Hipparcos
Encyclopedia
Hipparcos was a scientific mission of the European Space Agency
(ESA), launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry
, the accurate measurement of the positions of celestial objects on the sky. This permits the accurate determination of proper motion
s and parallax
es of stars, allowing a determination of their distance and tangential velocity. When combined with radial velocity measurements from spectroscopy
, this fixes all six quantities needed to determine the motion of the star. The Hipparcos Catalogue, a high-precision catalogue of more than 100,000 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.
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 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 lobby. 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, parallax
es, and annual proper motion
s 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 also reflected the name of the Greek astronomer Hipparchus
.
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 – 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 arcssec, 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 sun direction. The spacecraft spun around its Z-axis at the rate of 11.25 rev/day (168.75 arc-sec/sec) at an angle of 43° to the sun. The Z-axis rotated about the sun-satellite line at 6.4 rev/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 milli-Newton cold gas thrusters for attitude control, and gyroscope
s for attitude determination) ensured correct dynamic attitude control and determination during the operational lifetime.
. 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 Neuchatel; 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 Velizy; 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 Gotenborg; 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 the primary ground station at Odenwald 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.
The satellite was operated by the ESA operations control centre at ESOC, Darmstadt (Germany).
Including an estimate for the scientific activities related to the satellite observations and data processing, Hipparcos mission cost some 600 MEuro (2000 economic conditions), and its
execution involved some 200 European scientists and more than 2000 individuals in European industry.
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.
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.
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/yr 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 the 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.
s and multiple star
s 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.
s, 186 RR Lyr variables, 108 Delta Scuti variable
s, 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.
, 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 extant that they are presently known from independent ground-based surveys, can be found from the astronomical data base 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 plan 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/sec 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.
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 TDAC consortium. The Tycho Catalogue comprises more than one million stars with 20–30 milliarc-sec astrometry and two-colour (B and V band) photometry.
The final Hipparcos and Tycho Catalogues were completed in August 1996. The catalogues were published by ESA on behalf of the scientific teams in June 1997.
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
.
(a) the provision of an accurate reference frame: this has allowed the consistent and rigorous re-reduction of historical astrometric measurements, including those from Schmidt plates, meridian circles, the 100-year old Astrographic Catalogue, and 150 years of Earth-orientation measurements. These, in turn, have yielded a dense reference framework with high-accuracy long-term proper motions (the Tycho-2 Catalogue). Reduction of current state-of-the-art survey data has yielded the dense UCAC2 Catalogue of the US Naval Observatory on the same reference system, and improved astrometric data from recent surveys such as the Sloan Digital Sky Survey
and 2MASS
. Implicit in the high-accuracy reference frame is the measurement of General Relativistic light bending
, and the detection and characterisation of double and multiple stars;
(b) constraints on stellar structure
and stellar evolution
: the accurate distances and luminosities of 100,000 stars has provided the most comprehensive and accurate data set of fundamental stellar parameters to date, placing constraints on internal rotation, element diffusion, convective motions, and asteroseismology
. Combined with theoretical models and other data it yields evolutionary masses, radii, and ages for large numbers of stars covering a wide range of evolutionary states;
(c) Galactic kinematics and dynamics: the uniform and accurate distances and proper motions have provided a substantial advance in understanding of stellar kinematics
and the dynamical structure of the solar neighbourhood, ranging from the presence and evolution of clusters, associations and moving groups, the presence of resonance motions due to the Galaxy's central bar and spiral arms, determination of the parameters describing Galactic rotation
, discrimination of the disk and halo populations, evidence for halo accretion, and the measurement of space motions of runaway stars
, globular clusters, and many other types of star.
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. 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.
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
(ESA), launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry
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 our Solar System and our Galaxy, the Milky...
, the accurate measurement of the positions of celestial objects on the sky. This permits the accurate determination of proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
s and parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
es of stars, allowing a determination of their distance and tangential velocity. When combined with radial velocity measurements from spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
, this fixes all six quantities needed to determine the motion of the star. The Hipparcos Catalogue, a high-precision catalogue of more than 100,000 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
Tycho-2 Catalogue
The Tycho-2 Catalogue is a catalogue of more than 2.5 million of the brightest stars.- Catalogue :The astrometric reference catalogue contain positions, proper motions, and two-color photometric data for the 2,539,913 of the brightest stars in the Milky Way, of which about 5000 are visible to the...
of 2.5 million stars was published in 2000.
Background
By the second half of the 20th century, the accurate measurement of starStar
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
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
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
, but were compounded by complex optical terms, thermal and gravitational instrument flexures, and the absence of all-sky visibility. A proposal to make these exacting observations from space was first put forward in 1967.
Although originally proposed to the French space agency CNES
CNES
The is the French government space agency . Established under President Charles de Gaulle in 1961, its headquarters are located in central Paris and it is under the supervision of the French Ministries of Defence and Research...
, 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 lobby. 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
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
structure and kinematics, on a more secure empirical basis. Observationally, the objective was to provide the positions, parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
es, and annual proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
s 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 also reflected the name of the Greek astronomer Hipparchus
Hipparchus
Hipparchus, the common Latinization of the Greek Hipparkhos, can mean:* Hipparchus, the ancient Greek astronomer** Hipparchic cycle, an astronomical cycle he created** Hipparchus , a lunar crater named in his honour...
.
Satellite and payload
The spacecraft carried a single all-reflective eccentric Schmidt telescope, with an aperture of 29 cm. 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 dissectorImage dissector
An image dissector, also called a dissector tube, is a video camera tube in which photocathode emissions create an "electron image" which is then scanned to produce an electrical signal representing the visual image...
tube (photomultiplier
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
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).
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
system viewed a beam splitter in the optical path and was used as a star mapper – 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
UBV photometric system
UBV photometric system, also called the Johnson system , is a wide band photometric system for classifying stars according to their colors. It is the first known standardized photoelectric photometric system. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are...
. The positions of these latter stars were to be determined to a precision of 0.03 arcssec, 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 sun direction. The spacecraft spun around its Z-axis at the rate of 11.25 rev/day (168.75 arc-sec/sec) at an angle of 43° to the sun. The Z-axis rotated about the sun-satellite line at 6.4 rev/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
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
thrusters for course manoeuvres, 20 milli-Newton cold gas thrusters for attitude control, and gyroscope
Gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
s for attitude determination) ensured correct dynamic attitude control and determination during the operational lifetime.
Principles
Some key features of the observations were as follows:- through observations from space, the effects of astronomical seeingAstronomical seeingAstronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
due to the atmosphereAtmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
, instrumental gravitational flexure and thermal distortions could be obviated or minimised; - all-sky visibility permitted a direct linking of the stars observed all over the celestial sphere;
- the two viewing directions of the satellite, separated by a large and suitable angle (58°), resulted in a rigid connection between quasi-instantaneous one-dimensional observations in different parts of the sky. In turn, this led to parallaxParallaxParallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
determinations which are absolute (rather than relative, with respect to some unknown zero-point); - the continuous eclipticEclipticThe ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
-based scanning of the satellite resulted in an optimum use of the available observing time, with a resulting catalogue providing reasonably homogeneous sky density and uniform astrometric accuracy over the entire celestial sphere;
- the various geometrical scan configurations for each star, at multiple epochs throughout the 3-year observation programme, resulted in a dense network of one-dimensional positions from which the barycentricCenter of massIn physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
coordinate direction, the parallaxParallaxParallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
, and the object's proper motionProper motionThe proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
, could be solved for in what was effectively a global least squaresLeast squaresThe method of least squares is a standard approach to the approximate solution of overdetermined systems, i.e., sets of equations in which there are more equations than unknowns. "Least squares" means that the overall solution minimizes the sum of the squares of the errors made in solving every...
reduction of the totality of observations. The astrometric parameters as well as their standard errorsStandard error (statistics)The standard error is the standard deviation of the sampling distribution of a statistic. The term may also be used to refer to an estimate of that standard deviation, derived from a particular sample used to compute the estimate....
and correlation coefficientsCorrelationIn statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
were derived in the process; - since the number of independent geometrical observations per object was large (typically of order 30) compared with the number of unknowns for the standard model (five astrometric unknowns per star) astrometric solutions not complying with this simple five-parameter model, could be expanded to take into account the effects of double or multiple starsDouble starIn observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e...
, or non-linear photocentric motions ascribed to unresolved astrometric binaries; - a somewhat larger number of actual observations per object, of order 110, provided accurate and homogeneous photometric information for each star, from which mean magnitudes, variability amplitudes, and in many cases period and variability type classification could be undertaken.
Development, launch and operations
The Hipparcos satellite was financed and managed under the overall authority of the European Space AgencyEuropean Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
. The main industrial contractors were Matra Marconi Space
Matra Marconi Space
Matra Marconi Space was a Franco-British aerospace company.-History:Matra Marconi Space was established in 1990 as a joint venture between the space and telecommunication divisions of the Lagardère Group and the GEC group .The merged company was announced in December 1989 and was owned 51% by...
(now EADS Astrium
EADS Astrium
Astrium Satellites is one of the three business units of Astrium, a subsidiary of EADS. It is a European space manufacturer involved in the manufacture of spacecraft used for science, Earth observation and telecommunication, as well as the equipment and subsystems used therein and related ground...
) and Alenia Spazio (now Thales Alenia Space
Thales Alenia Space
Thales Alenia Space is an aerospace company born after the Thales Group bought the participation of Alcatel in the two joint-ventures between Alcatel and Finmeccanica, Alcatel Alenia Space and Telespazio.-History:...
).
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 Neuchatel; 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 Velizy; instrument switching mechanisms from Oerlikon-Contraves in Zurich; the image dissector tube and photomultiplier detectors assembled by the Dutch Space Research Organisation, SRON
Sròn
Sròn is the Scottish Gaelic word for nose and is the name of some hills in the Scottish Highlands. Before the abolition of the acute accent in Scottish Gaelic, it was sometimes spelt as srón...
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 Gotenborg; 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
Kourou
Kourou is a commune in French Guiana, an overseas region and department of France located in South America.Kourou is the location of the Guiana Space Centre, France and ESA's main spaceport.-Geography:...
, French Guiana, on 8 August 1989. Launched into a geostationary transfer orbit
Geostationary transfer orbit
A geosynchronous transfer orbit or geostationary transfer orbit is a Hohmann transfer orbit used to reach geosynchronous or geostationary orbit....
, the Mage-2 apogee boost motor failed to fire, and the intended geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
was never achieved. However, with the addition of further ground stations, in addition to the primary ground station at Odenwald 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.
The satellite was operated by the ESA operations control centre at ESOC, Darmstadt (Germany).
Including an estimate for the scientific activities related to the satellite observations and data processing, Hipparcos mission cost some 600 MEuro (2000 economic conditions), and its
execution involved some 200 European scientists and more than 2000 individuals in European industry.
Hipparcos Input Catalogue
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: 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 formAlthough 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
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
earlier than G5, and
V<7.3 + 1.1sin|b| for spectral types
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
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.
Data reductions
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
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
(stellar aberration
Aberration of light
The aberration of light is an astronomical phenomenon which produces an apparent motion of celestial objects about their real locations...
) made use of the corresponding satellite velocity. Modifications due to general relativistic
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
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 Hipparcos reference frame
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 quasarQuasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
3C273
3C273
3C 273 is a quasar located in the constellation Virgo. It was the first quasar ever to be identified.It is the optically brightest quasar in our sky , and one of the closest with a redshift, z, of 0.158. A luminosity distance of DL = may be calculated from z...
) the resulting rigid reference frame was transformed to an inertial frame of reference
Inertial frame of reference
In physics, an inertial frame of reference is a frame of reference that describes time homogeneously and space homogeneously, isotropically, and in a time-independent manner.All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not...
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.
MERLIN
The Multi-Element Radio Linked Interferometer Network is an interferometer array of radio telescopes spread across England. The array is run from Jodrell Bank Observatory in Cheshire by the University of Manchester on behalf of STFC as a National Facility.The array consists of up to seven radio...
and VLA
Very Large Array
The Very Large Array is a radio astronomy observatory located on the Plains of San Agustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA...
; observations of quasars relative to Hipparcos stars using CCDs
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
, photographic plates, and the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
; photographic programmes to determine stellar proper motions with respect to extragalactic objects (Bonn, Kiev, Lick, Potsdam, Yale/San Juan); and comparison of Earth rotation
Earth rotation
Earth's rotation is the rotation of the solid Earth around its own axis. The Earth rotates towards the east. As viewed from the North Star Polaris, the Earth turns counter-clockwise.- Rotation period :...
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/yr 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
International Celestial Reference System
The International Celestial Reference System is the current standard celestial reference system adopted by the International Astronomical Union . Its origin is at the barycenter of the solar system, with axes that are intended to be "fixed" with respect to space...
(the ICRS), and representing the best estimates at the time of the catalogue completion (in 1996). The resulting Hipparcos reference frame is thus the 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.
Double and multiple stars
Whilst of enormous astronomical importance, double starDouble star
In observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e...
s and multiple star
Multiple star
A multiple star consists of three or more stars which appear from the 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 physical, or this closeness may be merely apparent, in which case...
s 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:
- single-star solutions: 100,038 entries, of which 6,763 were flagged as suspected double
- component solutions (Annex C): 13,211 entries, comprising 24,588 components in 12,195 solutions
- acceleration solutions (Annex G): 2,622 solutions
- orbital solutions (Annex O): 235 entries
- variability-induced movers (Annex V): 288 entries
- stochastic solutions (Annex X): 1,561 entries
- no valid astrometric solution: 263 entries (of which 218 were flagged as suspected double)
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.
Photometric observations
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 variableCepheid variable
A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.Cepheid...
s, 186 RR Lyr variables, 108 Delta Scuti variable
Delta Scuti variable
A Delta Scuti variable is a variable star which exhibits variations in its luminosity due to both radial and non-radial pulsations of the star's surface. Typical brightness fluctuations are from 0.003 to 0.9 magnitudes in V over a period of a few hours, although the amplitude and period of the...
s, 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
Photometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...
, important for spectral classification and effective temperature
Effective temperature
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...
determination.
Radial velocities
Classical astrometry concerns only motions in the plane of the sky and ignores the star's radial velocityRadial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic 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
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
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 extant that they are presently known from independent ground-based surveys, can be found from the astronomical data base of the Centre de données astronomiques de Strasbourg
Centre de Données astronomiques de Strasbourg
The Centre de données astronomiques de Strasbourg is a data hub which collects and distributes astronomical information. It was established in 1972 under the name Centre de Données Stellaires...
.
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 plan 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/sec 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.
Published catalogues
| Property | Value |
|---|---|
| Common: | |
| Measurement period | 1989.8—1993.2 |
| Catalogue epoch | J1991.25 |
| Reference system | ICRS |
| • coincidence with ICRS (3 axes) | ±0.6 mas |
| • deviation from inertial (3 axes) | ±0.25 mas/yr |
| Hipparcos Catalogue: | |
| 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 |
| Completeness | V=7.3-9.0 mag |
| Tycho Catalogue: | |
| 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 TDAC consortium. The Tycho Catalogue comprises more than one million stars with 20–30 milliarc-sec astrometry and two-colour (B and V band) photometry.
The final Hipparcos and Tycho Catalogues were completed in August 1996. The catalogues were published by ESA on behalf of the scientific teams in June 1997.
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
Millennium Star Atlas
The Millennium Star Atlas was constructed as a collaboration betweena team at Sky & Telescope led by Roger Sinnott, and the European Space Agency'sHipparcos project, led by Michael Perryman. It was the first sky atlas to include the...
: 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
Centre de Données astronomiques de Strasbourg
The Centre de données astronomiques de Strasbourg is a data hub which collects and distributes astronomical information. It was established in 1972 under the name Centre de Données Stellaires...
.
Scientific results
The Hipparcos results impact a very broad range of astronomical research, which can be classified into three major themes:(a) the provision of an accurate reference frame: this has allowed the consistent and rigorous re-reduction of historical astrometric measurements, including those from Schmidt plates, meridian circles, the 100-year old Astrographic Catalogue, and 150 years of Earth-orientation measurements. These, in turn, have yielded a dense reference framework with high-accuracy long-term proper motions (the Tycho-2 Catalogue). Reduction of current state-of-the-art survey data has yielded the dense UCAC2 Catalogue of the US Naval Observatory on the same reference system, and improved astrometric data from recent surveys such as the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
and 2MASS
2MASS
Observations for the Two Micron All-Sky Survey began in 1997 and were completed in 2001 at two telescopes located one each in the northern and southern hemispheres to ensure coverage of the entire sky...
. Implicit in the high-accuracy reference frame is the measurement of General Relativistic light bending
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
, and the detection and characterisation of double and multiple stars;
(b) constraints on stellar structure
Stellar structure
Stars of different mass and age have varying internal structures. Stellar structure models describe the internal structure of a star in detail and make detailed predictions about the luminosity, the color and the future evolution of the star....
and stellar evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
: the accurate distances and luminosities of 100,000 stars has provided the most comprehensive and accurate data set of fundamental stellar parameters to date, placing constraints on internal rotation, element diffusion, convective motions, and asteroseismology
Asteroseismology
Asteroseismology also known as stellar seismology is the science that studies the internal structure of pulsating stars by the interpretation of their frequency spectra. Different oscillation modes penetrate to different depths inside the star...
. Combined with theoretical models and other data it yields evolutionary masses, radii, and ages for large numbers of stars covering a wide range of evolutionary states;
Stellar kinematics
Stellar kinematics is the study of the movement of stars without needing to understand how they acquired their motion. This differs from stellar dynamics, which takes into account gravitational effects...
and the dynamical structure of the solar neighbourhood, ranging from the presence and evolution of clusters, associations and moving groups, the presence of resonance motions due to the Galaxy's central bar and spiral arms, determination of the parameters describing Galactic rotation
Galaxy rotation curve
The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis....
, discrimination of the disk and halo populations, evidence for halo accretion, and the measurement of space motions of runaway stars
Stellar kinematics
Stellar kinematics is the study of the movement of stars without needing to understand how they acquired their motion. This differs from stellar dynamics, which takes into account gravitational effects...
, globular clusters, and many other types of star.
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
Chandler wobble
The Chandler wobble is a small motion in the Earth's axis of rotation relative to the Earth's surface, which was discovered by American astronomer Seth Carlo Chandler in 1891. It amounts to on the Earth's surface and has a period of 433 days...
, 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
Age of the universe
The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...
, the stellar initial mass function
Initial mass function
The initial mass function is an empirical function that describes the mass distribution of a population of stars in terms of their theoretical initial mass...
and star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
rates, and strategies for the search for extraterrestrial intelligence
SETI
The search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...
. 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. Some examples of notable results include (listed chronologically):
- studies of Galactic rotation from Cepheid variableCepheid variableA Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.Cepheid...
s
- the nature of Delta Scuti variableDelta Scuti variableA Delta Scuti variable is a variable star which exhibits variations in its luminosity due to both radial and non-radial pulsations of the star's surface. Typical brightness fluctuations are from 0.003 to 0.9 magnitudes in V over a period of a few hours, although the amplitude and period of the...
s
- studies of local stellar kinematicsStellar kinematicsStellar kinematics is the study of the movement of stars without needing to understand how they acquired their motion. This differs from stellar dynamics, which takes into account gravitational effects...
- testing the white dwarfWhite dwarfA white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
mass-radius relation
- the structure and dynamics of the Hyades clusterHyades (star cluster)The Hyades is the nearest open cluster to the Solar System and one of the best-studied of all star clusters. The Hipparcos satellite, the Hubble Space Telescope, and infrared color-magnitude diagram fitting have been used to establish a distance to the cluster's center of ~153 ly...
- kinematics of Wolf-Rayet starWolf-Rayet starWolf–Rayet stars are evolved, massive stars , which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s...
s and O-type runaway stars
- subdwarfSubdwarf starA subdwarf star, sometimes denoted by "sd", is luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type...
parallaxes: metal-rich clusters and the thick disk
- fine structure of the red giantRed giantA red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
clump and associated distance determinations
- unexpected stellar velocity distribution in the warped Galactic disk
- refining the Oort and Galactic constants
- Galactic disk dark matter, terrestrial impact cratering and the law of large numbers
- vertical motion and expansion of the Gould BeltGould BeltThe Gould Belt is a partial ring of stars in the Milky Way galaxy, about 3000 light years across, tilted toward the galactic plane by about 16 to 20 degrees. It contains many spectral class O- and B-type stars, and may represent the local spiral arm to which the Sun belongs—currently the Sun is...
- the use of gamma ray burstGamma ray burstGamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...
s as direction and time markers in SETISETIThe search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...
strategies
- evidence of a galaxy merger in the early formation history of the Milky Way
- study of nearby OB associations
- close approaches of stars to the Solar System
- studies of binary starBinary starA binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
orbits and masses
- the HD 209458HD 209458HD 209458 is an 8th magnitude star in the constellation Pegasus. It is very similar to our Sun, and it is classified as a yellow dwarf . Because it is located at a distance of about 150 light years, it is not visible to the unaided eye...
planetary transits
- formation of the stellar Galactic haloGalactic haloThe term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...
and thick disk
- the local density of matter in the Galaxy and the Oort limitOort LimitThere are two definitions of the Oort limit:* the outer boundary of the Oort cloud. The current estimate is about 100,000 astronomical units from the Sun, which is approximately 1/3 of the distance to the nearest star .* the estimate of the local density of matter in the Sun's vicinity.-...
- ice ageIce ageAn ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
epochs and the Sun's path through the Galaxy
- local kinematics of K and M giants and the concept of superclusters
- an improved reference frame for long-term Earth rotation studies
- the local stellar velocity field in the Galaxy
One controversial result has been the derived proximity, at about 120 parsecs, of the Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is 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...
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.
People
- Pierre Lacroute (Observatory of StrasbourgObservatory of StrasbourgThe Observatory of Strasbourg is an astronomical observatory in Strasbourg, France.Following the Franco-Prussian War of 1870–71, the city of Strasbourg became part of the German Empire. The University of Strasbourg was refounded in 1872 and a new observatory began construction in 1875...
): proposer of space astrometry in 1967 - Michael Perryman: ESA project scientist
- Catherine Turon (Observatoire de Paris-Meudon): leader of Input Catalogue Consortium
- Erik Høg: leader of the TDAC Consortium
- Lennart Lindegren: leader of the NDAC Consortium
- Jean Kovalevsky: leader of the FAST Consortium
- Adriaan Blaauw: chair of the observing programme selection committee
- Hipparcos Science Team: Uli Bastian, Pierluigi Bernacca, Michel Crézé, Francesco Donati, Michel Grenon, Michael Grewing, Erik Høg, Jean Kovalevsky, Floor van Leeuwen, Lennart Lindegren, Hans van der Marel, Francois MignardFrancois MignardFrançois Mignard is Director of CERGA of the Observatoire de la Côte d'Azur. Expert in space astrometry and solar system dynamics, Mignard is also involved in space missions such as the European Space Agency's Hipparcos and the Gaia mission...
, Andrew Murray, Michael Perryman (chair), Rudolf Le Poole, Hans Schrijver, Catherine Turon - Franco Emiliani: ESA project manager (1981–85)
- Hamid Hassan: ESA project manager (1985–89)
- Dietmar Heger: ESA/ESOC spacecraft operations manager
- Michel Bouffard: Matra Marconi Space project manager
- Bruno Strim: Alenia Spazio project manager