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113-416: 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

226-448: A binary black hole . A second gravitational wave was detected on 26 December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments. The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy . One of the oldest fields in astronomy, and in all of science,

339-666: A Delaware shipbuilder and state senator , and his second wife, Mary Jump. Cannon's mother was the first person to teach her the constellations and she encouraged her to follow her own interests, suggesting that she pursue studies in mathematics, chemistry, and biology at Wellesley College . Cannon and her mother used an old astronomy textbook to identify stars seen from their attic. Cannon's mother also taught her daughter household economics , which Cannon would later use to organize her research. Cannon took her mother's advice and pursued her love of astronomy . She lost most of her hearing sometime during her early adult years. Sources vary on

452-399: A complex classification system) and Williamina Fleming (who was overseeing the project for Pickering, and wanted a much more simple, straightforward approach). Cannon negotiated a compromise: she started by examining the bright southern hemisphere stars. To these stars, she applied a third system, a division of stars into the spectral classes O, B, A, F, G, K, M . Her scheme was based on

565-558: A few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars , and black hole candidates such as active galactic nuclei. In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth. In neutrino astronomy , astronomers use heavily shielded underground facilities such as SAGE , GALLEX , and Kamioka II/III for

678-478: A group of women hired by Harvard Observatory director Edward C. Pickering to complete the Henry Draper Catalogue , with the goal of mapping and defining every star in the sky to a photographic magnitude of about 9. In her notes, she referred to brightness as "Int" which was short for "intensity". In 1927, Pickering said that she was able to classify stars very quickly, "Miss Cannon is the only person in

791-523: A luminosity class of IIIb, while a luminosity class IIIa indicates a star slightly brighter than a typical giant. A sample of extreme V stars with strong absorption in He II λ4686 spectral lines have been given the Vz designation. An example star is HD 93129 B . Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, 59 Cygni

904-515: A magnifying glass, could classify stars down to the ninth magnitude, around 16 times fainter than the human eye can see. Her work was also highly accurate. Not long after work began on the Draper Catalogue, a disagreement developed as to how to classify the stars. The analysis was first started by Nettie Farrar, who left a few months later to be married. This left the problem to the ideas of Henry Draper's niece Antonia Maury (who insisted on

1017-552: A model allows astronomers to select between several alternative or conflicting models. Theorists also modify existing models to take into account new observations. In some cases, a large amount of observational data that is inconsistent with a model may lead to abandoning it largely or completely, as for geocentric theory , the existence of luminiferous aether , and the steady-state model of cosmic evolution. Phenomena modeled by theoretical astronomers include: Modern theoretical astronomy reflects dramatic advances in observation since

1130-663: A nearby observer. The modern classification system is known as the Morgan–Keenan (MK) classification. Each star is assigned a spectral class (from the older Harvard spectral classification, which did not include luminosity) 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 indices —the measured differences in three or more color magnitudes . Those numbers are given labels such as "U−V" or "B−V", which represent

1243-661: A number of important astronomers. Richard of Wallingford (1292–1336) made major contributions to astronomy and horology , including the invention of the first astronomical clock, the Rectangulus which allowed for the measurement of angles between planets and other astronomical bodies, as well as an equatorium called the Albion which could be used for astronomical calculations such as lunar , solar and planetary longitudes and could predict eclipses . Nicole Oresme (1320–1382) and Jean Buridan (1300–1361) first discussed evidence for

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1356-699: A pamphlet called "In the Footsteps of Columbus", published by the Blair Company and distributed as a souvenir at the Chicago World's Columbian Exposition of 1893. Soon afterward, Cannon was stricken with scarlet fever that rendered her nearly deaf. This hearing loss made it difficult for Cannon to socialize. As a result, she immersed herself in her work. In 1894, Cannon's mother died and life at home grew more difficult. She wrote to her former instructor at Wellesley, professor Sarah Frances Whiting , to see if there

1469-562: A repeating cycle known as a saros . Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena. In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun , and he proposed a model of

1582-610: A result, these subtypes are not evenly divided into any sort of mathematically representable intervals. The Yerkes spectral classification , also called the MK, or Morgan-Keenan (alternatively referred to as the MKK, or Morgan-Keenan-Kellman) system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan , Philip C. Keenan , and Edith Kellman from Yerkes Observatory . This two-dimensional ( temperature and luminosity ) classification scheme

1695-454: A series of twenty-two types numbered from I–XXII. Because the 22 Roman numeral groupings did not account for additional variations in spectra, three additional divisions were made to further specify differences: Lowercase letters were added to differentiate relative line appearance in spectra; the lines were defined as: Antonia Maury published her own stellar classification catalogue in 1897 called "Spectra of Bright Stars Photographed with

1808-608: A substantial amount of work in the realms of theoretical and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of dark matter , dark energy , and black holes ; whether or not time travel is possible, wormholes can form, or the multiverse exists; and the origin and ultimate fate of the universe . Topics also studied by theoretical astrophysicists include Solar System formation and evolution ; stellar dynamics and evolution ; galaxy formation and evolution ; magnetohydrodynamics ; large-scale structure of matter in

1921-427: Is visible light , or more generally electromagnetic radiation . Observational astronomy may be categorized according to the corresponding region of the electromagnetic spectrum on which the observations are made. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or outside the Earth's atmosphere. Specific information on these subfields

2034-580: Is a branch of astronomy that studies the universe as a whole. Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky . These include the Egyptians , Babylonians , Greeks , Indians , Chinese , Maya , and many ancient indigenous peoples of the Americas . In the past, astronomy included disciplines as diverse as astrometry , celestial navigation , observational astronomy , and

2147-407: Is a synonym for hotter , while "late" is a synonym for cooler . Depending on the context, "early" and "late" may be absolute or relative terms. "Early" as an absolute term would therefore refer to O or B, and possibly A stars. As a relative reference it relates to stars hotter than others, such as "early K" being perhaps K0, K1, K2 and K3. "Late" is used in the same way, with an unqualified use of

2260-584: Is absorbed by the Earth's atmosphere, requiring observations at these wavelengths to be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars ( OB stars ) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae , supernova remnants , and active galactic nuclei. However, as ultraviolet light

2373-591: Is also believed that the ruins at Great Zimbabwe and Timbuktu may have housed astronomical observatories. In Post-classical West Africa , Astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens as well as precise diagrams of orbits of the other planets based on complex mathematical calculations. Songhai historian Mahmud Kati documented a meteor shower in August 1583. Europeans had previously believed that there had been no astronomical observation in sub-Saharan Africa during

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2486-583: Is an inseparable part of the discipline of astrobiology. Astrobiology concerns itself with interpretation of existing scientific data , and although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories . This interdisciplinary field encompasses research on the origin of planetary systems , origins of organic compounds in space , rock-water-carbon interactions, abiogenesis on Earth, planetary habitability , research on biosignatures for life detection, and studies on

2599-553: Is based on spectral lines sensitive to stellar temperature and surface gravity , which is related to luminosity (whilst the Harvard classification is based on just surface temperature). Later, in 1953, after some revisions to the list of standard stars and classification criteria, the scheme was named the Morgan–Keenan classification , or MK , which remains in use today. Denser stars with higher surface gravity exhibit greater pressure broadening of spectral lines. The gravity, and hence

2712-427: Is easily absorbed by interstellar dust , an adjustment of ultraviolet measurements is necessary. X-ray astronomy uses X-ray wavelengths . Typically, X-ray radiation is produced by synchrotron emission (the result of electrons orbiting magnetic field lines), thermal emission from thin gases above 10 (10 million) kelvins , and thermal emission from thick gases above 10 Kelvin. Since X-rays are absorbed by

2825-414: Is founded on the detection and analysis of infrared radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing

2938-458: Is from these clouds that solar systems form. Studies in this field contribute to the understanding of the formation of the Solar System , Earth's origin and geology, abiogenesis , and the origin of climate and oceans. Astrobiology is an interdisciplinary scientific field concerned with the origins , early evolution , distribution, and future of life in the universe . Astrobiology considers

3051-416: Is given below. Radio astronomy uses radiation with wavelengths greater than approximately one millimeter, outside the visible range. Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons . Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this

3164-506: Is listed as spectral type B1.5Vnne, indicating a spectrum with the general classification B1.5V, as well as very broad absorption lines and certain emission lines. The reason for the odd arrangement of letters in the Harvard classification is historical, having evolved from the earlier Secchi classes and been progressively modified as understanding improved. During the 1860s and 1870s, pioneering stellar spectroscopist Angelo Secchi created

3277-645: Is not as easily done at shorter wavelengths. Although some radio waves are emitted directly by astronomical objects, a product of thermal emission , most of the radio emission that is observed is the result of synchrotron radiation , which is produced when electrons orbit magnetic fields . Additionally, a number of spectral lines produced by interstellar gas , notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths. A wide variety of other objects are observable at radio wavelengths, including supernovae , interstellar gas, pulsars , and active galactic nuclei . Infrared astronomy

3390-471: Is one of the few sciences in which amateurs play an active role . This is especially true for the discovery and observation of transient events . Amateur astronomers have helped with many important discoveries, such as finding new comets. Astronomy (from the Greek ἀστρονομία from ἄστρον astron , "star" and -νομία -nomia from νόμος nomos , "law" or "culture") means "law of the stars" (or "culture of

3503-476: Is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects , rather than their positions or motions in space". Among the objects studied are the Sun , other stars , galaxies , extrasolar planets , the interstellar medium and the cosmic microwave background . Their emissions are examined across all parts of the electromagnetic spectrum , and

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3616-499: Is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation (the use of celestial objects to guide navigation) and in the making of calendars . Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations , and an ability to determine past and future positions of

3729-472: The Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes . The Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere. Most gamma-ray emitting sources are actually gamma-ray bursts , objects which only produce gamma radiation for

3842-520: The Earth's atmosphere , all X-ray observations must be performed from high-altitude balloons , rockets , or X-ray astronomy satellites . Notable X-ray sources include X-ray binaries , pulsars , supernova remnants , elliptical galaxies , clusters of galaxies , and active galactic nuclei . Gamma ray astronomy observes astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as

3955-576: The He  II λ4541 disappears. However, with modern equipment, the line is still apparent in the early B-type stars. Today for main-sequence stars, the B class is instead defined by the intensity of the He ;I violet spectrum, with the maximum intensity corresponding to class B2. For supergiants, lines of silicon are used instead; the Si ;IV λ4089 and Si III λ4552 lines are indicative of early B. At mid-B,

4068-636: The International Astronomical Union passed the resolution to formally adopt Cannon's stellar classification system; with only minor changes, it is still being used for classification today. Also in 1922, Cannon spent six months in Arequipa, Peru , to photograph stars in the Southern hemisphere . In 1925, she became the first woman to receive an honorary doctorate of science from Oxford University . In 1933, she represented professional women at

4181-542: The Kelvin–Helmholtz mechanism , which is now known to not apply to main-sequence stars . If that were true, then stars would start their lives as very hot "early-type" stars and then gradually cool down into "late-type" stars. This mechanism provided ages of the Sun that were much smaller than what is observed in the geologic record , and was rendered obsolete by the discovery that stars are powered by nuclear fusion . The terms "early" and "late" were carried over, beyond

4294-939: The Milky Way , as its own group of stars was only proven in the 20th century, along with the existence of "external" galaxies. The observed recession of those galaxies led to the discovery of the expansion of the Universe . In 1919, when the Hooker Telescope was completed, the prevailing view was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope, Edwin Hubble identified Cepheid variables in several spiral nebulae and in 1922–1923 proved conclusively that Andromeda Nebula and Triangulum among others, were entire galaxies outside our own, thus proving that

4407-762: The Muslim world by the early 9th century. In 964, the Andromeda Galaxy , the largest galaxy in the Local Group , was described by the Persian Muslim astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars . The SN 1006 supernova , the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and Chinese astronomers in 1006. Iranian scholar Al-Biruni observed that, contrary to Ptolemy ,

4520-555: The Renaissance , Nicolaus Copernicus proposed a heliocentric model of the solar system. His work was defended by Galileo Galilei and expanded upon by Johannes Kepler . Kepler was the first to devise a system that correctly described the details of the motion of the planets around the Sun. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was Isaac Newton , with his invention of celestial dynamics and his law of gravitation , who finally explained

4633-495: The Secchi classes in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra, shown in the table below. In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article. The Roman numerals used for Secchi classes should not be confused with the completely unrelated Roman numerals used for Yerkes luminosity classes and

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4746-555: The Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model. In the 2nd century BC, Hipparchus discovered precession , calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe . Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy. The Antikythera mechanism ( c.  150 –80 BC)

4859-496: The Sun is then G2V, indicating a main-sequence star with a surface temperature around 5,800 K. The conventional colour description takes into account only the peak of the stellar spectrum. In actuality, however, stars radiate in all parts of the spectrum. Because all spectral colours combined appear white, the actual apparent colours the human eye would observe are far lighter than the conventional colour descriptions would suggest. This characteristic of 'lightness' indicates that

4972-427: The interstellar medium . The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites , is also called cosmochemistry , while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it

5085-629: The 11 inch Draper Telescope as Part of the Henry Draper Memorial", which included 4,800 photographs and Maury's analyses of 681 bright northern stars. This was the first instance in which a woman was credited for an observatory publication. In 1901, Annie Jump Cannon returned to the lettered types, but dropped all letters except O, B, A, F, G, K, M, and N used in that order, as well as P for planetary nebulae and Q for some peculiar spectra. She also used types such as B5A for stars halfway between types B and A, F2G for stars one fifth of

5198-442: The 1990s, including studies of the cosmic microwave background , distant supernovae and galaxy redshifts , which have led to the development of a standard model of cosmology . This model requires the universe to contain large amounts of dark matter and dark energy whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations. Astrophysics

5311-450: The B2 subclass, and moderate hydrogen lines. As O- and B-type stars are so energetic, they only live for a relatively short time. Thus, due to the low probability of kinematic interaction during their lifetime, they are unable to stray far from the area in which they formed, apart from runaway stars . The transition from class O to class B was originally defined to be the point at which

5424-451: The Catalogue a long-term project to obtain the optical spectra of as many stars as possible and to index and classify stars by spectra. When Cannon first started cataloging the stars, she was able to classify 1,000 stars in three years, but by 1913, she was able to work on 200 stars an hour. Cannon could classify three stars a minute just by looking at their spectral patterns and, if using

5537-403: The Earth's atmosphere and of their physical and chemical properties", while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena". In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu , "astronomy" may be used to describe the qualitative study of

5650-686: 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 subgiants , class  V for main-sequence stars , class  sd (or VI ) for subdwarfs , and class  D (or VII ) for white dwarfs . The full spectral class for

5763-522: The Sun's apogee (highest point in the heavens) was mobile, not fixed. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani , Thebit , Abd al-Rahman al-Sufi , Biruni , Abū Ishāq Ibrāhīm al-Zarqālī , Al-Birjandi , and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars . It

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5876-574: The Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Universe, or the Ptolemaic system , named after Ptolemy . A particularly important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians , who laid the foundations for the later astronomical traditions that developed in many other civilizations. The Babylonians discovered that lunar eclipses recurred in

5989-663: The US, where she studied physics and astronomy. Cannon studied under Sarah Frances Whiting , one of the few women physicists in the United States at the time, and went on to become the valedictorian at Wellesley College. She graduated with a degree in physics in 1884 and returned home to Delaware for a decade. During these years, Cannon developed her skills in the new art of photography . In 1892, she traveled through Europe taking photographs with her Blair box camera . After she returned home her prose and photos from Spain were published in

6102-807: The World's Fair in Chicago ( Century of Progress ). In 1935, she created the Annie J. Cannon Prize for "the woman of any country, whose contributions to the science of astronomy are the most distinguished." In 1938, she became the William C. Bond Astronomer at Harvard University. The astronomer Cecilia Payne collaborated with Cannon and used Cannon's data to show that the stars were composed mainly of hydrogen and helium. Annie Jump Cannon's career in astronomy lasted for more than 40 years, until her retirement in 1940. Despite her retirement, she continued to actively work on astronomy in

6215-485: The abundance of that element. The strengths of the different spectral lines vary mainly 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 primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature. Most stars are currently classified under the Morgan–Keenan (MK) system using

6328-521: The age of the Universe and size of the Observable Universe. Theoretical astronomy led to speculations on the existence of objects such as black holes and neutron stars , which have been used to explain such observed phenomena as quasars , pulsars , blazars , and radio galaxies . Physical cosmology made huge advances during the 20th century. In the early 1900s the model of the Big Bang theory

6441-513: The alphabet. This classification system was later modified by Annie Jump Cannon and Antonia Maury to produce the Harvard spectral classification scheme. In 1897, another astronomer at Harvard, Antonia Maury , placed the Orion subtype of Secchi class I ahead of the remainder of Secchi class I, thus placing the modern type B ahead of the modern type A. She was the first to do so, although she did not use lettered spectral types, but rather

6554-486: The atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space. Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets. Historically, optical astronomy, which has been also called visible light astronomy, is the oldest form of astronomy. Images of observations were originally drawn by hand. In

6667-871: The brighter stars of the constellation Orion . About 1 in 800 (0.125%) of the main-sequence stars in the solar neighborhood are B-type main-sequence stars . B-type stars are relatively uncommon and the closest is Regulus, at around 80 light years. Astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics , physics , and chemistry in order to explain their origin and their overall evolution . Objects of interest include planets , moons , stars , nebulae , galaxies , meteoroids , asteroids , and comets . Relevant phenomena include supernova explosions, gamma ray bursts , quasars , blazars , pulsars , and cosmic microwave background radiation . More generally, astronomy studies everything that originates beyond Earth's atmosphere . Cosmology

6780-519: The classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest. A common mnemonic for remembering the order of the spectral type letters, from hottest to coolest, is " O h, B e A F ine G uy/ G irl: K iss M e!", or another one is " O ur B right A stronomers F requently G enerate K iller M nemonics!" . The spectral classes O through M, as well as other more specialized classes discussed later, are subdivided by Arabic numerals (0–9), where 0 denotes

6893-636: The colors passed by two standard filters (e.g. U ltraviolet, B lue and V isual). The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon , who re-ordered and simplified the prior alphabetical system by Draper (see History ). 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 approximately 2,000 to 50,000  K , whereas more-evolved stars – in particular, newly-formed white dwarfs – can have surface temperatures above 100,000 K. Physically,

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7006-523: The demise of the model they were based on. O-type stars are very hot and extremely luminous, with most of their radiated output in the ultraviolet range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar neighborhood are O-type stars. Some of the most massive stars lie within this spectral class. O-type stars frequently have complicated surroundings that make measurement of their spectra difficult. O-type spectra formerly were defined by

7119-408: The department is historically affiliated with a physics department, and many professional astronomers have physics rather than astronomy degrees. Some titles of the leading scientific journals in this field include The Astronomical Journal , The Astrophysical Journal , and Astronomy & Astrophysics . In early historic times, astronomy only consisted of the observation and predictions of

7232-474: The detection of neutrinos . The vast majority of the neutrinos streaming through the Earth originate from the Sun , but 24 neutrinos were also detected from supernova 1987A . Cosmic rays , which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of secondary particles which can be detected by current observatories. Some future neutrino detectors may also be sensitive to

7345-665: The extreme velocity of their stellar wind , which may reach 2,000 km/s. Because they are so massive, O-type stars have very hot cores and burn through their hydrogen fuel very quickly, so they are the first stars to leave the main sequence . When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. The MKK scheme was extended to O9.7 in 1971 and O4 in 1978, and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced. Spectral standards: B-type stars are very luminous and blue. Their spectra have neutral helium lines, which are most prominent at

7458-611: The help of the Harvard computers , especially Williamina Fleming , the first iteration of the Henry Draper catalogue was devised to replace the Roman-numeral scheme established by Angelo Secchi. The catalogue used a scheme in which the previously used Secchi classes (I to V) were subdivided into more specific classes, given letters from A to P. Also, the letter Q was used for stars not fitting into any other class. Fleming worked with Pickering to differentiate 17 different classes based on

7571-402: The hottest stars of a given class. For example, A0 denotes the hottest stars in class A and A9 denotes the coolest ones. Fractional numbers are allowed; for example, the star Mu Normae is classified as O9.7. The Sun is classified as G2. The fact that the Harvard classification of a star indicated its surface or photospheric temperature (or more precisely, its effective temperature )

7684-408: The intensity of hydrogen spectral lines, which causes variation in the wavelengths emanated from stars and results in variation in color appearance. The spectra in class A tended to produce the strongest hydrogen absorption lines while spectra in class O produced virtually no visible lines. The lettering system displayed the gradual decrease in hydrogen absorption in the spectral classes when moving down

7797-427: The intensity of the latter relative to that of Si II λλ4128-30 is the defining characteristic, while for late B, it is the intensity of Mg II λ4481 relative to that of He I λ4471. These stars tend to be found in their originating OB associations , which are associated with giant molecular clouds . The Orion OB1 association occupies a large portion of a spiral arm of the Milky Way and contains many of

7910-581: The international community and assumed an ambassador-like role outside of it. In 1911 she was made the Curator of Astronomical Photographs at Harvard. In 1914, she was admitted as an honorary member of the Royal Astronomical Society . In 1921, she became one of the first women to receive an honorary doctorate from a European university when she was awarded an honorary doctor's degree in math and astronomy from Groningen University . On May 9, 1922,

8023-414: The introduction of new technology, including the spectroscope and photography . Joseph von Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859, Gustav Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of temperatures , masses , and sizes. The existence of the Earth's galaxy,

8136-579: The late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using charge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 Å to 7000 Å (400 nm to 700 nm), that same equipment can be used to observe some near-ultraviolet and near-infrared radiation. Ultraviolet astronomy employs ultraviolet wavelengths between approximately 100 and 3200 Å (10 to 320 nm). Light at those wavelengths

8249-544: The letters O , B , A , F , G , K , and M , a sequence from the hottest ( O type) to the coolest ( M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g., A8, A9, F0, and F1 form a sequence from hotter to cooler). The sequence has been expanded with three classes for other stars that do not fit in the classical system: W , S and C . Some non-stellar objects have also been assigned letters: D for white dwarfs and L , T and Y for Brown dwarfs . In

8362-493: The level of assistant in this line of work at the time and many were paid only 25 cents an hour to work seven hours a day, six days a week. Leavitt, another woman in the observatory who made significant contributions, shared with Cannon the experience of being deaf. Cannon dominated this field because of her "tidiness" and patience for the tedious work and even helped the men in the observatory gain popularity. Cannon helped broker partnerships and exchanges of equipment between men in

8475-419: The main sequence). Nominal luminosity class VII (and sometimes higher numerals) is now rarely used for white dwarf or "hot sub-dwarf" classes, since the temperature-letters of the main sequence and giant stars no longer apply to white dwarfs. Occasionally, letters a and b are applied to luminosity classes other than supergiants; for example, a giant star slightly less luminous than typical may be given

8588-576: The making of calendars . Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects. This data is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. These two fields complement each other. Theoretical astronomy seeks to explain observational results and observations are used to confirm theoretical results. Astronomy

8701-484: The modern definition uses the ratio of the nitrogen line N IV λ4058 to N III λλ4634-40-42. O-type stars have dominant lines of absorption and sometimes emission for He  II lines, prominent ionized ( Si  IV, O  III, N  III, and C  III) and neutral helium lines, strengthening from O5 to O9, and prominent hydrogen Balmer lines , although not as strong as in later types. Higher-mass O-type stars do not retain extensive atmospheres due to

8814-589: The motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that may have had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year. As civilizations developed, most notably in Egypt , Mesopotamia , Greece , Persia , India , China , and Central America , astronomical observatories were assembled and ideas on

8927-417: The motions of the planets. Newton also developed the reflecting telescope . Improvements in the size and quality of the telescope led to further discoveries. The English astronomer John Flamsteed catalogued over 3000 stars. More extensive star catalogues were produced by Nicolas Louis de Lacaille . The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered

9040-406: The nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as astrometry . From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the Universe were explored philosophically. The Earth was believed to be the center of the Universe with

9153-480: The observation of young stars embedded in molecular clouds and the cores of galaxies. Observations from the Wide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous galactic protostars and their host star clusters . With the exception of infrared wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as

9266-480: The observatory up until a few weeks before she died. During her career, Cannon helped women gain acceptance and respect within the scientific community. Her calm and hardworking attitude and demeanor helped her gain respect throughout her lifetime and paved the path for future women astronomers. Cannon died on April 13, 1941, in Cambridge , Massachusetts, at the age of 77. She died in a hospital after being ill for over

9379-534: The particles produced when cosmic rays hit the Earth's atmosphere. Gravitational-wave astronomy is an emerging field of astronomy that employs gravitational-wave detectors to collect observational data about distant massive objects. A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory LIGO . LIGO made its first detection on 14 September 2015, observing gravitational waves from

9492-497: The planet Uranus , the first new planet found. During the 18–19th centuries, the study of the three-body problem by Leonhard Euler , Alexis Claude Clairaut , and Jean le Rond d'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Joseph-Louis Lagrange and Pierre Simon Laplace , allowing the masses of the planets and moons to be estimated from their perturbations. Significant advances in astronomy came about with

9605-470: The planets with great accuracy, a field known as celestial mechanics . More recently the tracking of near-Earth objects will allow for predictions of close encounters or potential collisions of the Earth with those objects. The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for

9718-518: The potential for life to adapt to challenges on Earth and in outer space . Cosmology (from the Greek κόσμος ( kosmos ) "world, universe" and λόγος ( logos ) "word, study" or literally "logic") could be considered the study of the Universe as a whole. Annie Jump Cannon Cannon was born on December 11, 1863, in Dover, Delaware . She was the eldest of three daughters born to Wilson Cannon,

9831-576: The pre-colonial Middle Ages, but modern discoveries show otherwise. For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), the Roman Catholic Church gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the date for Easter . Medieval Europe housed

9944-401: The pressure, on the surface of a giant star is much lower than for a dwarf star because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as luminosity effects and a luminosity class can be assigned purely from examination of the spectrum. A number of different luminosity classes are distinguished, as listed in

10057-458: The properties examined include luminosity , density , temperature , and chemical composition. Because astrophysics is a very broad subject, astrophysicists typically apply many disciplines of physics, including mechanics , electromagnetism , statistical mechanics , thermodynamics , quantum mechanics , relativity , nuclear and particle physics , and atomic and molecular physics . In practice, modern astronomical research often involves

10170-468: The properties of more distant stars, as their properties can be compared. Measurements of the radial velocity and proper motion of stars allow astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated dark matter in the galaxy. During the 1990s, the measurement of the stellar wobble of nearby stars

10283-618: The proposed neutron star classes. In the 1880s, the astronomer Edward C. Pickering began to make a survey of stellar spectra at the Harvard College Observatory , using the objective-prism method. A first result of this work was the Draper Catalogue of Stellar Spectra , published in 1890. Williamina Fleming classified most of the spectra in this catalogue and was credited with classifying over 10,000 featured stars and discovering 10 novae and more than 200 variable stars. With

10396-459: The question of whether extraterrestrial life exists, and how humans can detect it if it does. The term exobiology is similar. Astrobiology makes use of molecular biology , biophysics , biochemistry , chemistry , astronomy, physical cosmology , exoplanetology and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from that on Earth. The origin and early evolution of life

10509-428: The ratio of the strength of the He  II λ4541 relative to that of He I λ4471, where λ is the radiation wavelength . Spectral type O7 was defined to be the point at which the two intensities are equal, with the He I line weakening towards earlier types. Type O3 was, by definition, the point at which said line disappears altogether, although it can be seen very faintly with modern technology. Due to this,

10622-425: The rotation of the Earth, furthermore, Buridan also developed the theory of impetus (predecessor of the modern scientific theory of inertia ) which was able to show planets were capable of motion without the intervention of angels. Georg von Peuerbach (1423–1461) and Regiomontanus (1436–1476) helped make astronomical progress instrumental to Copernicus's development of the heliocentric model decades later. During

10735-410: The simplified assignment of colours within the spectrum can be misleading. Excluding colour-contrast effects in dim light, in typical viewing conditions there are no green, cyan, indigo, or violet stars. "Yellow" dwarfs such as the Sun are white, "red" dwarfs are a deep shade of yellow/orange, and "brown" dwarfs do not literally appear brown, but hypothetically would appear dim red or grey/black to

10848-513: The solar chromosphere, then to stellar spectra. Harvard astronomer Cecilia Payne then demonstrated that the O-B-A-F-G-K-M spectral sequence is actually 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 (largely subjective) estimates of the strengths of absorption features in stellar spectra. As

10961-436: The stars" depending on the translation). Astronomy should not be confused with astrology , the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, they are now entirely distinct. "Astronomy" and " astrophysics " are synonyms. Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside

11074-570: The strength of the Balmer absorption lines . After absorption lines were understood in terms of stellar temperatures, her initial classification system was rearranged to avoid having to update star catalogs. In 1901, Cannon published her first catalog of stellar spectra. Cannon and the other women at the observatory, including Henrietta Swan Leavitt , Antonia Maury , and Florence Cushman , were criticized at first for being "out of their place" and not being housewives. Women did not commonly rise beyond

11187-477: The subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject. However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. Some fields, such as astrometry , are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics", partly depending on whether

11300-462: The table below. Marginal cases are allowed; for example, a star may be either a supergiant or a bright giant, or may be in between the subgiant and main-sequence classifications. In these cases, two special symbols are used: For example, a star classified as A3-4III/IV would be in between spectral types A3 and A4, while being either a giant star or a subgiant. Sub-dwarf classes have also been used: VI for sub-dwarfs (stars slightly less luminous than

11413-483: The term indicating stars with spectral types such as K and M, but it can also be used for stars that are cool relative to other stars, as in using "late G" to refer to G7, G8, and G9. In the relative sense, "early" means a lower Arabic numeral following the class letter, and "late" means a higher number. This obscure terminology is a hold-over from a late nineteenth century model of stellar evolution , which supposed that stars were powered by gravitational contraction via

11526-482: The time frame and actual cause, although it is often attributed to scarlet fever . Cannon's personality has been described as "ebullient". She chose not to marry or have children. At Wilmington Conference Academy (later known as Wesley College ), Cannon was a promising student, particularly in mathematics. In 1880, Cannon was sent to Wellesley College in Massachusetts, one of the top academic schools for women in

11639-534: The universe consists of a multitude of galaxies. With this Hubble formulated the Hubble constant , which allowed for the first time a calculation of the age of the Universe and size of the Observable Universe, which became increasingly precise with better meassurements, starting at 2 billion years and 280 million light-years, until 2006 when data of the Hubble Space Telescope allowed a very accurate calculation of

11752-409: The universe; origin of cosmic rays ; general relativity and physical cosmology , including string cosmology and astroparticle physics . Astrochemistry is the study of the abundance and reactions of molecules in the Universe , and their interaction with radiation . The discipline is an overlap of astronomy and chemistry . The word "astrochemistry" may be applied to both the Solar System and

11865-471: The way from F to G, and so on. Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc. This is essentially the modern form of the Harvard classification system. This system was developed through the analysis of spectra on photographic plates, which could convert light emanated from stars into a readable spectrum. A luminosity classification known as the Mount Wilson system

11978-414: The world—man or woman—who can do this work so quickly." Mary Anna Draper , the widow of wealthy physician and amateur astronomer Henry Draper , had set up a fund to support the work. Men at the laboratory did the labor of operating the telescopes and taking photographs while the women examined the data, carried out astronomical calculations, and cataloged those photographs during the day. Pickering made

12091-640: Was used to detect large extrasolar planets orbiting those stars. Theoretical astronomers use several tools including analytical models and computational numerical simulations ; each has its particular advantages. Analytical models of a process are better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved. Theorists in astronomy endeavor to create theoretical models that are based on existing observations and known physics, and to predict observational consequences of those models. The observation of phenomena predicted by

12204-409: Was a job opening. Whiting hired her as a junior physics teacher at the college. This opportunity allowed Cannon to take graduate courses at the college in physics and astronomy. Whiting also inspired Cannon to learn about spectroscopy . In order to gain access to a better telescope, Cannon enrolled at Radcliffe College in 1894 as a "special student", continuing her studies of astronomy. Radcliffe

12317-511: Was an early analog computer designed to calculate the location of the Sun , Moon , and planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe. Astronomy flourished in the Islamic world and other parts of the world. This led to the emergence of the first astronomical observatories in

12430-532: Was formulated, heavily evidenced by cosmic microwave background radiation , Hubble's law , and the cosmological abundances of elements . Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere. In February 2016, it was revealed that the LIGO project had detected evidence of gravitational waves in the previous September. The main source of information about celestial bodies and other objects

12543-456: Was not fully understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated (by 1914), this was generally 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

12656-535: Was set up near Harvard College for Harvard professors to repeat their lectures to the young Radcliffe women. This relationship gave Cannon access to the Harvard College Observatory . In 1896, Edward C. Pickering hired her as his assistant at the Observatory. In 1907, Cannon finished her studies and received her master's degree from Wellesley College. In 1896, Cannon became a member of the Harvard Computers ,

12769-483: Was used to distinguish between stars of different luminosities. This notation system is still sometimes seen on modern spectra. The stellar classification system is taxonomic , based on type specimens , similar to classification of species in biology : The categories are defined by one or more standard stars for each category and sub-category, with an associated description of the distinguishing features. Stars are often referred to as early or late types. "Early"

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