Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1932, when Karl Jansky at Bell Telephone Laboratories observed radiation coming from the Milky Way. Subsequent observations have identified a number of different sources of radio emission; these include stars and galaxies, as well as new classes of objects, such as radio galaxies, quasars and masers. The discovery of the cosmic microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy. Radio astronomy is conducted using large radio antennas referred to as radio telescopes, that are either used singularly, or with multiple linked telescopes utilizing the techniques of radio interferometry and aperture synthesis; the use of interferometry allows radio astronomy to achieve high angular resolution, as the resolving power of an interferometer is set by the distance between its components, rather than the size of its components.
Before Jansky observed the Milky Way in the 1930s, physicists speculated that radio waves could be observed from astronomical sources. In the 1860s, James Clerk Maxwell's equations had shown that electromagnetic radiation is associated with electricity and magnetism, could exist at any wavelength. Several attempts were made to detect radio emission from the Sun including an experiment by German astrophysicists Johannes Wilsing and Julius Scheiner in 1896 and a centimeter wave radiation apparatus set up by Oliver Lodge between 1897 and 1900; these attempts were unable to detect any emission due to technical limitations of the instruments. The discovery of the radio reflecting ionosphere in 1902, led physicists to conclude that the layer would bounce any astronomical radio transmission back into space, making them undetectable. Karl Jansky made the discovery of the first astronomical radio source serendipitously in the early 1930s; as an engineer with Bell Telephone Laboratories, he was investigating static that interfered with short wave transatlantic voice transmissions.
Using a large directional antenna, Jansky noticed that his analog pen-and-paper recording system kept recording a repeating signal of unknown origin. Since the signal peaked about every 24 hours, Jansky suspected the source of the interference was the Sun crossing the view of his directional antenna. Continued analysis showed that the source was not following the 24-hour daily cycle of the Sun but instead repeating on a cycle of 23 hours and 56 minutes. Jansky discussed the puzzling phenomena with his friend and teacher Albert Melvin Skellett, who pointed out that the time between the signal peaks was the exact length of a sidereal day. By comparing his observations with optical astronomical maps, Jansky concluded that the radiation source peaked when his antenna was aimed at the densest part of the Milky Way in the constellation of Sagittarius, he concluded that since the Sun were not large emitters of radio noise, the strange radio interference may be generated by interstellar gas and dust in the galaxy.
Jansky announced his discovery in 1933. He wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs reassigned him to another project, so he did no further work in the field of astronomy, his pioneering efforts in the field of radio astronomy have been recognized by the naming of the fundamental unit of flux density, the jansky, after him. Grote Reber was inspired by Jansky's work, built a parabolic radio telescope 9m in diameter in his backyard in 1937, he began by repeating Jansky's observations, conducted the first sky survey in the radio frequencies. On February 27, 1942, James Stanley Hey, a British Army research officer, made the first detection of radio waves emitted by the Sun; that year George Clark Southworth, at Bell Labs like Jansky detected radiowaves from the sun. Both researchers were bound by wartime security surrounding radar, so Reber, not, published his 1944 findings first. Several other people independently discovered solar radiowaves, including E. Schott in Denmark and Elizabeth Alexander working on Norfolk Island.
At Cambridge University, where ionospheric research had taken place during World War II, J. A. Ratcliffe along with other members of the Telecommunications Research Establishment that had carried out wartime research into radar, created a radiophysics group at the university where radio wave emissions from the Sun were observed and studied; this early research soon branched out into the observation of other celestial radio sources and interferometry techniques were pioneered to isolate the angular source of the detected emissions. Martin Ryle and Antony Hewish at the Cavendish Astrophysics Group developed the technique of Earth-rotation aperture synthesis; the radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s. During the late 1960s and early 1970s, as computers became capable of handling the computationally intensive Fourier transform inversions required, they used aperture synthesis to create a'One-Mile' and a'5 km' effective
In physics, electromagnetic radiation refers to the waves of the electromagnetic field, propagating through space, carrying electromagnetic radiant energy. It includes radio waves, infrared, ultraviolet, X-rays, gamma rays. Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light, which, in a vacuum, is denoted c. In homogeneous, isotropic media, the oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave; the wavefront of electromagnetic waves emitted from a point source is a sphere. The position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves of different frequency are called by different names since they have different sources and effects on matter. In order of increasing frequency and decreasing wavelength these are: radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
Electromagnetic waves are emitted by electrically charged particles undergoing acceleration, these waves can subsequently interact with other charged particles, exerting force on them. EM waves carry energy and angular momentum away from their source particle and can impart those quantities to matter with which they interact. Electromagnetic radiation is associated with those EM waves that are free to propagate themselves without the continuing influence of the moving charges that produced them, because they have achieved sufficient distance from those charges. Thus, EMR is sometimes referred to as the far field. In this language, the near field refers to EM fields near the charges and current that directly produced them electromagnetic induction and electrostatic induction phenomena. In quantum mechanics, an alternate way of viewing EMR is that it consists of photons, uncharged elementary particles with zero rest mass which are the quanta of the electromagnetic force, responsible for all electromagnetic interactions.
Quantum electrodynamics is the theory of. Quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation; the energy of an individual photon is greater for photons of higher frequency. This relationship is given by Planck's equation E = hν, where E is the energy per photon, ν is the frequency of the photon, h is Planck's constant. A single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light; the effects of EMR upon chemical compounds and biological organisms depend both upon the radiation's power and its frequency. EMR of visible or lower frequencies is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules or break chemical bonds; the effects of these radiations on chemical systems and living tissue are caused by heating effects from the combined energy transfer of many photons. In contrast, high frequency ultraviolet, X-rays and gamma rays are called ionizing radiation, since individual photons of such high frequency have enough energy to ionize molecules or break chemical bonds.
These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, can be a health hazard. James Clerk Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields and their symmetry; because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave. Maxwell's equations were confirmed by Heinrich Hertz through experiments with radio waves. According to Maxwell's equations, a spatially varying electric field is always associated with a magnetic field that changes over time. A spatially varying magnetic field is associated with specific changes over time in the electric field. In an electromagnetic wave, the changes in the electric field are always accompanied by a wave in the magnetic field in one direction, vice versa; this relationship between the two occurs without either type of field causing the other.
In fact, magnetic fields can be viewed as electric fields in another frame of reference, electric fields can be viewed as magnetic fields in another frame of reference, but they have equal significance as physics is the same in all frames of reference, so the close relationship between space and time changes here is more than an analogy. Together, these fields form a propagating electromagnetic wave, which moves out into space and need never again interact with the source; the distant EM field formed in this way by the acceleration of a charge carries energy with it that "radiates" away through space, hence the term. Maxwell's equations established that some charges and currents produce a local type of electromagnetic field near them that does not have the behaviour of EMR. Currents directly produce a magnetic field, but it is of a magnetic dipole type that dies out with distance from the current. In a similar manner, moving charges pushed apart in a conductor by a changing electrical potential produce an electric dipole type electric
The Galaxy Evolution Explorer is an orbiting ultraviolet space telescope launched on April 28, 2003, operated until early 2012. An airlaunched Pegasus rocket placed the craft into a nearly circular orbit at an altitude of 697 kilometres and an inclination to the Earth's equator of 29 degrees; the first observation was dedicated to the crew of the Space Shuttle Columbia, being images in the constellation Hercules taken on May 21, 2003. This region was selected because it had been directly overhead the shuttle at the time of its last contact with the NASA Mission Control Center. After its primary mission of 29 months, observation operations were extended to 9 years with NASA placing it into standby mode on 7 Feb 2012. NASA cut off financial support for operations of GALEX in early February 2011 as it was ranked lower than other projects which were seeking a limited supply of funding; the mission's life-cycle cost to NASA was $150.6 million. The California Institute of Technology negotiated to transfer control of GALEX and its associated ground control equipment to the California Institute of Technology in keeping with the Stevenson-Wydler Technology Innovation Act.
Under this Act, excess research equipment owned by the US government can be transferred to educational institutions and non-profit organizations. In May 2012, GALEX operations were transferred to Caltech. On June 28, 2013 NASA decommissioned GALEX, it is expected that the spacecraft will remain in orbit for at least 65 years before it will re-enter the atmosphere. During its initial 29-month mission, extended, it made observations in ultraviolet wavelengths to measure the history of star formation in the universe 80 percent of the way back to the Big Bang. Since scientists believe the Universe to be about 13.8 billion years old, the mission will study galaxies and stars across about 10 billion years of cosmic history. The spacecraft's mission is to observe hundreds of thousands of galaxies, with the goal of determining the distance of each galaxy from Earth and the rate of star formation in each galaxy. Near- and far-UV emissions as measured by GALEX can indicate the presence of young stars, but may originate from old stellar populations.
Partnering with the NASA Jet Propulsion Laboratory on the mission are the California Institute of Technology, Orbital Sciences Corporation, University of California, Yonsei University, Johns Hopkins University, Columbia University, Laboratoire d'Astrophysique de Marseille, France. The observatory participated in GOALS with Spitzer and Hubble. GOALS stands for Great Observatories All-sky LIRG Survey, Luminous Infrared Galaxies were studied at the multiple wavelengths allowed by the telescopes; the telescope has a 50 cm diameter aperture primary, in a Richey-Chretien f/6 configuration. It can see light wavelengths from 135 nanometers to 280 nm, with a field of view of 1.2 degrees wide. It has gallium-arsenide solar cells. GALEX Arecibo SDSS Survey Arecibo Observatory GALEX website by the California Institute of Technology GALEX website by the Jet Propulsion Laboratory GALEX data archive by the STScI / MAST GALEXView Search Tool by the STScI / MAST GALEX Ultraviolet Sky Survey at Wikisky.org
Williamina Paton Stevens Fleming was a Scottish astronomer. During her career, she helped develop a common designation system for stars and cataloged thousands of stars and other astronomical phenomena. Among several career achievements that advanced astronomy, Fleming is noted for her discovery of the Horsehead Nebula in 1888 Williamina Stevens was born in Dundee, Scotland on May 15, 1857, to Mary Walker and Robert Stevens, a carver and gilder. There, in 1877, she married an accountant and widower of Dundee, she worked as a teacher a short time before the couple emigrated to Boston, Massachusetts, US, when she was 21. After she and her child were abandoned by her husband, Williamina Fleming worked as a maid in the home of Professor Edward Charles Pickering, director of the Harvard College Observatory; the story was told that Pickering became frustrated with the performance of the men working at the HCO and would complain loudly: "My Scottish maid could do better!"Pickering's wife Lizzie recommended Williamina as having talents beyond custodial and maternal arts, in 1879 Pickering hired Fleming to conduct part-time administrative work at the observatory.
In 1881, Pickering invited Fleming to formally join the HCO and taught her how to analyze stellar spectra. She became one of the founding members of the Harvard Computers, an all-women cadre of human computers hired by Pickering to compute mathematical classifications and edit the observatory's publications. In 1886, Fleming was placed in charge of the group. Pickering challenged Fleming to improve a preexisting classification of stars; the latest Harvard College Observatory images contained photographed spectra of stars that extended into the ultraviolet range, which allowed much more accurate classifications than recording spectra by hand through an instrument at night. Fleming devised a system for classifying stars according to the relative amount of hydrogen observed in their spectra, known as the Pickering-Fleming system. Stars showing hydrogen as the most abundant element were classified A, her colleague Annie Jump Cannon reordered the classification system based upon the surface temperature of stars, resulting in the Harvard system for classifying stars, still in use today.
As a result of years of work by their female computer team, the HCO published the first Henry Draper Catalog in 1890, a catalog with more than 10,000 stars classified according to their spectrum. The majority of these classifications were done by Fleming. Fleming made it possible to go back and compare recorded plates, by organizing thousands of photographs by telescope along with other identifying factors. In 1898, she was appointed Curator of Astronomical Photographss at Harvard, the first woman to hold the position. During her career, Fleming discovered a total of 59 gaseous nebulae, over 310 variable stars, 10 novae. Most notably, in 1888, Fleming discovered the Horsehead Nebula on a telescope-photogrammetry plate made by astronomer W. H. Pickering, brother of E. C. Pickering, she described the bright nebula as having "a semicircular indentation 5 minutes in diameter 30 minutes south of Zeta Orionis". Subsequent professional publications neglected to give credit to Fleming for the discovery.
The first Dreyer Index Catalogue omitted Fleming's name from the list of contributors having discovered sky objects at Harvard, attributing the entire work to "Pickering". However, by the time the second Dreyer Index Catalogue was published in 1908, Fleming and her female colleagues at the HCO were sufficiently well-known and received proper credit for their discoveries. Fleming is credited with the discovery of the first white dwarf: The first person who knew of the existence of white dwarfs was Mrs. Fleming. Pickering and I. With characteristic generosity, Pickering had volunteered to have the spectra of the stars which I had observed for parallax looked up on the Harvard plates. All those of faint absolute magnitude turned out to be of class G or later. Moved with curiosity I asked him about the companion of 40 Eridani. Characteristically, again, he telephoned to Mrs. Fleming who reported within an hour or so, that it was of Class A. Fleming published her discovery of white dwarf stars in 1910.
Her other notable publications include A Photographic Study of Variable Stars, a list of 222 variable stars she had discovered. Fleming advocated for other women in the sciences in her talk "A Field for Woman's Work in Astronomy" at the 1893 World's Fair in Chicago, where she promoted the hiring of female assistants in astronomy, her speech suggested she agreed with the prevailing idea that women were inferior, but felt that, if given greater opportunities, they would be able to become equals. In 1906, she was made an honorary member of the Royal Astronomical Society of London, the first American woman to be so honored. Soon after she was appointed honorary fellow in astronomy of Wellesley College. Shortly before her death the Astronomical Society of Mexico awarded her the Guadalupe Almendaro medal for her discovery of new stars, she died in Boston on May 21, 1911. The Fleming lunar crater was jointly named after her and Alexander Fleming Made an honorary member of the Royal Astronomical Society of London in 1906, the first American woman to be elected Awarded the Guadalupe Almendaro Medal by the Astronomical Society of Mexico for her discovery of new stars Appointed an h
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
The Sharpless catalog is a list of 313 HII regions, intended to be comprehensive north of declination −27°. The first edition was published in 1953 with 142 objects, the second and final version was published by US astronomer Stewart Sharpless in 1959 with 312 objects. Sharpless includes some planetary nebulae and supernova remnants, in addition to HII regions. In 1953 Stewart Sharpless joined the staff of the United States Naval Observatory Flagstaff Station, where he surveyed and cataloged H II regions of the Milky Way 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 312 nebulae. Sharpless coordinates are based on the star catalogs Bonner Durchmusterung and Cordoba Durchmusterung, but the second release was adjusted to the 1900 epoch. In the second release, some coordinates for southern hemisphere regions have an uncertainty over 1 minute of arc.
This can make them difficult to find, so a revised catalog called BFS was released. BFS has about 20 removals. Most of the removed items were taken out because they were remnants; the 312 items in Sharpless sometimes overlap with the 110 Messier objects, 7,840 objects in the New General Catalogue, the Caldwell catalogue, the RCW catalog. Contemporary catalogs were Gum and RCW, but they covered the southern hemisphere. Examples of second Sharpless catalog.
William Parsons, 3rd Earl of Rosse
William Parsons, 3rd Earl of Rosse HFRSE, was an Anglo-Irish astronomer who had several telescopes built. His 72-inch telescope, built in 1845 and colloquially known as the "Leviathan of Parsonstown", was the world's largest telescope, in terms of aperture size, until the early 20th century. From April 1807 until February 1841, he was styled as Baron Oxmantown, he was born in York, the son of Sir Lawrence Parsons. He was educated at Trinity College and Oxford University's Magdalen College, graduating with first-class honours in mathematics in 1822, he inherited an earldom and a large estate in King's County in Ireland when his father, Lawrence, 2nd Earl of Rosse, died in February 1841. Lord Rosse married Mary Field, daughter of John Wilmer Field, on 14 April 1836, they had a total of thirteen children, but only four sons survived to adulthood: Lawrence, 4th Earl of Rosse. The Rev. Randal Parsons; the Hon. Richard Clere Parsons known for developing railways in South America; the Hon. Sir Charles Algernon Parsons, known for inventing the steam turbine.
In addition to his astronomical interests, Rosse served as a Member of Parliament for King's County from 1821 to 1834, president of the British Association in 1843–1844, an Irish representative peer after 1845, president of the Royal Society, chancellor of Trinity College, Dublin. During the 1840s, he had the Leviathan of Parsonstown built, a 72-inch telescope at Birr Castle, County Offaly; the 72-inch telescope replaced a 36-inch telescope. He had to invent many of the techniques he used for constructing the Leviathan, both because its size was without precedent and because earlier telescope builders had guarded their secrets or had failed to publish their methods. Details of the metal, casting and polishing of the 3-ton'speculum' were presented in 1844 at the Belfast Natural History Society. Rosse's telescope was considered a marvellous technical and architectural achievement, images of it were circulated within the British commonwealth. Building of the Leviathan began in 1842 and it was first used in 1845.
It was the world's largest telescope, until the early 20th century. Using this telescope Rosse catalogued a large number of nebulae. Lord Rosse performed astronomical studies and discovered the spiral nature of some nebulas, today known to be spiral galaxies. Rosse's telescope Leviathan was the first to reveal the spiral structure of M51, a galaxy nicknamed as the "Whirlpool Galaxy", his drawings of it resemble modern photographs. Rosse named the Crab Nebula, based on an earlier drawing made with his older 36-inch telescope in which it resembled a crab. A few years when the 72-inch telescope was in service, he produced an improved drawing of different appearance, but the original name continued to be used. A main component of Rosse's nebular research was his attempt to resolve the nebular hypothesis, which posited that planets and stars were formed by gravity acting on gaseous nebulae. Rosse himself did not believe that nebulas were gaseous, arguing rather that they were made of such an amount of fine stars that most telescopes could not resolve them individually.
In 1845 Rosse and his technicians claimed to have resolved the Orion nebula into its individual stars using the Leviathan, a claim which had considerable cosmological and philosophical implications, as at the time there was considerable debate over whether or not the universe was "evolved", a concept which the nebular hypothesis supported and with which Rosse disagreed strongly. Rosse's primary opponent in this was John Herschel, who used his own instruments to claim that the Orion nebula was a "true" nebula, discounted Rosse's instruments as flawed. Neither man could establish sufficiently scientific results to resolve the question. One of Rosse's telescope admirers was Thomas Langlois Lefroy, a fellow Irish MP, who said, "The planet Jupiter, which through an ordinary glass is no larger than a good star, is seen twice as large as the moon appears to the naked eye... But the genius displayed in all the contrivances for wielding this mighty monster surpasses the design and execution of it; the telescope weighs sixteen tons, yet Lord Rosse raised it single-handed off its resting place, two men with ease raised it to any height."Lord Rosse's son published his father's findings, including the discovery of 226 NGC objects in the publication Observations of Nebulae and Clusters of Stars Made With the Six-foot and Three-foot Reflectors at Birr Castle From the Year 1848 up to the Year 1878, Scientific Transactions of the Royal Dublin Society Vol. II, 1878.
Lord Rosse had a variety of optical reflecting telescopes built. Rosse's telescopes used cast speculum metal ground parabolically and polished. 15-inch 24-inch 36-inch 72-inch, started in 18