was defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. In other words, it was defined such that the triple point of water is 273.16 K. On 16 November 2018, a new definition was adopted, in terms of a fixed value of the Boltzmann constant. For legal metrology purposes, the new definition will come into force on 20 May 2019; the Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin, who wrote of the need for an "absolute thermometric scale". Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or written as a degree; the kelvin is the primary unit of temperature measurement in the physical sciences, but is used in conjunction with the degree Celsius, which has the same magnitude.
The definition implies that absolute zero is equivalent to −273.15 °C. In 1848, William Thomson, made Lord Kelvin, wrote in his paper, On an Absolute Thermometric Scale, of the need for a scale whereby "infinite cold" was the scale's null point, which used the degree Celsius for its unit increment. Kelvin calculated; this absolute scale is known today as the Kelvin thermodynamic temperature scale. Kelvin's value of "−273" was the negative reciprocal of 0.00366—the accepted expansion coefficient of gas per degree Celsius relative to the ice point, giving a remarkable consistency to the accepted value. In 1954, Resolution 3 of the 10th General Conference on Weights and Measures gave the Kelvin scale its modern definition by designating the triple point of water as its second defining point and assigned its temperature to 273.16 kelvins. In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K. Furthermore, feeling it useful to more explicitly define the magnitude of the unit increment, the 13th CGPM held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."In 2005, the Comité International des Poids et Mesures, a committee of the CGPM, affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the Kelvin thermodynamic temperature scale would refer to water having an isotopic composition specified as Vienna Standard Mean Ocean Water.
In 2018, Resolution A of the 26th CGPM adopted a significant redefinition of SI base units which included redefining the Kelvin in terms of a fixed value for the Boltzmann constant of 1.380649×10−23 J/K. When spelled out or spoken, the unit is pluralised using the same grammatical rules as for other SI units such as the volt or ohm; when reference is made to the "Kelvin scale", the word "kelvin"—which is a noun—functions adjectivally to modify the noun "scale" and is capitalized. As with most other SI unit symbols there is a space between the kelvin symbol. Before the 13th CGPM in 1967–1968, the unit kelvin was called a "degree", the same as with the other temperature scales at the time, it was distinguished from the other scales with either the adjective suffix "Kelvin" or with "absolute" and its symbol was °K. The latter term, the unit's official name from 1948 until 1954, was ambiguous since it could be interpreted as referring to the Rankine scale. Before the 13th CGPM, the plural form was "degrees absolute".
The 13th CGPM changed the unit name to "kelvin". The omission of "degree" indicates that it is not relative to an arbitrary reference point like the Celsius and Fahrenheit scales, but rather an absolute unit of measure which can be manipulated algebraically. In science and engineering, degrees Celsius and kelvins are used in the same article, where absolute temperatures are given in degrees Celsius, but temperature intervals are given in kelvins. E.g. "its measured value was 0.01028 °C with an uncertainty of 60 µK." This practice is permissible because the degree Celsius is a special name for the kelvin for use in expressing relative temperatures, the magnitude of the degree Celsius is equal to that of the kelvin. Notwithstanding that the official endorsement provided by Resolution 3 of the 13th CGPM states "a temperature interval may be expressed in degrees Celsius", the practice of using both °C and K is widespread throughout the scientific world; the use of SI prefixed forms of the degree Celsius to express a temperature interval has not been adopted.
In 2005 the CIPM embarked on a programme to redefine the kelvin using a more experimentally rigorous methodology. In particular, the committee proposed redefining the kelvin such that Boltzmann's constant takes the exact value 1.3806505×10−23 J/K. The committee had hoped tha