The Big Dipper or the Plough is a large asterism consisting of seven bright stars of the constellation Ursa Major. Four define a "bowl" or "body" and three define a "handle" or "head", it is recognized as a distinct grouping in many cultures. The North Star, the current northern pole star and the tip of the handle of the Little Dipper, can be located by extending an imaginary line through the front two stars of the asterism and Dubhe; this makes it useful in celestial navigation. The constellation of Ursa Major has been seen as a wagon, or a ladle; the "bear" tradition is Greek, but the name "bear" has parallels in Siberian or North American traditions. The name "Bear" is Homeric, native to Greece, while the "Wain" tradition is Mesopotamian. Book XVIII of Homer's Iliad mentions it as "the Bear, which men call the Wain". In Latin, these seven stars were known as the "Seven Oxen"; the classical mythographer identified the "Bear" as the nymph Callisto, changed into a she-bear by Hera, the jealous wife of Zeus.
In Ireland and the United Kingdom, this pattern is known as the Plough. The symbol of the Starry Plough has been used as a political symbol by Irish Republican and left wing movements. Former names include Butcher's Cleaver; the terms Charles's Wain and Charles his Wain are derived from the still older Carlswæn. A folk etymology holds that this derived from Charlemagne, but the name is common to all the Germanic languages and intended the churls' wagon, in contrast with the women's wagon. An older "Odin's Wain" may have preceded these Nordic designations. In German, it is known as the "Great Wagon" and, less the "Great Bear". In Scandinavia, it is known by variations of "Charles's Wagon", but the "Great Bear". In Dutch, its official name is the "Great Bear", but it is popularly known as the "Saucepan". In Italian, too, it is called the "Great Wagon". In Romanian and most Slavic languages, it is known as the "Great Wagon" as well. In Hungarian, it is called "Göncöl's Wagon" or, less "Big Göncöl" after a táltos in Hungarian mythology who carried medicine that could cure any disease.
In Finnish, the figure is known as Otava with established etymology in the archaic meaning'salmon net', although other uses of the word refer to'bear' and'wheel'. The bear relation is claimed to stem from the animal's resemblance to—and mythical origin from—the asterism rather than vice versa. In the Lithuanian language, the stars of Ursa Major are known as Didieji Grįžulo Ratai. Other names for the constellation include Perkūno Ratai, Kaušas, Vežimas, Samtis. In traditional Chinese astronomy, which continues to be used throughout East Asia, these stars are considered to compose the Right Wall of the Purple Forbidden Enclosure which surrounds the Northern Celestial Pole, although numerous other groupings and names have been made over the centuries; each star has a distinct name, which has varied over time and depending upon the asterism being constructed. The Western asterism is now known as the "Northern Dipper" or the "Seven Stars of the Northern Dipper"; the personification of the Big Dipper itself is known as "Doumu" in Chinese folk religion and Taoism, Marici in Buddhism.
In Shinto, the seven largest stars of Ursa Major belong to Amenominakanushi, the oldest and most powerful of all kami. In North Korea, the constellation is featured on the flag of the country's special forces. In South Korea, the constellation is referred to as "the seven stars of the north". In the related myth, a widow with seven sons found comfort with a widower, but to get to his house required crossing a stream; the seven sons, sympathetic to their mother, placed stepping stones in the river. Their mother, not knowing who put the stones in place, blessed them and, when they died, they became the constellation. In Malay, it is known as the "Boat Constellation". In Burmese, these stars are known as Pucwan Tārā. Pucwan is a general term for a crustacean, such as prawn, crab, etc. In Javanese, as known as "Bintang Kartika"; this name comes from Sanskrit. In ancient Javanese this brightest seven stars are known as Lintang Wuluh means "seven stars"; this star cluster is so popular because its emergence into the sky signals the time marker for planting.
In Hindu astronomy, it is referred to as the "Collection of Seven Great Sages", as each star is named after a mythical Hindu sage. An Arabian story has the four stars of the Plough's bowl as a coffin, with the three stars in the handle as mourners, following it. In Mongolian, it is known as the "Seven Gods". In Kazakh, they are known as the Jetiqaraqshi and, in Kyrgyz, as the Jetigen. While its Western origins come from its resemblance to the kitchen utensil, In Filipino, the Big Dipper and its sister constellation Little Dipper are more associated with the tabo, a hygiene tool akin to a bucket with a handl
In astronomy, a circumpolar constellation is a constellation that never sets below the horizon, as viewed from a location on Earth. Due to Earth's rotation and axial tilt with respect to the Sun, the stars and constellations can be divided into two categories; those stars and constellations that never rise or set are called circumpolar. The rest are divided into seasonal constellations; the stars and constellations that are circumpolar depends on the observer's latitude. In the Northern Hemisphere, certain stars and constellations will always be visible in the northern circumpolar sky; the same holds true in the Southern Hemisphere, where certain stars and constellations will always be visible in the southern circumpolar sky. The celestial north pole marked by Polaris less than 1° away, always has an azimuth equal to zero; the pole's altitude for a given latitude Ø is fixed, its value is given by the following formula: A = 90° - Ø. All stars with a declination less than A are not circumpolar; as viewed from the North Pole, all visible constellations north of the celestial equator are circumpolar, for constellations south of the celestial equator as viewed from the South Pole.
As viewed from the Equator, no circumpolar constellations are visible. As viewed from mid-northern latitudes, circumpolar constellations may include Ursa Major, Ursa Minor, Cepheus and the less-known Camelopardalis
Proper motion is the astronomical measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more distant stars. The components for proper motion in the equatorial coordinate system are given in the direction of right ascension and of declination, their combined value is computed as the total proper motion. It has dimensions of angle per time arcseconds per year or milliarcseconds per year. Knowledge of the proper motion and radial velocity allows calculations of true stellar motion or velocity in space in respect to the Sun, by coordinate transformation, the motion in respect to the Milky Way. Proper motion is not "proper", because it includes a component due to the motion of the Solar System itself. Over the course of centuries, stars appear to maintain nearly fixed positions with respect to each other, so that they form the same constellations over historical time.
Ursa Major or Crux, for example, looks nearly the same now. However, precise long-term observations show that the constellations change shape, albeit slowly, that each star has an independent motion; this motion is caused by the movement of the stars relative to the Solar System. The Sun travels in a nearly circular orbit about the center of the Milky Way at a speed of about 220 km/s at a radius of 8 kPc from the center, which can be taken as the rate of rotation of the Milky Way itself at this radius; the proper motion is a two-dimensional vector and is thus defined by two quantities: its position angle and its magnitude. The first quantity indicates the direction of the proper motion on the celestial sphere, the second quantity is the motion's magnitude expressed in arcseconds per year or milliarcsecond per year. Proper motion may alternatively be defined by the angular changes per year in the star's right ascension and declination, using a constant epoch in defining these; the components of proper motion by convention are arrived at.
Suppose an object moves from coordinates to coordinates in a time Δt. The proper motions are given by: μ α = α 2 − α 1 Δ t, μ δ = δ 2 − δ 1 Δ t; the magnitude of the proper motion μ is given by the Pythagorean theorem: μ 2 = μ δ 2 + μ α 2 ⋅ cos 2 δ, μ 2 = μ δ 2 + μ α ∗ 2, where δ is the declination. The factor in cos2δ accounts for the fact that the radius from the axis of the sphere to its surface varies as cosδ, for example, zero at the pole. Thus, the component of velocity parallel to the equator corresponding to a given angular change in α is smaller the further north the object's location; the change μα, which must be multiplied by cosδ to become a component of the proper motion, is sometimes called the "proper motion in right ascension", μδ the "proper motion in declination". If the proper motion in right ascension has been converted by cosδ, the result is designated μα*. For example, the proper motion results in right ascension in the Hipparcos Catalogue have been converted. Hence, the individual proper motions in right ascension and declination are made equivalent for straightforward calculations of various other stellar motions.
The position angle θ is related to these components by: μ sin θ = μ α cos δ = μ α ∗, μ cos θ = μ δ. Motions in equatorial coordinates can be converted to motions in galactic coordinates. For the majority of stars seen in the sky, the observed proper motions are small and unremarkable; such stars are either faint or are distant, have changes of below 10 milliarcseconds per year, do not appear to move appreciably over many millennia. A few do have significant motions, are called high-proper motion stars. Motions can be in seemingly random directions. Two or more stars, double stars or open star clusters, which are moving in similar directions, exhibit so-called shared or common proper motion, suggesting they may be gravitationally attached or share similar motion in space. Barnard's Star has the largest proper motion of all stars, moving at 10.3 seconds of arc per year. L
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
Stellar parallax is the apparent shift of position of any nearby star against the background of distant objects. Created by the different orbital positions of Earth, the small observed shift is largest at time intervals of about six months, when Earth arrives at opposite sides of the Sun in its orbit, giving a baseline distance of about two astronomical units between observations; the parallax itself is considered to be half of this maximum, about equivalent to the observational shift that would occur due to the different positions of Earth and the Sun, a baseline of one astronomical unit. Stellar parallax is so difficult to detect that its existence was the subject of much debate in astronomy for hundreds of years, it was first observed in 1806 by Giuseppe Calandrelli who reported parallax in α-Lyrae in his work "Osservazione e riflessione sulla parallasse annua dall’alfa della Lira". In 1838 Friedrich Bessel made the first successful parallax measurement, for the star 61 Cygni, using a Fraunhofer heliometer at Königsberg Observatory.
Once a star's parallax is known, its distance from Earth can be computed trigonometrically. But the more distant an object is, the smaller its parallax. With 21st-century techniques in astrometry, the limits of accurate measurement make distances farther away than about 100 parsecs too approximate to be useful when obtained by this technique; this limits the applicability of parallax as a measurement of distance to objects that are close on a galactic scale. Other techniques, such as spectral red-shift, are required to measure the distance of more remote objects. Stellar parallax measures are given in the tiny units of arcseconds, or in thousandths of arcseconds; the distance unit parsec is defined as the length of the leg of a right triangle adjacent to the angle of one arcsecond at one vertex, where the other leg is 1 AU long. Because stellar parallaxes and distances all involve such skinny right triangles, a convenient trigonometric approximation can be used to convert parallaxes to distance.
The approximate distance is the reciprocal of the parallax: d ≃ 1 / p. For example, Proxima Centauri, whose parallax is 0.7687, is 1 / 0.7687 parsecs = 1.3009 parsecs distant. Stellar parallax is so small that its apparent absence was used as a scientific argument against heliocentrism during the early modern age, it is clear from Euclid's geometry that the effect would be undetectable if the stars were far enough away, but for various reasons such gigantic distances involved seemed implausible: it was one of Tycho Brahe's principal objections to Copernican heliocentrism that in order for it to be compatible with the lack of observable stellar parallax, there would have to be an enormous and unlikely void between the orbit of Saturn and the eighth sphere. James Bradley first tried to measure stellar parallaxes in 1729; the stellar movement proved too insignificant for his telescope, but he instead discovered the aberration of light and the nutation of Earth's axis, catalogued 3222 stars. Stellar parallax is most measured using annual parallax, defined as the difference in position of a star as seen from Earth and Sun, i.e. the angle subtended at a star by the mean radius of Earth's orbit around the Sun.
The parsec is defined as the distance. Annual parallax is measured by observing the position of a star at different times of the year as Earth moves through its orbit. Measurement of annual parallax was the first reliable way to determine the distances to the closest stars; the first successful measurements of stellar parallax were made by Friedrich Bessel in 1838 for the star 61 Cygni using a heliometer. Being difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century by use of the filar micrometer. Astrographs using astronomical photographic plates sped the process in the early 20th century. Automated plate-measuring machines and more sophisticated computer technology of the 1960s allowed more efficient compilation of star catalogues. In the 1980s, charge-coupled devices replaced photographic plates and reduced optical uncertainties to one milliarcsecond. Stellar parallax remains the standard for calibrating other measurement methods. Accurate calculations of distance based on stellar parallax require a measurement of the distance from Earth to the Sun, now known to exquisite accuracy based on radar reflection off the surfaces of planets.
The angles involved in these calculations are small and thus difficult to measure. The nearest star to the Sun, Proxima Centauri, has a parallax of 0.7687 ± 0.0003 arcsec. This angle is that subtended by an object 2 centimeters in diameter located 5.3 kilometers away. In 1989 the satellite Hipparcos was launched for obtaining parallaxes and proper motions of nearby stars, increasing the number of stellar parallaxes measured to milliarcsecond accuracy a thousandfold. So, Hipparcos is only able to measure parallax angles for stars up to about 1,600 light-years away, a little more than one percent of the diameter of the Milky Way Galaxy; the Hubble telescope WFC3 now has a precision of 20 to 40 microarcseconds, enabling reliable distance measurements u
A debris disk is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks have been found around both mature and young stars, as well as at least one debris disk in orbit around an evolved neutron star. Younger debris disks can constitute a phase in the formation of a planetary system following the protoplanetary disk phase, when terrestrial planets may finish growing, they can be produced and maintained as the remnants of collisions between planetesimals, otherwise known as asteroids and comets. By 2001, over 900 candidate stars had been found to possess a debris disk, they are discovered by examining the star system in infrared light and looking for an excess of radiation beyond that emitted by the star. This excess is inferred to be radiation from the star, absorbed by the dust in the disk re-radiated away as infrared energy. Debris disks are described as massive analogs to the debris in the Solar System.
Most known debris disks have radii of 10–100 astronomical units. Some debris disks contain a component of warmer dust located within 10 AU from the central star; this dust is sometimes called exozodiacal dust by analogy to zodiacal dust in the Solar System. In 1984 a debris disk was detected around the star Vega using the IRAS satellite; this was believed to be a protoplanetary disk, but it is now known to be a debris disk due to the lack of gas in the disk and the age of the star. The first four debris disks discovered with IRAS are known as the "fabulous four": Vega, Beta Pictoris and Epsilon Eridani. Subsequently, direct images of the Beta Pictoris disk showed irregularities in the dust, which were attributed to gravitational perturbations by an unseen exoplanet; that explanation was confirmed with the 2008 discovery of the exoplanet Beta Pictoris b. Other exoplanet-hosting stars, including the first discovered by direct imaging, are known to host debris disks; the nearby star 55 Cancri, a system, known to contain five planets, was reported to have a debris disk, but that detection could not be confirmed.
Structures in the debris disk around Epsilon Eridani suggest perturbations by a planetary body in orbit around that star, which may be used to constrain the mass and orbit of the planet. On 24 April 2014, NASA reported detecting debris disks in archival images of several young stars, HD 141943 and HD 191089, first viewed between 1999 and 2006 with the Hubble Space Telescope, by using newly improved imaging processes. During the formation of a Sun-like star, the object passes through the T-Tauri phase during which it is surrounded by a gas-rich, disk-shaped nebula. Out of this material are formed planetesimals, which can continue accreting other planetesimals and disk material to form planets; the nebula continues to orbit the pre-main-sequence star for a period of 1–20 million years until it is cleared out by radiation pressure and other processes. Second generation dust may be generated about the star by collisions between the planetesimals, which forms a disk out of the resulting debris. At some point during their lifetime, at least 45% of these stars are surrounded by a debris disk, which can be detected by the thermal emission of the dust using an infrared telescope.
Repeated collisions can cause a disk to persist for much of the lifetime of a star. Typical debris disks contain small grains 1–100 μm in size. Collisions will grind down these grains to sub-micrometre sizes, which will be removed from the system by radiation pressure from the host star. In tenuous disks like the ones in the Solar System, the Poynting–Robertson effect can cause particles to spiral inward instead. Both processes limit the lifetime of the disk to 10 Myr or less. Thus, for a disk to remain intact, a process is needed to continually replenish the disk; this can occur, for example, by means of collisions between larger bodies, followed by a cascade that grinds down the objects to the observed small grains. For collisions to occur in a debris disk, the bodies must be gravitationally perturbed sufficiently to create large collisional velocities. A planetary system around the star can cause such perturbations, as can a binary star companion or the close approach of another star; the presence of a debris disk may indicate a high likelihood of exoplanets orbiting the star.
Furthermore, many debris disks show structures within the dust that point to the presence of one or more exoplanets within the disk. Belts of dust or debris have been detected around many stars, including the Sun, including the following: The orbital distance of the belt is an estimated mean distance or range, based either on direct measurement from imaging or derived from the temperature of the belt; the Earth has an average distance from the Sun of 1 AU. Accretion disc Asteroid belt Protoplanetary disk McCabe, Caer. "Catalog of Resolved Circumstellar Disks". NASA JPL. Retrieved 2019-03-08
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is used as an estimate of a body's surface temperature when the body's emissivity curve is not known; when the star's or planet's net emissivity in the relevant wavelength band is less than unity, the actual temperature of the body will be higher than the effective temperature. The net emissivity may be low due to surface or atmospheric properties, including greenhouse effect; the effective temperature of a star is the temperature of a black body with the same luminosity per surface area as the star and is defined according to the Stefan–Boltzmann law FBol = σTeff4. Notice that the total luminosity of a star is L = 4πR2σTeff4, where R is the stellar radius; the definition of the stellar radius is not straightforward. More rigorously the effective temperature corresponds to the temperature at the radius, defined by a certain value of the Rosseland optical depth within the stellar atmosphere.
The effective temperature and the bolometric luminosity are the two fundamental physical parameters needed to place a star on the Hertzsprung–Russell diagram. Both effective temperature and bolometric luminosity depend on the chemical composition of a star; the effective temperature of our Sun is around 5780 kelvins. Stars have a decreasing temperature gradient; the "core temperature" of the Sun—the temperature at the centre of the Sun where nuclear reactions take place—is estimated to be 15,000,000 K. The color index of a star indicates its temperature from the cool—by stellar standards—red M stars that radiate in the infrared to the hot blue O stars that radiate in the ultraviolet; the effective temperature of a star indicates the amount of heat that the star radiates per unit of surface area. From the warmest surfaces to the coolest is the sequence of stellar classifications known as O, B, A, F, G, K, M. A red star could be a tiny red dwarf, a star of feeble energy production and a small surface or a bloated giant or supergiant star such as Antares or Betelgeuse, either of which generates far greater energy but passes it through a surface so large that the star radiates little per unit of surface area.
A star near the middle of the spectrum, such as the modest Sun or the giant Capella radiates more energy per unit of surface area than the feeble red dwarf stars or the bloated supergiants, but much less than such a white or blue star as Vega or Rigel. To find the effective temperature of a planet, it can be calculated by equating the power received by the planet to the known power emitted by a blackbody of temperature T. Take the case of a planet at a distance D from the star, of luminosity L. Assuming the star radiates isotropically and that the planet is a long way from the star, the power absorbed by the planet is given by treating the planet as a disc of radius r, which intercepts some of the power, spread over the surface of a sphere of radius D; the calculation assumes the planet reflects some of the incoming radiation by incorporating a parameter called the albedo. An albedo of 1 means that all the radiation is reflected, an albedo of 0 means all of it is absorbed; the expression for absorbed power is then: P a b s = L r 2 4 D 2 The next assumption we can make is that the entire planet is at the same temperature T, that the planet radiates as a blackbody.
The Stefan–Boltzmann law gives an expression for the power radiated by the planet: P r a d = 4 π r 2 σ T 4 Equating these two expressions and rearranging gives an expression for the effective temperature: T = L 16 π σ D 2 4 Note that the planet's radius has cancelled out of the final expression. The effective temperature for Jupiter from this calculation is 88 K and 51 Pegasi b is 1,258 K. A better estimate of effective temperature for some planets, such as Jupiter, would need to include the internal heating as a power input; the actual temperature depends on atmosphere effects. The actual temperature from spectroscopic analysis for HD 209458 b is 1,130 K, but the effective temperature is 1,359 K; the internal heating within Jupiter raises the effective temperature to about 152 K. The surface temperature of a planet can be estimated by modifying the effective-temperature calculation to account for emissivity and temperature variation; the area of the planet that absorbs the power from the star is Aabs, some fraction of the total surface area Atotal = 4πr2, where r is the radius of the planet.
This area intercepts some of the power, spread over the surface of a sphere of radius D. We allow the planet to reflect some of the incoming radiation by incorporating a parameter a called the albedo. An albedo of 1 means that all the radiation is reflected, an albedo