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43-429: The Castor Moving Group (CMG), or Castor stream, is a moving group of stars in the solar vicinity sharing similar velocities and directions. The stars that have been identified as part of the group include Castor , Fomalhaut , Vega , α Cephei and α Librae . While the reported group members are young (typically with ages of hundreds of millions of years), recent results suggest that the velocity differences amongst

86-469: A u ≈ 206 264.81   a u . {\displaystyle {\begin{aligned}\mathrm {SD} &={\frac {\mathrm {ES} }{\tan 1''}}\\&={\frac {\mathrm {ES} }{\tan \left({\frac {1}{60\times 60}}\times {\frac {\pi }{180}}\right)}}\\&\approx {\frac {1\,\mathrm {au} }{{\frac {1}{60\times 60}}\times {\frac {\pi }{180}}}}={\frac {648\,000}{\pi }}\,\mathrm {au} \approx 206\,264.81~\mathrm {au} .\end{aligned}}} Because

129-793: A u = 180 × 60 × 60 × 149 597 870 700   m = 96 939 420 213 600 000   m {\displaystyle \pi ~\mathrm {pc} =180\times 60\times 60~\mathrm {au} =180\times 60\times 60\times 149\,597\,870\,700~\mathrm {m} =96\,939\,420\,213\,600\,000~\mathrm {m} } (exact by the 2015 definition) Therefore, 1   p c = 96 939 420 213 600 000 π   m = 30 856 775 814 913 673   m {\displaystyle 1~\mathrm {pc} ={\frac {96\,939\,420\,213\,600\,000}{\pi }}~\mathrm {m} =30\,856\,775\,814\,913\,673~\mathrm {m} } (to

172-663: A degree ). The nearest star, Proxima Centauri , is about 1.3 parsecs (4.2 light-years) from the Sun : from that distance, the gap between the Earth and the Sun spans slightly less than ⁠ 1 / 3600 ⁠ of one degree of view. Most stars visible to the naked eye are within a few hundred parsecs of the Sun, with the most distant at a few thousand parsecs, and the Andromeda Galaxy at over 700,000 parsecs. The word parsec

215-489: A bit amongst studies. There is controversy over whether the Castor Moving Group constitutes a physical group of stars of shared origin (e.g. an "association") or a group of stars of heterogeneous age and chemical composition that happen to have somewhat similar velocities (e.g. a "dynamical stream"). In their 2013 paper describing the discovery of Fomalhaut C, Mamajek et al. point out that the 3D velocities of

258-424: A common origin. Stellar association Stellar associations were discovered by Victor Ambartsumian in 1947. The conventional name for an association uses the names or abbreviations of the constellation (or constellations) in which they are located; the association type, and, sometimes, a numerical identifier. Victor Ambartsumian first categorized stellar associations into two groups, OB and T, based on

301-447: A distance d using the formula d ≈ ⁠ c / H ⁠ × z . One gigaparsec (Gpc) is one billion parsecs — one of the largest units of length commonly used. One gigaparsec is about 3.26 billion ly, or roughly ⁠ 1 / 14 ⁠ of the distance to the horizon of the observable universe (dictated by the cosmic microwave background radiation ). Astronomers typically use gigaparsecs to express

344-458: A name suggested by Sidney van den Bergh after he discovered that the stars in these nebulae had a non-uniform distribution. These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed. This allows the properties of the surrounding dark cloud to be examined by astronomers. Because R-associations are more plentiful than OB associations, they can be used to trace out

387-466: A number of infant T Tauri stars that are still in the process of entering the main sequence . These sparse populations of up to a thousand T Tauri stars are known as T associations . The nearest example is the Taurus-Auriga T association (Tau-Aur T association), located at a distance of 140 parsecs from the Sun. Other examples of T associations include the R Corona Australis T association ,

430-472: Is a portmanteau of "parallax of one second" and was coined by the British astronomer Herbert Hall Turner in 1913 to simplify astronomers' calculations of astronomical distances from only raw observational data. Partly for this reason, it is the unit preferred in astronomy and astrophysics , though the light-year remains prominent in popular science texts and common usage. Although parsecs are used for

473-514: Is a unit of length used to measure the large distances to astronomical objects outside the Solar System , approximately equal to 3.26 light-years or 206,265 astronomical units (AU), i.e. 30.9  trillion kilometres (19.2 trillion miles ). The parsec unit is obtained by the use of parallax and trigonometry , and is defined as the distance at which 1 AU subtends an angle of one arcsecond ( ⁠ 1 / 3600 ⁠ of

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516-419: Is determined in a similar fashion. To determine the number of galaxies in superclusters , volumes in cubic megaparsecs (Mpc ) are selected. All the galaxies in these volumes are classified and tallied. The total number of galaxies can then be determined statistically. The huge Boötes void is measured in cubic megaparsecs. In physical cosmology , volumes of cubic gigaparsecs (Gpc ) are selected to determine

559-506: Is intended to measure one billion stellar distances to within 20 microarcsecond s, producing errors of 10% in measurements as far as the Galactic Centre , about 8000 pc away in the constellation of Sagittarius . Distances expressed in fractions of a parsec usually involve objects within a single star system. So, for example: Distances expressed in parsecs (pc) include distances between nearby stars, such as those in

602-576: Is one million parsecs, or about 3,260,000 light years. Sometimes, galactic distances are given in units of Mpc/ h (as in "50/ h  Mpc", also written " 50 Mpc h "). h is a constant (the " dimensionless Hubble constant ") in the range 0.5 < h < 0.75 reflecting the uncertainty in the value of the Hubble constant H for the rate of expansion of the universe: h = ⁠ H / 100 (km/s)/Mpc ⁠ . The Hubble constant becomes relevant when converting an observed redshift z into

645-435: Is the fundamental calibration step for distance determination in astrophysics ; however, the accuracy of ground-based telescope measurements of parallax angle is limited to about 0.01″ , and thus to stars no more than 100 pc distant. This is because the Earth's atmosphere limits the sharpness of a star's image. Space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond

688-415: Is very low (roughly 0.4% of the mass density of the local Galactic disk ), hence the systems are not bound to one another, and the high velocity dispersion can not be attributed to the dynamical interactions amongst the moving group members themselves. They conclude that at least those famous "members" of the Castor Moving Group ( Fomalhaut , Vega , Castor , LP 944-20 ) are extremely unlikely to have shared

731-680: The Lupus T association , the Chamaeleon T association and the Velorum T association . T associations are often found in the vicinity of the molecular cloud from which they formed. Some, but not all, include O-B class stars. To summarize the characteristics of Moving groups members: they have the same age and origin, the same chemical composition and they have the same amplitude and direction in their vector of velocity. Associations of stars that illuminate reflection nebulae are called R associations ,

774-625: The Sun at about 5 billion years.) The Hipparcos satellite provided measurements that located a dozen OB associations within 650 parsecs of the Sun. The nearest OB association is the Scorpius–Centaurus association , located about 400 light-years from the Sun . OB associations have also been found in the Large Magellanic Cloud and the Andromeda Galaxy . These associations can be quite sparse, spanning 1,500 light-years in diameter. Young stellar groups can contain

817-413: The 3.5-parsec distance of 61 Cygni . The parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the subtended angle, from that star's perspective, of the semimajor axis of the Earth's orbit. Substituting the star's parallax for the one arcsecond angle in the imaginary right triangle,

860-536: The Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the parallax angle, which is formed by lines from the Sun and Earth to the star at the distant vertex . Then the distance to the star could be calculated using trigonometry. The first successful published direct measurements of an object at interstellar distances were undertaken by German astronomer Friedrich Wilhelm Bessel in 1838, who used this approach to calculate

903-507: The Milky Way were formed in OB associations. O class stars are short-lived, and will expire as supernovae after roughly one to fifteen million years, depending on the mass of the star. As a result, OB associations are generally only a few million years in age or less. The O-B stars in the association will have burned all their fuel within 10 million years. (Compare this to the current age of

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946-493: The astronomical unit is defined to be 149 597 870 700   m , the following can be calculated: Therefore, if 1  ly ≈ 9.46 × 10  m, A corollary states that a parsec is also the distance from which a disc that is one au in diameter must be viewed for it to have an angular diameter of one arcsecond (by placing the observer at D and a disc spanning ES ). Mathematically, to calculate distance, given obtained angular measurements from instruments in arcseconds,

989-417: The categorization is not always clear-cut. Young associations will contain 10–100 massive stars of spectral class O and B , and are known as OB associations . These are believed to form within the same small volume inside a giant molecular cloud . Once the surrounding dust and gas is blown away, the remaining stars become unbound and begin to drift apart. It is believed that the majority of all stars in

1032-532: The definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as exactly ⁠ 648 000 / π ⁠  au, or approximately 3.085 677 581 491 3673 × 10  metres (based on the IAU 2012 definition of the astronomical unit). This corresponds to the small-angle definition of the parsec found in many astronomical references. Imagining an elongated right triangle in space, where

1075-509: The distance SD is calculated as follows: S D = E S tan ⁡ 1 ″ = E S tan ⁡ ( 1 60 × 60 × π 180 ) ≈ 1 a u 1 60 × 60 × π 180 = 648 000 π

1118-402: The distribution of matter in the visible universe and to determine the number of galaxies and quasars. The Sun is currently the only star in its cubic parsec, (pc ) but in globular clusters the stellar density could be from 100–1000 pc . The observational volume of gravitational wave interferometers (e.g., LIGO , Virgo ) is stated in terms of cubic megaparsecs (Mpc ) and is essentially

1161-407: The famous Castor Moving Group members Fomalhaut , Vega , Castor , and LP 944-20 are now very well-determined. They find that the star-to-star velocity differences are so large (typically ~5–10 km/s) that the stars were not within 100 pc of each other as recently as 10 Myr ago, let alone at the purported group age of ~200 million years. The local density of purported Castor Moving Group members

1204-473: The formula would be: Distance star = Distance earth-sun tan ⁡ θ 3600 {\displaystyle {\text{Distance}}_{\text{star}}={\frac {{\text{Distance}}_{\text{earth-sun}}}{\tan {\frac {\theta }{3600}}}}} where θ is the measured angle in arcseconds, Distance earth-sun is a constant ( 1 au or 1.5813 × 10  ly). The calculated stellar distance will be in

1247-531: The limit of ground-based observations. Between 1989 and 1993, the Hipparcos satellite, launched by the European Space Agency (ESA), measured parallaxes for about 100 000 stars with an astrometric precision of about 0.97  mas , and obtained accurate measurements for stellar distances of stars up to 1000 pc away. ESA's Gaia satellite , which launched on 19 December 2013,

1290-433: The long leg of the triangle will measure the distance from the Sun to the star. A parsec can be defined as the length of the right triangle side adjacent to the vertex occupied by a star whose parallax angle is one arcsecond. The use of the parsec as a unit of distance follows naturally from Bessel's method, because the distance in parsecs can be computed simply as the reciprocal of the parallax angle in arcseconds (i.e.: if

1333-411: The moving group stars are too large for them to have possibly shared a common origin. The moving group was first proposed by J. P. Anosova, and V. V. Orlov in 1990. Anosova and Orlov originally proposed 15 members. In 1999, Barrado y Navascues presented a membership list of 16 stars, and estimated a group age of 200 million years. The membership of the group has not been well-established and varies quite

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1376-426: The nearest metre ). Approximately, In the diagram above (not to scale), S represents the Sun, and E the Earth at one point in its orbit (such as to form a right angle at S ). Thus the distance ES is one astronomical unit (au). The angle SDE is one arcsecond ( ⁠ 1 / 3600 ⁠ of a degree ) so by definition D is a point in space at a distance of one parsec from the Sun. Through trigonometry,

1419-592: The need of a name for that unit of distance. He proposed the name astron , but mentioned that Carl Charlier had suggested siriometer and Herbert Hall Turner had proposed parsec . It was Turner's proposal that stuck. By the 2015 definition, 1 au of arc length subtends an angle of 1″ at the center of the circle of radius 1 pc . That is, 1 pc = 1 au/tan( 1″ ) ≈ 206,264.8 au by definition. Converting from degree/minute/second units to radians , Therefore, π   p c = 180 × 60 × 60  

1462-399: The oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is taken approximately half a year later, when the Earth is on the opposite side of the Sun. The distance between the two positions of

1505-521: The parallax angle is 1 arcsecond, the object is 1 pc from the Sun; if the parallax angle is 0.5 arcseconds, the object is 2 pc away; etc.). No trigonometric functions are required in this relationship because the very small angles involved mean that the approximate solution of the skinny triangle can be applied. Though it may have been used before, the term parsec was first mentioned in an astronomical publication in 1913. Astronomer Royal Frank Watson Dyson expressed his concern for

1548-415: The properties of their stars. A third category, R, was later suggested by Sidney van den Bergh for associations that illuminate reflection nebulae . The OB, T, and R associations form a continuum of young stellar groupings. But it is currently uncertain whether they are an evolutionary sequence, or represent some other factor at work. Some groups also display properties of both OB and T associations, so

1591-399: The same spiral arm or globular cluster . A distance of 1,000 parsecs (3,262 ly) is denoted by the kiloparsec (kpc). Astronomers typically use kiloparsecs to express distances between parts of a galaxy or within groups of galaxies . So, for example : Astronomers typically express the distances between neighbouring galaxies and galaxy clusters in megaparsecs (Mpc). A megaparsec

1634-525: The same measurement unit as used in Distance earth-sun (e.g. if Distance earth-sun = 1 au , unit for Distance star is in astronomical units; if Distance earth-sun = 1.5813 × 10  ly, unit for Distance star is in light-years). The length of the parsec used in IAU 2015 Resolution B2 (exactly ⁠ 648 000 / π ⁠ astronomical units) corresponds exactly to that derived using

1677-576: The shorter distances within the Milky Way , multiples of parsecs are required for the larger scales in the universe, including kilo parsecs (kpc) for the more distant objects within and around the Milky Way, mega parsecs (Mpc) for mid-distance galaxies, and giga parsecs (Gpc) for many quasars and the most distant galaxies. In August 2015, the International Astronomical Union (IAU) passed Resolution B2 which, as part of

1720-436: The shorter leg measures one au ( astronomical unit , the average Earth – Sun distance) and the subtended angle of the vertex opposite that leg measures one arcsecond ( 1 ⁄ 3600 of a degree), the parsec is defined as the length of the adjacent leg. The value of a parsec can be derived through the rules of trigonometry . The distance from Earth whereupon the radius of its solar orbit subtends one arcsecond. One of

1763-539: The sizes of large-scale structures such as the size of, and distance to, the CfA2 Great Wall ; the distances between galaxy clusters; and the distance to quasars . For example: To determine the number of stars in the Milky Way, volumes in cubic kiloparsecs (kpc ) are selected in various directions. All the stars in these volumes are counted and the total number of stars statistically determined. The number of globular clusters, dust clouds, and interstellar gas

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1806-424: The small-angle calculation. This differs from the classic inverse- tangent definition by about 200 km , i.e.: only after the 11th significant figure . As the astronomical unit was defined by the IAU (2012) as an exact length in metres, so now the parsec corresponds to an exact length in metres. To the nearest meter, the small-angle parsec corresponds to 30 856 775 814 913 673  m . The parallax method

1849-480: The structure of the galactic spiral arms. An example of an R-association is Monoceros R2 , located 830 ± 50 parsecs from the Sun. The Ursa Major Moving Group is one example of a stellar association. (Except for α Ursae Majoris and η Ursae Majoris , all the stars in the Plough/Big Dipper are part of that group.) Other young moving groups include: Parsec The parsec (symbol: pc )

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