Hipparcos was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky. This permitted the accurate determination of proper motions and parallaxes of stars, allowing a determination of their distance and tangential velocity. When combined with radial velocity measurements from spectroscopy, this pinpointed all six quantities needed to determine the motion of stars. The resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, while the enhanced Tycho-2 Catalogue of 2.5 million stars was published in 2000. Hipparcos' follow-up mission, Gaia, was launched in 2013.
The word "Hipparcos" is an acronym for HIgh Precision PARallax COllecting Satellite and also a reference to the ancient Greek astronomer Hipparchus of Nicaea, who is noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.
Hipparcos satellite in the Large Solar Simulator, ESTEC, February 1988
|Mission type||Astrometric observatory|
|Mission duration||4 years, 1 week|
Matra Marconi Space
|Launch mass||1,140 kg (2,510 lb) |
|Dry mass||635 kg (1,400 lb) |
|Payload mass||210 kg (460 lb) |
|Power||295 watts |
|Start of mission|
|Launch date||23:25:53, August 8, 1989 (UTC)|
|Rocket||Ariane 4 44LP (V-33/405)|
|Launch site||Kourou ELA-2|
|End of mission|
|Deactivated||August 15, 1993|
|Regime||Geostationary transfer orbit|
|Semi-major axis||24,519 km (15,235 mi)|
|Perigee||500.3 km (310.9 mi)|
|Apogee||35,797.5 km (22,243.5 mi)|
|Argument of perigee||161.89 degrees|
|Mean anomaly||250.97 degrees|
|Mean motion||2.26 rev/day|
|Epoch||16 June 2015, 13:45:39 UTC|
|Diameter||29 cm (11 in)|
|Focal length||1.4 m (4.6 ft)|
Legacy ESA insignia for the Hipparcos mission
By the second half of the 20th century, the accurate measurement of star positions from the ground was running into essentially insurmountable barriers to improvements in accuracy, especially for large-angle measurements and systematic terms. Problems were dominated by the effects of the Earth's atmosphere, but were compounded by complex optical terms, thermal and gravitational instrument flexures, and the absence of all-sky visibility. A formal proposal to make these exacting observations from space was first put forward in 1967.
Although originally proposed to the French space agency CNES, it was considered too complex and expensive for a single national programme. Its acceptance within the European Space Agency's scientific programme, in 1980, was the result of a lengthy process of study and lobbying. The underlying scientific motivation was to determine the physical properties of the stars through the measurement of their distances and space motions, and thus to place theoretical studies of stellar structure and evolution, and studies of galactic structure and kinematics, on a more secure empirical basis. Observationally, the objective was to provide the positions, parallaxes, and annual proper motions for some 100,000 stars with an unprecedented accuracy of 0.002 arcseconds, a target in practice eventually surpassed by a factor of two. The name of the space telescope, "Hipparcos" was an acronym for High Precision Parallax Collecting Satellite, and it also reflected the name of the ancient Greek astronomer Hipparchus, who is considered the founder of trigonometry and the discoverer of the precession of the equinoxes (due to the Earth wobbling on its axis).
The spacecraft carried a single all-reflective, eccentric Schmidt telescope, with an aperture of 29 cm (11.4 in). A special beam-combining mirror superimposed two fields of view, 58 degrees apart, into the common focal plane. This complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the rectangular entrance pupil, and providing an unvignetted field of view of about 1°×1°. The telescope used a system of grids, at the focal surface, composed of 2688 alternate opaque and transparent bands, with a period of 1.208 arc-sec (8.2 micrometre). Behind this grid system, an image dissector tube (photomultiplier type detector) with a sensitive field of view of about 38-arc-sec diameter converted the modulated light into a sequence of photon counts (with a sampling frequency of 1200 Hz) from which the phase of the entire pulse train from a star could be derived. The apparent angle between two stars in the combined fields of view, modulo the grid period, was obtained from the phase difference of the two star pulse trains. Originally targeting the observation of some 100,000 stars, with an astrometric accuracy of about 0.002 arc-sec, the final Hipparcos Catalogue comprised nearly 120,000 stars with a median accuracy of slightly better than 0.001 arc-sec (1 milliarc-sec).
An additional photomultiplier system viewed a beam splitter in the optical path and was used as a star mapper. Its purpose was to monitor and determine the satellite attitude, and in the process, to gather photometric and astrometric data of all stars down to about 11th magnitude. These measurements were made in two broad bands approximately corresponding to B and V in the (Johnson) UBV photometric system. The positions of these latter stars were to be determined to a precision of 0.03 arc-sec, which is a factor of 25 less than the main mission stars. Originally targeting the observation of around 400,000 stars, the resulting Tycho Catalogue comprised just over 1 million stars, with a subsequent analysis extending this to the Tycho-2 Catalogue of about 2.5 million stars.
The attitude of the spacecraft about its center of gravity was controlled to scan the celestial sphere in a regular precessional motion maintaining a constant inclination between the spin axis and the direction to the Sun. The spacecraft spun around its Z-axis at the rate of 11.25 revolutions/day (168.75 arc-sec/s) at an angle of 43° to the Sun. The Z-axis rotated about the sun-satellite line at 6.4 revolutions/year.
The spacecraft consisted of two platforms and six vertical panels, all made of aluminum honeycomb. The solar array consisted of three deployable sections, generating around 300 W in total. Two S-band antennas were located on the top and bottom of the spacecraft, providing an omni-directional downlink data rate of 24 kbit/s. An attitude and orbit-control subsystem (comprising 5-newton hydrazine thrusters for course manoeuvres, 20-millinewton cold gas thrusters for attitude control, and gyroscopes for attitude determination) ensured correct dynamic attitude control and determination during the operational lifetime.
Some key features of the observations were as follows:
The Hipparcos satellite was financed and managed under the overall authority of the European Space Agency. The main industrial contractors were Matra Marconi Space (now EADS Astrium) and Alenia Spazio (now Thales Alenia Space).
Other hardware components were supplied as follows: the beam-combining mirror from REOSC at Saint Pierre du Perray; the spherical, folding and relay mirrors from Carl Zeiss AG in Oberkochen; the external straylight baffles from CASA in Madrid; the modulating grid from CSEM in Neuchâtel; the mechanism control system and the thermal control electronics from Dornier Satellite Systems in Friedrichshafen; optical filters, the experiment structures and the attitude and orbit control system from Matra Marconi Space in Vélizy; instrument switching mechanisms from Oerlikon-Contraves in Zurich; the image dissector tube and photomultiplier detectors assembled by the Dutch Space Research Organisation, SRON in The Netherlands; the refocusing assembly mechanism designed by TNO-TPD in Delft; the electrical power subsystem from British Aerospace in Bristol; the structure and reaction control system from Daimler-Benz Aerospace in Bremen; the solar arrays and thermal control system from Fokker Space System in Leiden; the data handling and telecommunications system from Saab Ericsson Space in Gothenburg; and the apogee boost motor from SEP in France. Groups from the Institut d'Astrophysique in Liege and the Laboratoire d'Astronomie Spatiale in Marseille contributed optical performance, calibration and alignment test procedures; Captec in Dublin and Logica in London contributed to the on-board software and calibration.
The Hipparcos satellite was launched (with the direct broadcast satellite TV-SAT2 as co-passenger) on an Ariane 4 launch vehicle, flight V33, from Kourou, French Guiana, on 8 August 1989. Launched into a geostationary transfer orbit, the Mage-2 apogee boost motor failed to fire, and the intended geostationary orbit was never achieved. However, with the addition of further ground stations, in addition to ESA operations control centre at ESOC in Germany, the satellite was successfully operated in its geostationary transfer orbit for almost 3.5 years. All of the original mission goals were, eventually, exceeded.
Including an estimate for the scientific activities related to the satellite observations and data processing, Hipparcos mission cost about €600 million (2000 economic conditions), and its execution involved some 200 European scientists and more than 2000 individuals in European industry.
The satellite observations relied on a pre-defined list of target stars. Stars were observed as the satellite rotated, by a sensitive region of the image dissector tube detector. This pre-defined star list formed the Hipparcos Input Catalogue (HIC): each star in the final Hipparcos Catalogue was contained in the Input Catalogue. The Input Catalogue was compiled by the INCA Consortium over the period 1982–89, finalised pre-launch, and published both digitally and in printed form.  Although fully superseded by the satellite results, it nevertheless includes supplemental information on multiple system components as well as compilations of radial velocities and spectral types which, not observed by the satellite, were not included in the published Hipparcos Catalogue.
Constraints on total observing time, and on the uniformity of stars across the celestial sphere for satellite operations and data analysis, led to an Input Catalogue of some 118,000 stars. It merged two components: first, a survey of around 58,000 objects as complete as possible to the following limiting magnitudes: V<7.9 + 1.1sin|b| for spectral types earlier than G5, and V<7.3 + 1.1sin|b| for spectral types later than G5 (b is the Galactic latitude). Stars constituting this survey are flagged in the Hipparcos Catalogue.
The second component comprised additional stars selected according to their scientific interest, with none fainter than about magnitude V=13 mag. These were selected from around 200 scientific proposals submitted on the basis of an Invitation for Proposals issued by ESA in 1982, and prioritised by the Scientific Proposal Selection Committee in consultation with the Input Catalogue Consortium. This selection had to balance 'a priori' scientific interest, and the observing programme's limiting magnitude, total observing time, and sky uniformity constraints.
For the main mission results, the data analysis was carried out by two independent scientific teams, NDAC and FAST, together comprising some 100 astronomers and scientists, mostly from European (ESA-member state) institutes. The analyses, proceeding from nearly 1000 Gbit of satellite data acquired over 3.5 years, incorporated a comprehensive system of cross-checking and validation, and is described in detail in the published catalogue.
A detailed optical calibration model was included to map the transformation from sky to instrumental coordinates. Its adequacy could be verified by the detailed measurement residuals. The Earth's orbit, and the satellite's orbit with respect to the Earth, were essential for describing the location of the observer at each epoch of observation, and were supplied by an appropriate Earth ephemeris combined with accurate satellite ranging. Corrections due to special relativity (stellar aberration) made use of the corresponding satellite velocity. Modifications due to general relativistic light bending were significant (4 milliarc-sec at 90° to the ecliptic) and corrected for deterministically assuming γ=1 in the PPN formalism. Residuals were examined to establish limits on any deviations from this general relativistic value, and no significant discrepancies were found.
The satellite observations essentially yielded highly accurate relative positions of stars with respect to each other, throughout the measurement period (1989–93). In the absence of direct observations of extragalactic sources (apart from marginal observations of quasar 3C273) the resulting rigid reference frame was transformed to an inertial frame of reference linked to extragalactic sources. This allows surveys at different wavelengths to be directly correlated with the Hipparcos stars, and ensures that the catalogue proper motions are, as far as possible, kinematically non-rotating. The determination of the relevant three solid-body rotation angles, and the three time-dependent rotation rates, was conducted and completed in advance of the catalogue publication. This resulted in an accurate but indirect link to an inertial, extragalactic, reference frame.
A variety of methods to establish this reference frame link before catalogue publication were included and appropriately weighted: interferometric observations of radio stars by VLBI networks, MERLIN and VLA; observations of quasars relative to Hipparcos stars using CCDs, photographic plates, and the Hubble Space Telescope; photographic programmes to determine stellar proper motions with respect to extragalactic objects (Bonn, Kiev, Lick, Potsdam, Yale/San Juan); and comparison of Earth rotation parameters obtained by VLBI and by ground-based optical observations of Hipparcos stars. Although very different in terms of instruments, observational methods and objects involved, the various techniques generally agreed to within 10 milliarc-sec in the orientation and 1 milliarc-sec/year in the rotation of the system. From appropriate weighting, the coordinate axes defined by the published catalogue are believed to be aligned with the extragalactic radio frame to within ±0.6 milliarc-sec at the epoch J1991.25, and non-rotating with respect to distant extragalactic objects to within ±0.25 milliarc-sec/yr.
The Hipparcos and Tycho Catalogues were then constructed such that the Hipparcos reference frame coincides, to within observational uncertainties, with the International Celestial Reference System (the ICRS), and representing the best estimates at the time of the catalogue completion (in 1996). The resulting Hipparcos reference frame is thus a materialisation of the ICRS in the optical. It extends and improves the J2000 (FK5) system, retaining approximately the global orientation of that system but without its regional errors.
Whilst of enormous astronomical importance, double stars and multiple stars provided considerable complications to the observations (due to the finite size and profile of the detector's sensitive field of view) and to the data analysis. The data processing classified the astrometric solutions as follows:
If a binary star has a long orbital period such that non-linear motions of the photocentre were insignificant over the short (3-year) measurement duration, the binary nature of the star would pass unrecognised by Hipparcos, but could show as a Hipparcos proper motion discrepant compared to those established from long temporal baseline proper motion programmes on ground. Higher-order photocentric motions could be represented by a 7-parameter, or even 9-parameter model fit (compared to the standard 5-parameter model), and typically such models could be enhanced in complexity until suitable fits were obtained. A complete orbit, requiring 7 elements, was determined for 45 systems. Orbital periods close to one year can become degenerate with the parallax, resulting in unreliable solutions for both. Triple or higher-order systems provided further challenges to the data processing.
The highest accuracy photometric data were provided as a by-product of the main mission astrometric observations. They were made in a broad-band visible light passband, specific to Hipparcos, and designated Hp. The median photometric precision, for Hp<9 mag, was 0.0015 mag, with typically 110 distinct observations per star throughout the 3.5-year observation period. As part of the data reductions and catalogue production, new variables were identified and designated with appropriate variable star identifiers. Variable stars were classified as periodic or unsolved variables; the former were published with estimates of their period, variability amplitude, and variability type. In total some 11,597 variable objects were detected, of which 8237 were newly classified as variable. There are, for example, 273 Cepheid variables, 186 RR Lyr variables, 108 Delta Scuti variables, and 917 eclipsing binary stars. The star mapper observations, constituting the Tycho (and Tycho-2) Catalogue, provided two colours, roughly B and V in the Johnson UBV photometric system, important for spectral classification and effective temperature determination.
Classical astrometry concerns only motions in the plane of the sky and ignores the star's radial velocity, i.e. its space motion along the line-of-sight. Whilst critical for an understanding of stellar kinematics, and hence population dynamics, its effect is generally imperceptible to astrometric measurements (in the plane of the sky), and therefore it is generally ignored in large-scale astrometric surveys. In practice, it can be measured as a Doppler shift of the spectral lines. More strictly, however, the radial velocity does enter a rigorous astrometric formulation. Specifically, a space velocity along the line-of-sight means that the transformation from tangential linear velocity to (angular) proper motion is a function of time. The resulting effect of secular or perspective acceleration is the interpretation of a transverse acceleration actually arising from a purely linear space velocity with a significant radial component, with the positional effect proportional to the product of the parallax, the proper motion, and the radial velocity. At the accuracy levels of Hipparcos it is of (marginal) importance only for the nearest stars with the largest radial velocities and proper motions, but was accounted for in the 21 cases for which the accumulated positional effect over two years exceeds 0.1 milliarc-sec. Radial velocities for Hipparcos Catalogue stars, to the extent that they are presently known from independent ground-based surveys, can be found from the astronomical database of the Centre de données astronomiques de Strasbourg.
The absence of reliable distances for the majority of stars means that the angular measurements made, astrometrically, in the plane of the sky, cannot generally be converted into true space velocities in the plane of the sky. For this reason, astrometry characterises the transverse motions of stars in angular measure (e.g. arcsec per year) rather than in km/s or equivalent. Similarly, the typical absence of reliable radial velocities means that the transverse space motion (when known) is, in any case, only a component of the complete, three-dimensional, space velocity.
|• coincidence with ICRS (3 axes)||±0.6 mas|
|• deviation from inertial (3 axes)||±0.25 mas/yr|
|Number of entries||118,218|
|• with associated astrometry||117,955|
|• with associated photometry||118,204|
|Mean sky density||≈3 per sq deg|
|Limiting magnitude||V≈12.4 mag|
|Number of entries||1,058,332|
|• based on Tycho data||1,052,031|
|• with only Hipparcos data||6301|
|Mean sky density||25 per sq deg|
|Limiting magnitude||V≈11.5 mag|
|Completeness to 90 per cent||V≈10.5 mag|
|Completeness to 99.9 per cent||V≈10.0 mag|
|Tycho 2 Catalogue:|
|Number of entries||2,539,913|
|Mean sky density:|
|• at b=0°||≈150 per sq deg|
|• at b=±30°||≈50 per sq deg|
|• at b=±90°||≈25 per sq deg|
|Completeness to 90 per cent||V≈11.5 mag|
|Completeness to 99 per cent||V≈11.0 mag|
The final Hipparcos Catalogue was the result of the critical comparison and merging of the two (NDAC and FAST consortia) analyses, and contains 118,218 entries (stars or multiple stars), corresponding to an average of some three stars per square degree over the entire sky. Median precision of the five astrometric parameters (Hp<9 mag) exceeded the original mission goals, and are between 0.6–1.0 mas. Some 20,000 distances were determined to better than 10%, and 50,000 to better than 20%. The inferred ratio of external to standard errors is ≈1.0–1.2, and estimated systematic errors are below 0.1 mas. The number of solved or suspected double or multiple stars is 23,882. Photometric observations yielded multi-epoch photometry with a mean number of 110 observations per star, and a median photometric precision (Hp<9 mag) of 0.0015 mag, with 11,597 entries were identified as variable or possibly-variable.
For the star mapper results, the data analysis was carried out by the Tycho Data Analysis Consortium (TDAC). The Tycho Catalogue comprises more than one million stars with 20–30 milliarc-sec astrometry and two-colour (B and V band) photometry.
A more extensive analysis of the star mapper (Tycho) data extracted additional faint stars from the data stream. Combined with old photographic plate observations made several decades earlier as part of the Astrographic Catalogue programme, the Tycho-2 Catalogue of more than 2.5 million stars (and fully superseding the original Tycho Catalogue) was published in 2000.
The Hipparcos and Tycho-1 Catalogues were used to create the Millennium Star Atlas: an all-sky atlas of one million stars to visual magnitude 11. Some 10,000 nonstellar objects are also included to complement the catalogue data.
Between 1997 and 2007, investigations into subtle effects in the satellite attitude and instrument calibration continued. A number of effects in the data that had not been fully accounted for were studied, such as scan-phase discontinuities and micrometeoroid-induced attitude jumps. A re-reduction of the associated steps of the analysis was eventually undertaken. This has led to improved astrometric accuracies for stars brighter than Hp=9.0 mag, reaching a factor of about three for the brightest stars (Hp<4.5 mag), while also underlining the conclusion that the Hipparcos Catalogue as originally published is generally reliable within the quoted accuracies.
All catalogue data are available online from the Centre de Données astronomiques de Strasbourg.
The Hipparcos results have affected a very broad range of astronomical research, which can be classified into three major themes:
Associated with these major themes, Hipparcos has provided results in topics as diverse as Solar System science, including mass determinations of asteroids, Earth's rotation and Chandler Wobble; the internal structure of white dwarfs; the masses of brown dwarfs; the characterisation of extra-solar planets and their host stars; the height of the Sun above the Galactic mid-plane; the age of the Universe; the stellar initial mass function and star formation rates; and strategies for the search for extraterrestrial intelligence. The high-precision multi-epoch photometry has been used to measure variability and stellar pulsations in many classes of objects. The Hipparcos and Tycho catalogues are now routinely used to point ground-based telescopes, navigate space missions, and drive public planetaria.
Since 1997, several thousand scientific papers have been published making use of the Hipparcos and Tycho catalogues. A detailed review of the Hipparcos scientific literature between 1997–2007 was published in 2009, and a popular account of the project in 2010. Some examples of notable results include (listed chronologically):
One controversial result has been the derived proximity, at about 120 parsecs, of the Pleiades cluster, established both from the original catalogue as well as from the revised analysis. This has been contested by various other recent work, placing the mean cluster distance at around 130 parsecs.
According to 2012 paper the anomaly was due to the use of a weighted mean when there is a correlation between distances and distance errors for stars in clusters. It is resolved by using an unweighted mean. There is no systematic bias in the Hipparcos data when it comes to star clusters.
In August 2014, the discrepancy between the cluster distance of 120.2±1.5 parsecs (pc) as measured by Hipparcos and the distance of 133.5±1.2 pc derived with other techniques was confirmed by parallax measurements made using VLBI, which gave 136.2±1.2 pc, the most accurate and precise distance yet presented for the cluster.
Another distance debate set-off by Hipparcos is for the distance to the star Polaris
101 Virginis is a red giant variable star in the Boötes constellation. It was originally catalogued as 101 Virginis by Flamsteed due to an error in the position. When it was confirmed as a variable star, it was actually within the border of the constellation Bootes and given the name CY Boötis.The variability is not strongly defined but a primary period of 23 days and a secondary period of 340 days have been reported.CY Boo is listed in the Hipparcos catalogue as a "problem binary", a star which was suspected of being multiple but for which the Hipparcos observations did not give a satisfactory solution. Further observations have always shown it to be single.12 Aquarii
12 Aquarii (abbreviated 12 Aqr) is a double star in the constellation Aquarius. 12 Aquarii is the Flamsteed designation. It consists of a K-type giant and an A-type main-sequence star. Parallax measurements by Hipparcos put it at a distance of some 500 light-years, or 150 parsecs away.19 Aquarii
19 Aquarii is a star in the constellation of Aquarius. With an apparent magnitude of about 5.7, the star is barely visible to the naked eye (see Bortle scale). Parallax estimates made by the Hipparcos spacecraft put it at a distance of about 300 light years (92 parsecs) away from the Earth.19 Aquarii has a spectral type of A8V, meaning it is an A-type main-sequence star. These types of stars are bluish-white colored, and have effective temperatures between 7100 and 11500 K. It rotates fairly fast, as its projected rotational velocity is about 155 km/s, so it must be rotating at least that fast.20 Cygni
20 Cygni is a single, orange-hued star in the northern constellation of Cygnus. It is a faint star but is visible to the naked eye with an apparent visual magnitude of 5.03. The distance to 20 Cygni can be estimated from its annual parallax shift of 16 mas, which yields a range of 202 light years. It is moving closer to the Earth with a heliocentric radial velocity of −22 km/s.This is an aging red giant star with a stellar classification of K3 III CN2, a star that has used up its core hydrogen and is expanding. The suffix notation indicates there are unusually strong lines of cyanogen in the spectrum. 20 Cyg is listed as one of the least variable stars in the Hipparcos catalogue, changing its brightness by no more than 0.01 magnitude. It has 1.28 times the mass of the Sun and has expanded to 13 times the Sun's radius. The star is radiating 57.5 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,337 K.32 Ophiuchi
32 Ophiuchi (32 Oph) is a star in the constellation Hercules. Its apparent magnitude is 5.00.39 Boötis
39 Boötis (abbreviated as 39 Boo) or ADS 9406 B is a star in the constellation Boötes. A multiple star system, it has a combined apparent magnitude of 5.68. The system is 224 ± 8 light-years distant.
It is composed of BD+49 2326A, which has an apparent magnitude of 6.248 and spectral type F8V, and BD+49 2326B, which has an apparent magnitude of 6.62 and spectral type F7V. Both stars are themselves spectroscopic binaries.41 Cygni
41 Cygni (41 Cyg) is a star in the constellation Cygnus. Its apparent magnitude is 4.02.60 Andromedae
60 Andromedae (abbreviated 60 And) is a star system in the northern constellation of Andromeda, located to the west-northwest of Gamma Andromedae. 60 Andromedae is the Flamsteed designation though the star also bears the Bayer designation b Andromedae. It is bright enough to be seen by the naked eye with an apparent visual magnitude of 4.82. Based upon parallax measurements made during the Hipparcos mission, it is at a distance of roughly 530 light-years (160 parsecs) from Earth.This system is known to have three components. The primary is a giant star with a stellar classification of K3.5 III Ba0.4, meaning that an overabundance of barium ionized one time is observed in the spectrum of the star, making it a barium star. The secondary component is likely a white dwarf with a period of 748.2 days and an eccentricity of 0.34. There is a third component at an angular separation of 0.22 arcseconds.Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of the Solar System and our galaxy, the Milky Way.Beta Cancri
Beta Cancri (β Cancri, abbreviated Beta Cnc, β Cnc), also named Tarf , is the brightest star in the zodiacal constellation of Cancer. It has an apparent visual magnitude of +3.5 and an absolute magnitude of −1.2. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 290 light-years distant from the Sun. An exoplanet, designated Beta Cancri b, is believed to be orbiting the star.Beta Cancri has a companion listed and together they are designated WDS J08165+0911. As the primary, Beta Cancri bears the designation WDS J08165+0911A. The companion is designated WDS J08165+0911B.Chi Andromedae
Chi Andromedae (χ Andromedae, χ And) is the Bayer designation for a star in the northern constellation of Andromeda. It has an apparent visual magnitude of +5.2, which is relatively faint for a naked-eye star. Based upon parallax measurements made during the Hipparcos mission, Chi Andromedae is located around 250 light-years (77 parsecs) from Earth.It with φ And composed the Chinese asterism 軍南門 (Keun Nan Mun, Mandarin jūnnánmén), "the South Gate of the Camp". χ Andromedae is a member of 天大將軍 (Tiān Dà Jiāng Jūn), meaning Heaven's Great General, together with γ Andromedae, φ Persei, 51 Andromedae, 49 Andromedae, θ Andromedae, τ Andromedae, 56 Andromedae, β Trianguli, γ Trianguli and δ Trianguli. Consequently, the Chinese name for χ Andromedae itself is 天大將軍五 (Tiān Dà Jiāng Jūn wǔ, English: the Fifth Star of Heaven's Great General.)This is most likely a spectroscopic binary system with an estimated orbital period of 21.5 years and an eccentricity of 0.37. The primary component has a stellar classification of G8 III, which indicates it is a giant star that has exhausted the supply of hydrogen at its core and evolved away from the main sequence. The outer envelope has expanded to about nine times the radius of the Sun and it is radiating 47 times the luminosity of the Sun at an effective temperature of 5,070 K. This heat gives the star the yellow-hued glow of a G-type star. It appears to be rotating very slowly with no measurable projected rotational velocity.HD 125351
HD 125351 or A Boötis (A Boo) is spectroscopic binary in the constellation Boötes. The system has an apparent magnitude of +4.97, with a spectrum matching a K-type giant star. It is approximately 233 light years from Earth.HD 181342
HD 181342 is a star in the constellation of Sagittarius. With an apparent magnitude of 7.55, it cannot be seen with the naked eye. Parallax measurements made by Hipparcos put the star at a distance of 360 light-years (111 parsecs) away.HD 181342 is a K-type red giant star. It was formerly an A-type main-sequence star, but at an age of 1.56 billion years it has swelled up to a size of 4.55 solar radii. It is currently 1.78 times the mass of the Sun, 16.2 times as luminous, and its surface temperature is 4976 K.International Celestial Reference System
The International Celestial Reference System (ICRS) is the current standard celestial reference system adopted by the International Astronomical Union (IAU). Its origin is at the barycenter of the Solar System, with axes that are intended to be "fixed" with respect to space. ICRS coordinates are approximately the same as equatorial coordinates: the mean pole at J2000.0 in the ICRS lies at 17.3±0.2 mas in the direction 12 h and 5.1±0.2 mas in the direction 18 h. The mean equinox of J2000.0 is shifted from the ICRS right ascension origin by 78±10 mas (direct rotation around the polar axis).
The defining extragalactic reference frame of the ICRS is the International Celestial Reference Frame (currently ICRF3) based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky. Because they are so distant, they are apparently stationary to our current technology, yet their positions can be measured with the utmost accuracy by Very Long Baseline Interferometry (VLBI). The positions of most are known to 0.001 arcsecond or better. At optical wavelengths, the ICRS is currently realized by the Hipparcos Celestial Reference Frame (HCRF), a subset of about 100,000 stars in the Hipparcos Catalogue. A more accurate optical realization of the ICRS (Gaia-CRF2), based on the observation by the Gaia spacecraft of almost 500,000 extragalactic objects believed to be quasars, is under preparation.Iota Andromedae
Iota Andromedae (ι And, ι Andromedae) is a star in the constellation Andromeda. It has an apparent magnitude of +4.29 and is approximately 500 light years from Earth.Iota Andromedae is a B-type main sequence star with a stellar classification of B8 V. It is among the least variable stars observed during the Hipparcos mission.Pleiades
The Pleiades (), also known as the Seven Sisters and Messier 45, are an open star cluster containing middle-aged, hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky.
The cluster is dominated by hot blue and luminous stars that have formed within the last 100 million years. Reflection nebulae around the brightest stars were once thought to be left over material from the formation of the cluster, but are now considered likely to be an unrelated dust cloud in the interstellar medium through which the stars are currently passing.Computer simulations have shown that the Pleiades were probably formed from a compact configuration that resembled the Orion Nebula. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.Polaris
Polaris, designated α Ursae Minoris (Alpha Ursae Minoris, abbreviated Alpha UMi, α UMi), commonly the North Star or Pole Star, is the brightest star in the constellation of Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star. The revised Hipparcos parallax gives a distance to Polaris of about 433 light-years (133 parsecs), while calculations by other methods derive distances around 30% closer.
Polaris is a triple star system, composed of the primary star, Polaris Aa (a yellow supergiant), in orbit with a smaller companion (Polaris Ab); the pair in orbit with Polaris B (discovered in August 1779 by William Herschel). There were once thought to be two more distant components—Polaris C and Polaris D—but these have been shown not to be physically associated with the Polaris system.Przybylski's Star
Przybylski's Star , or HD 101065, is a rapidly oscillating Ap star at roughly 355 light-years (109 parsecs) from the Sun in the southern constellation of Centaurus.V385 Andromedae
V385 Andromedae is a variable star in the constellation Andromeda, about 360 parsecs (1,200 ly) away. It is a red giant over a hundred times larger than the sun. It has an apparent magnitude around 6.4, just about visible to the naked eye in ideal conditions.
V385 Andromedae was identified as a long-period variable in 1999 from analysis of Hipparcos photometry. It was classified as a slow irregular variable, but analysis of its light curve identified a possible 36 day period. It varies by about 0.1 magnitudes.
|Cancelled and proposed|
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