A star catalogue or star catalog, is an astronomical catalogue that lists stars. In astronomy, many stars are referred to by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, this article covers only some of the more quoted ones. Star catalogues were compiled by many different ancient people, including the Babylonians, Chinese and Arabs, they were sometimes accompanied by a star chart for illustration. Most modern catalogues are available in electronic format and can be downloaded from space agencies data centres. Completeness and accuracy is described by the weakest apparent magnitude V and the accuracy of the positions. From their existing records, it is known that the ancient Egyptians recorded the names of only a few identifiable constellations and a list of thirty-six decans that were used as a star clock; the Egyptians called the circumpolar star "the star that cannot perish" and, although they made no known formal star catalogues, they nonetheless created extensive star charts of the night sky which adorn the coffins and ceilings of tomb chambers.
Although the ancient Sumerians were the first to record the names of constellations on clay tablets, the earliest known star catalogues were compiled by the ancient Babylonians of Mesopotamia in the late 2nd millennium BC, during the Kassite Period. They are better known by their Assyrian-era name'Three Stars Each'; these star catalogues, written on clay tablets, listed thirty-six stars: twelve for "Anu" along the celestial equator, twelve for "Ea" south of that, twelve for "Enlil" to the north. The Mul. Apin lists, dated to sometime before the Neo-Babylonian Empire, are direct textual descendants of the "Three Stars Each" lists and their constellation patterns show similarities to those of Greek civilization. In Ancient Greece, the astronomer and mathematician Eudoxus laid down a full set of the classical constellations around 370 BC, his catalogue Phaenomena, rewritten by Aratus of Soli between 275 and 250 BC as a didactic poem, became one of the most consulted astronomical texts in antiquity and beyond.
It contains descriptions of the positions of the stars, the shapes of the constellations and provided information on their relative times of rising and setting. In the 3rd century BC, the Greek astronomers Timocharis of Alexandria and Aristillus created another star catalogue. Hipparchus completed his star catalogue in 129 BC, which he compared to Timocharis' and discovered that the longitude of the stars had changed over time; this led him to determine the first value of the precession of the equinoxes. In the 2nd century, Ptolemy of Roman Egypt published a star catalogue as part of his Almagest, which listed 1,022 stars visible from Alexandria. Ptolemy's catalogue was based entirely on an earlier one by Hipparchus, it remained the standard star catalogue in the Arab worlds for over eight centuries. The Islamic astronomer al-Sufi updated it in 964, the star positions were redetermined by Ulugh Beg in 1437, but it was not superseded until the appearance of the thousand-star catalogue of Tycho Brahe in 1598.
Although the ancient Vedas of India specified how the ecliptic was to be divided into twenty-eight nakshatra, Indian constellation patterns were borrowed from Greek ones sometime after Alexander's conquests in Asia in the 4th century BC. The earliest known inscriptions for Chinese star names were written on oracle bones and date to the Shang Dynasty. Sources dating from the Zhou Dynasty which provide star names include the Zuo Zhuan, the Shi Jing, the "Canon of Yao" in the Book of Documents; the Lüshi Chunqiu written by the Qin statesman Lü Buwei provides most of the names for the twenty-eight mansions. An earlier lacquerware chest found in the Tomb of Marquis Yi of Zeng contains a complete list of the names of the twenty-eight mansions. Star catalogues are traditionally attributed to Shi Shen and Gan De, two rather obscure Chinese astronomers who may have been active in the 4th century BC of the Warring States period; the Shi Shen astronomy is attributed to Shi Shen, the Astronomic star observation to Gan De.
It was not until the Han Dynasty that astronomers started to observe and record names for all the stars that were apparent in the night sky, not just those around the ecliptic. A star catalogue is featured in one of the chapters of the late 2nd-century-BC history work Records of the Grand Historian by Sima Qian and contains the "schools" of Shi Shen and Gan De's work. Sima's catalogue—the Book of Celestial Offices —includes some 90 constellations, the stars therein named after temples, ideas in philosophy, locations such as markets and shops, different people such as farmers and soldiers. For his Spiritual Constitution of the Universe of 120 AD, the astronomer Zhang Heng compiled a star catalogue comprising 124 constellations. Chinese constellation names were adopted by the Koreans and Japanese. A large number of star catalogues were published by Muslim astronomers in the medieval Islamic world; these were Zij treatises, including Arzachel's Tables of Toledo, the Maragheh observatory's Zij-i Ilkhani and Ulugh Beg's Zij-i-Sultani.
The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptic orbit, 1 is a parabolic escape orbit, greater than 1 is a hyperbola; the term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is used for the isolated two-body problem, but extensions exist for objects following a Klemperer rosette orbit through the galaxy. In a two-body problem with inverse-square-law force, every orbit is a Kepler orbit; the eccentricity of this Kepler orbit is a non-negative number. The eccentricity may take the following values: circular orbit: e = 0 elliptic orbit: 0 < e < 1 parabolic trajectory: e = 1 hyperbolic trajectory: e > 1 The eccentricity e is given by e = 1 + 2 E L 2 m red α 2 where E is the total orbital energy, L is the angular momentum, mred is the reduced mass, α the coefficient of the inverse-square law central force such as gravity or electrostatics in classical physics: F = α r 2 or in the case of a gravitational force: e = 1 + 2 ε h 2 μ 2 where ε is the specific orbital energy, μ the standard gravitational parameter based on the total mass, h the specific relative angular momentum.
For values of e from 0 to 1 the orbit's shape is an elongated ellipse. The limit case between an ellipse and a hyperbola, when e equals 1, is parabola. Radial trajectories are classified as elliptic, parabolic, or hyperbolic based on the energy of the orbit, not the eccentricity. Radial orbits hence eccentricity equal to one. Keeping the energy constant and reducing the angular momentum, elliptic and hyperbolic orbits each tend to the corresponding type of radial trajectory while e tends to 1. For a repulsive force only the hyperbolic trajectory, including the radial version, is applicable. For elliptical orbits, a simple proof shows that arcsin yields the projection angle of a perfect circle to an ellipse of eccentricity e. For example, to view the eccentricity of the planet Mercury, one must calculate the inverse sine to find the projection angle of 11.86 degrees. Next, tilt any circular object by that angle and the apparent ellipse projected to your eye will be of that same eccentricity; the word "eccentricity" comes from Medieval Latin eccentricus, derived from Greek ἔκκεντρος ekkentros "out of the center", from ἐκ- ek-, "out of" + κέντρον kentron "center".
"Eccentric" first appeared in English in 1551, with the definition "a circle in which the earth, sun. Etc. deviates from its center". By five years in 1556, an adjectival form of the word had developed; the eccentricity of an orbit can be calculated from the orbital state vectors as the magnitude of the eccentricity vector: e = | e | where: e is the eccentricity vector. For elliptical orbits it can be calculated from the periapsis and apoapsis since rp = a and ra = a, where a is the semimajor axis. E = r a − r p r a + r p = 1 − 2 r a r p + 1 where: ra is the radius at apoapsis. Rp is the radius at periapsis; the eccentricity of an elliptical orbit can be used to obtain the ratio of the periapsis to the apoapsis: r p r a = 1 − e 1 + e For Earth, orbital eccentricity ≈ 0.0167, apoapsis= aphelion and periapsis= perihelion relative to sun. For Earth's annual orbit path, ra/rp ratio = longest_radius / shortest_radius ≈ 1.034 relative to center point of path. The eccentricity of the Earth's orbit is about 0.0167.
Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times. Earth's axis of rotation is tilted with respect to its orbital plane; the gravitational interaction between Earth and the Moon causes ocean tides, stabilizes Earth's orientation on its axis, slows its rotation. Earth is the largest of the four terrestrial planets. Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is covered with water by oceans; the remaining 29% is land consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere.
The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, a convecting mantle that drives plate tectonics. Within the first billion years of Earth's history, life appeared in the oceans and began to affect the Earth's atmosphere and surface, leading to the proliferation of aerobic and anaerobic organisms; some geological evidence indicates. Since the combination of Earth's distance from the Sun, physical properties, geological history have allowed life to evolve and thrive. In the history of the Earth, biodiversity has gone through long periods of expansion punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely. Over 7.6 billion humans live on Earth and depend on its biosphere and natural resources for their survival.
Humans have developed diverse cultures. The modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most spelled eorðe, it has cognates in every Germanic language, their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was being used to translate the many senses of Latin terra and Greek γῆ: the ground, its soil, dry land, the human world, the surface of the world, the globe itself; as with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus, Norse mythology included Jörð, a giantess given as the mother of Thor. Earth was written in lowercase, from early Middle English, its definite sense as "the globe" was expressed as the earth. By Early Modern English, many nouns were capitalized, the earth became the Earth when referenced along with other heavenly bodies. More the name is sometimes given as Earth, by analogy with the names of the other planets.
House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name but writes it in lowercase when preceded by the, it always appears in lowercase in colloquial expressions such as "what on earth are you doing?" The oldest material found in the Solar System is dated to 4.5672±0.0006 billion years ago. By 4.54±0.04 Bya the primordial Earth had formed. The bodies in the Solar System evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, dust. According to nebular theory, planetesimals formed by accretion, with the primordial Earth taking 10–20 million years to form. A subject of research is the formation of some 4.53 Bya. A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, hit Earth.
In this view, the mass of Theia was 10 percent of Earth, it hit Earth with a glancing blow and some of its mass merged with Earth. Between 4.1 and 3.8 Bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth. Earth's atmosphere and oceans were formed by volcanic outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. By 3.5 Bya, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind. A crust formed; the two models that explain land mass propose either a steady growth to the present-day forms or, more a rapid growth early in Earth history followed by a long-term steady continental area. Continents formed by plate tectonics
SIMBAD is an astronomical database of objects beyond the Solar System. It is maintained by the Centre de données astronomiques de France. SIMBAD was created by merging the Catalog of Stellar Identifications and the Bibliographic Star Index as they existed at the Meudon Computer Centre until 1979, expanded by additional source data from other catalogues and the academic literature; the first on-line interactive version, known as Version 2, was made available in 1981. Version 3, developed in the C language and running on UNIX stations at the Strasbourg Observatory, was released in 1990. Fall of 2006 saw the release of Version 4 of the database, now stored in PostgreSQL, the supporting software, now written in Java; as of 10 February 2017, SIMBAD contains information for 9,099,070 objects under 24,529,080 different names, with 327,634 bibliographical references and 15,511,733 bibliographic citations. The minor planet 4692 SIMBAD was named in its honour. Planetary Data System – NASA's database of information on SSSB, maintained by JPL and Caltech.
NASA/IPAC Extragalactic Database – a database of information on objects outside the Milky Way maintained by JPL. NASA Exoplanet Archive – an online astronomical exoplanet catalog and data service Bibcode SIMBAD, Strasbourg SIMBAD, Harvard
The radial velocity of an object with respect to a given point is the rate of change of the distance between the object and the point. That is, the radial velocity is the component of the object's velocity that points in the direction of the radius connecting the object and the point. In astronomy, the point is taken to be the observer on Earth, so the radial velocity denotes the speed with which the object moves away from or approaches the Earth. In astronomy, radial velocity is measured to the first order of approximation by Doppler spectroscopy; the quantity obtained by this method may be called the barycentric radial-velocity measure or spectroscopic radial velocity. However, due to relativistic and cosmological effects over the great distances that light travels to reach the observer from an astronomical object, this measure cannot be transformed to a geometric radial velocity without additional assumptions about the object and the space between it and the observer. By contrast, astrometric radial velocity is determined by astrometric observations.
Light from an object with a substantial relative radial velocity at emission will be subject to the Doppler effect, so the frequency of the light decreases for objects that were receding and increases for objects that were approaching. The radial velocity of a star or other luminous distant objects can be measured by taking a high-resolution spectrum and comparing the measured wavelengths of known spectral lines to wavelengths from laboratory measurements. A positive radial velocity indicates the distance between the objects was increasing. In many binary stars, the orbital motion causes radial velocity variations of several kilometers per second; as the spectra of these stars vary due to the Doppler effect, they are called spectroscopic binaries. Radial velocity can be used to estimate the ratio of the masses of the stars, some orbital elements, such as eccentricity and semimajor axis; the same method has been used to detect planets around stars, in the way that the movement's measurement determines the planet's orbital period, while the resulting radial-velocity amplitude allows the calculation of the lower bound on a planet's mass using the binary mass function.
Radial velocity methods alone may only reveal a lower bound, since a large planet orbiting at a high angle to the line of sight will perturb its star radially as much as a much smaller planet with an orbital plane on the line of sight. It has been suggested that planets with high eccentricities calculated by this method may in fact be two-planet systems of circular or near-circular resonant orbit; the radial velocity method to detect exoplanets is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an exoplanet as it orbits the star. When the star moves towards us, its spectrum is blueshifted, while it is redshifted when it moves away from us. By looking at the spectrum of a star—and so, measuring its velocity—it can be determined if it moves periodically due to the influence of an exoplanet companion. From the instrumental perspective, velocities are measured relative to the telescope's motion. So an important first step of the data reduction is to remove the contributions of the Earth's elliptic motion around the sun at ± 30 km/s, a monthly rotation of ± 13 m/s of the Earth around the center of gravity of the Earth-Moon system, the daily rotation of the telescope with the Earth crust around the Earth axis, up to ±460 m/s at the equator and proportional to the cosine of the telescope's geographic latitude, small contributions from the Earth polar motion at the level of mm/s, contributions of 230 km/s from the motion around the Galactic center and associated proper motions.
In the case of spectroscopic measurements corrections of the order of ±20 cm/s with respect to aberration. Proper motion Peculiar velocity Relative velocity Space velocity The Radial Velocity Equation in the Search for Exoplanets
A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is called a star cluster or galaxy, broadly speaking, they are star systems. Star systems are not to be confused with planetary systems, which include planets and similar bodies A star system of two stars is known as a binary star, binary star system or physical double star. If there are no tidal effects, no perturbation from other forces, no transfer of mass from one star to the other, such a system is stable, both stars will trace out an elliptic orbit around the barycenter of the system indefinitely.. Examples of binary systems are Sirius and Cygnus X-1, the last of which consists of a star and a black hole. A multiple star system consists of three or more stars that appear from Earth to be close to one another in the sky; this may result from the stars being physically close and gravitationally bound to each other, in which case it is a physical multiple star, or this closeness may be apparent, in which case it is an optical multiple star.
Physical multiple stars are commonly called multiple stars or multiple star systems. Most multiple star systems are triple stars. Systems with four or more components are less to occur. Multiple-star systems are called trinary or ternary if they contain three stars; these systems are smaller than open star clusters, which have more complex dynamics and have from 100 to 1,000 stars. Most multiple star systems known are triple. For example, in the 1999 revision of Tokovinin's catalog of physical multiple stars, 551 out of the 728 systems described are triple. However, because of selection effects, knowledge of these statistics is incomplete. Multiple-star systems can be divided into two main dynamical classes: hierarchical systems which are stable and consist of nested orbits that don't interact much and so each level of the hierarchy can be treated as a Two-body problem, or the trapezia which have unstable interacting orbits and are modelled as an n-body problem, exhibiting chaotic behavior. Most multiple-star systems are organized in what is called a hierarchical system: the stars in the system can be divided into two smaller groups, each of which traverses a larger orbit around the system's center of mass.
Each of these smaller groups must be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, so on. Each level of the hierarchy can be treated as a two-body problem by considering close pairs as if they were a single star. In these systems there is little interaction between the orbits and the stars' motion will continue to approximate stable Keplerian orbits around the system's center of mass, unlike the unstable trapezia systems or the more complex dynamics of the large number of stars in star clusters and galaxies. In a physical triple star system, each star orbits the center of mass of the system. Two of the stars form a close binary system, the third orbits this pair at a distance much larger than that of the binary orbit; this arrangement is called hierarchical. The reason for this is that if the inner and outer orbits are comparable in size, the system may become dynamically unstable, leading to a star being ejected from the system.
Triple stars that are not all gravitationally bound might comprise a physical binary and an optical companion, such as Beta Cephei, or a purely optical triple star, such as Gamma Serpentis. Hierarchical multiple star systems with more than three stars can produce a number of more complicated arrangements, which can be illustrated by what Evans has called a mobile diagram; these are similar to ornamental mobiles hung from the ceiling. Some examples can be seen in the figure to the right; each level of the diagram illustrates the decomposition of the system into two or more systems with smaller size. Evans calls a diagram multiplex if there is a node with more than two children, i.e. if the decomposition of some subsystem involves two or more orbits with comparable size. Because, as we have seen for triple stars, this may be unstable, multiple stars are expected to be simplex, meaning that at each level there are two children. Evans calls the number of levels in the diagram its hierarchy. A simplex diagram of hierarchy 1, as in, describes a binary system.
A simplex diagram of hierarchy 2 may describe a quadruple system, as in. A simplex diagram of hierarchy 3 may describe a system with anywhere from four to eight components; the mobile diagram in shows an example of a quadruple system with hierarchy 3, consisting of a single distant component orbiting a close binary system, with one of the components of the close binary being an closer binary. A real example of a system with hierarchy 3 is Castor known as Alpha Geminorum or α Gem, it consists of what appears to be a visual binary star which, upon closer inspection, can be seen to consist of two spectroscopic binary stars. By itself, this would be a quadruple hierarchy 2 system as in, but it is orbited b
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