Science Highlights

This page highlights some scientific results based on data from the Hipparcos and Tycho Catalogues and the Tycho-2 Catalogue. For a more complete overview, visit the Publications page.

Hipparcos-related papers feature in Astronomy & Astrophysics special 40th anniversary issue
[July 2009]

To mark its 40th anniversary, Astronomy & Astrophysics has published a special issue (volume 500). It reprints the 40 most cited articles according to ADS for the last 40 years, together with commentaries that highlight their context and impact in astrophysics. Amongst them, two Hipparcos-related papers make the top 40: The Hipparcos Catalogue (Perryman, M. A. C., Lindegren, L., Kovalevsky, J., et al., 1997, A&A, 323, L49); and The Tycho-2 Catalogue of the 2.5 million brightest stars (Høg, E., Fabricius, C., Makarov, V.V., et al., 2000, A&A, 355, L27).

Astronomical Applications of Astrometry: 10 Years of Exploitation of the Hipparcos Satellite Data
[December 2008]

"Astronomical Applications of Astrometry: Ten Years of Exploitation of the Hipparcos Satellite Data", by the ESA project scientist, Michael Perryman, was published in December 2008 by Cambridge University Press.

It is a comprehensive textbook style review of the enormous range of applications of the Hipparcos data in different areas, describing each topic and the state-of-the-art before Hipparcos, and summarising all major contributions made by the satellite, including reference to some 3000 publications between 1997-2007. It also contains a detailed overview of the Hipparcos and Tycho Catalogues, their annexes and their updates; and a critical compilation of fundamental astrometric quantities used in the interpretation of the data. For more information (and some of the first reviews), follow this link.

New Hipparcos Hp bolometric corrections available
[October 2007]

The first, and incomplete, assessment of Hipparcos Hp bolometric corrections was made in 1997 by R. Cayrel et al. based on Kurucz ATLAS9 model atmospheres (see ads link to paper ). Recently, a new and more complete set of corrections has been established and made available by M. Bessell. The new data are based on Castelli models for main-sequence stars, Plez models for giants, and Plez models for solar-composition M-dwarfs. The tables can be retrieved from M. Bessell's web site.

New reduction of the Hipparcos raw data available
[September 2007]

"Hipparcos, the new Reduction of the Raw Data" is the title of a new publication, by Floor van Leeuwen, in the Astrophysics and Space Science Library Series from Springer publishers due for release on 27 September 2007. Based on an improved understanding of peculiarities of the satellite's dynamics, and on a better understanding of the connectivity requirements in the along scan attitude, the new reduction of the astrometric data provides an improvement by a factor of 2.2 in the total weight compared to the catalogue of 1997, and provides improved data for a wide range of studies on stellar luminosities and local galactic kinematics. An outline of the motivation behind the new reduction can be read here. Details of the validation of the new Hipparcos reduction are discussed here.

Revised Hipparcos parallaxes for Cepheids: the PL relation and the Hubble Constant
[May 2007]

Revised Hipparcos parallaxes for classical Cepheids, together with some HST-based parallaxes, have been applied to a new analysis to describe the period-luminosity relation for Cepheids in our Galaxy, and to derive the Hubble Constant. The analysis shows that, within the uncertainities, the value of the slope for our Galaxy is the same as for the LMC. Applying the new PL description to the determination of distance modulii for a number of galaxies yielded a Hubble Constant of (70±5), compatible with values from WMAP data and a ΛCDM model. Further details are available in Cepheid Parallaxes and the Hubble Constant, by van Leeuwen et al.

Using Hipparcos stars to recover the local star formation rate
[August 2006]

The Hipparcos catalogue provides a rich data source for detailed studies of the solar neighbourhood. Cignoni et al., in their paper on Recovering the star formation rate in the solar neighbourhood demonstrate how the large sample of precise positions, velocities and photometric data facilitates the study of the colour-magnitude diagram for local stars in a statistical sense. Further details are available here.

Elemental abundance survey of the Galactic thick disk
[May 2006]

Precise astrometric data for a sample of nearby, thick-disk candidate stars from the Hipparcos catalogue are combined with accurate radial velocity measurements for these stars to derive kinematic and Galactic orbital parameters. These parameters allow the stars to be better defined as belonging to the thin-disk, thick-disk or halo. Abundances for these stars have been obtained and a detailed analysis of the characteristics of the elemental abundance for the different populations performed. The observed compositions of the thin and thick disks seem to be consistent with models of galaxy formation by hierarchical clustering in a ΛCDM universe. Further details are available here.

Hipparcos data reveals stellar companions to be brown dwarfs
[December 2005]

Using Hipparcos Intermediate Astrometric Data Reffert and Quirrenbach have determined the masses of the outer companions to HD38529 (HIP 27253) and HD168443 (HIP 89844). Radial velocity measurements had determined that these companions were substellar bodies but only lower limits could be placed on their masses. Fitting an orbital solution to the Intermediate Data using known parameters from the radial velocity measurements allowed the authors to determine the masses of the companions. The resulting mass measurements point to the companions being brown dwarfs.

Tracing the Sun's path through the Galaxy - with the help of Hipparcos
[June 2005]

A recent calculation of the Sun's motion through the Galaxy has shown that, during the past 500 million years, the Sun has traversed four spiral arms at times that appear to be consistent with long-duration cold periods on Earth. The calculations are based on the Sun's current motion relative to the local standard of rest as determined from parallaxes and proper motions from Hipparcos, and on a realistic model of the Galactic potential. Integrating back in time over 500 million years allows the Sun's passage through the Galaxy to be tracked. Details can be found in the article Ice Age Epochs and the Sun's Path Through the Galaxy by Gies and Helsel; see also the Gaia Picture of the Week.

Discovery of lost Hipparchus catalogue on the Farnase Atlas
[May 2005]

A study by Bradley Schaefer of Louisiana State University has shown that the constellations depicted on the sky globe of the Farnese Atlas are those of the original Hipparchus star catalogue. Using accurate measurements of the positions of the stars on the sky globe Schaefer determined that the date of the constellations was 125 BC. This date is consistent with Hipparchus having been the observer. In addition, a detailed comparison between the stellar positions on the globe and those recorded in Hipparchus's only surviving work support this assertion.

Christiaan Huygens prize awarded to Hipparcos researcher
[October 2004]

The 2004 Christiaan Huygens Prize for Science has been awarded to Amina Helmi for her ground-breaking work on the formation of our Galaxy, the Milky Way. Galaxies are believed to have been built up from the merging of smaller precursor structures. Using data from the Hipparcos catalogues Dr. Helmi and her colleagues searched for evidence of these precursor structures in the current distribution of stars. They found that about 10 per cent of the metal-poor stars in the Milky Way halo come from a single coherent structure that was disrupted during or soon after the Galaxy's formation. This is clear evidence supporting the theory that galaxies are formed by the amalgamation of smaller assemblies of stars.

ESA's Hipparcos finds rebels with a cause
[October 2004]

A team of European astronomers has discovered that many stars in the vicinity of the Sun have unusual motions caused by the spiral arms of our galaxy, the Milky Way. According to this research, based on data from ESA's Hipparcos observatory, our stellar neighbourhood is the crossroads of streams of stars coming from several directions. Some of the stars hosting planetary systems could be immigrants from more central regions of the Milky Way. Further details can be found in the ESA Science News Release 23-2004 or in the article on this site.

New survey of Solar neighbourhood uses Hipparcos and Tycho-2 data
[January 2004]

An important new survey of our Solar neighbourhood, which includes Hipparcos parallaxes and Tycho-2 proper motions, has been completed. The Geneva-Copenhagen survey provides new determinations of metallicity, rotation, age, kinematics and Galactic orbits for a complete, magnitude-limited, and kinematically unbiased sample of nearby F and G dwarf stars. A number of basic relations of the Galactic disk can be studied with this new data. Details are provided in The Geneva-Copenhagen survey of the Solar Neighbourhood, by Nordstrom et al.

Hipparcos parallaxes and the distance to the Pleiades
[January 2004]

The Pleiades "distance problem" was identified shortly after the publication of the Hipparcos Catalogue in 1997, and remains unresolved. Essentially, the mean cluster distance derived from a number of individual Hipparcos parallaxes is some 10% smaller than the distance determined through main-sequence fitting. A recent Nature paper implies that the Hipparcos results are in error. The situation is not so clear-cut: see article on this web site.

Star Award for the Hipparcos web site
[June 2000]

The Hipparcos Space Astrometry Mission web site has been selected to receive the Griffith Observatory Star Award for July 2-8 2000, for excellence in promoting astronomy to the public through the World Wide Web.

Doubling the Hipparcos star count: the Tycho-2 Catalogue
[updated February 2000]
The Tycho-2 Catalogue, an astrometric and photometric reference catalogue of the 2.5 million brightest stars on the entire sky, was released on February 8th, 2000. The catalogue contains positions, motions, brightness and colours for 2,539,913 stars, more than doubling the number of stars in the original Tycho Catalogue.

When Hipparcos saw the shadow of an alien planet
[updated December 1999]

Astronomers have just realised that news of a planet orbiting a distant star came from ESA's Hipparcos satellite eight years ago, although no one noticed it until now. Analysis of the Hipparcos measurements of the star HD209548, combined with recent ground-based observations, have now provided astronomers with an amazingly accurate measure of the orbital period of the planet. Details of these Hipparcos results will be published in the January 2000 issue of Astronomy & Astrophysics. See also the ESA Science web site for more on this story.

Hipparcos provides evidence for a Milky Way invasion
[updated November 1999]
Astronomers from Leiden Observatory (in the Netherlands) and the Max-Planck-Institut fur Astrophysik (in Munich, Germany) have used Hipparcos proper motion data along with radial velocities and photometric parallaxes to show that about ten per cent of the metal-poor stars in the halo of the Milky Way come from a single coherent structure that was disrupted during or soon after the Milky Way formed. This research is published the November 4th (1999) issue of Nature (Nature Vol. 402 pp53-55: Debris Streams in the solar neighbourhood as relicts from the formation of the Milky Way by A. Helmi et al). See also the ESA Science web site for more on this story.

Hipparcos Scientists awarded the ESA Director of Science Medal
[updated May 1999]

The first recipients of the ESA Director of Science Medal were announced on 20th May 1999 in Bern, Switzerland. In recognition of their outstanding contributions to the Hipparcos mission, medals were awarded to Catherine Turon (Observatoire de Paris-Meudon), Jean Kovalevsky (Observatoire de la Côte d'Azur), Lennart Lindegren (Lund Observatory) and Erik Høg (Copenhagen University Observatory). For further details on the awards see the ESA Space Science report.

"Success Story - 30 Discoveries from ESA's science missions in space": the Hipparcos contributions
[updated May 1999]
A new brochure (May 1999) from the ESA Publications Division (reference ESA BR-147) highlights a selection of discoveries from the European Space Agency space science missions (from 1984 to 1999). The Hipparcos mission figures prominently in the brochure, under the title "Hipparcos-stars in 3D", with features on:

"A riddle solved: older Universe, younger stars";
"New puzzle: key stars are closer than expected";
"Fixed: an amazing star that flashes gamma-rays";
"The Milky Way: it's floppy and it changes shape".

The brochure may also be viewed online at the ESA Space Science site.


Globular Clusters 15 September 1998

Distances and Ages of Globular Clusters using Hipparcos Parallaxes of Local Subdwarfs
R.G. Gratton, E. Carretta and G. Clementini
(Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press)

Abstract: We discuss the impact of Population II and Globular Cluster (GCs) stars on the derivation of the age of the Universe, and on the study of the formation and early evolution of galaxies, our own in particular. The long-standing problem of the actual distance scale to Population II stars and GCs is addressed, and a variety of different methods commonly used to derive distances to Population II stars are briefly reviewed. Emphasis is given to the discussion of distances and ages for GCs derived using Hipparcos parallaxes of local subdwarfs. Results obtained by different authors are slightly different, depending on different assumptions about metallicity scale, reddenings, and corrections for undetected binaries. These and other uncertainties present in the method are discussed. Finally, we outline progress expected in the near future.
This paper is available here from the astro-ph preprint server.

Globular Cluster Distance Determinations
B. Chaboyer
(Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press)

Abstract: The present status of the distance scale to Galactic globular clusters is reviewed. Six distance determination techniques which are deemed to be most reliable are discussed in depth. These different techniques are used to calibrate the absolute magnitude of the RR Lyrae stars. The various calibrations fall into three groups. Main sequence fitting using Hipparcos parallaxes, theoretical HB models and the RR Lyrae in the LMC all favor a bright calibration, implying a `long' globular cluster distance scale. White dwarf fitting and the astrometric distances yield a somewhat fainter RR Lyrae calibration, while the statistical parallax solution yields faint RR Lyrae stars implying a `short' distance scale to globular clusters. Various secondary distance indicators discussed all favor the long distance scale. The `long' and `short' distance scales differ by (0.31+/-0.16) mag. Averaging together all of the different distance determinations yields Mv(RR) = (0.23+/-0.04)([Fe/H] + 1.6) + (0.56+/-0.12) mag.
This paper is available here from the astro-ph preprint server.

The Age of the Universe 15 September 1998

The Age of the Universe
B. Chaboyer
(invited review to appear in Physics Reports)

Abstract: A minimum age of the universe can be estimated directly by determining the age of the oldest objects in the our Galaxy. These objects are the metal-poor stars in the halo of the Milky Way. Recent work on nucleochronology finds that the oldest stars are 15.2+/-3.7 Gyr old. White dwarf cooling curves have found a minimum age for the oldest stars of 8 Gyr. Currently, the best estimate for the age of the oldest stars is based upon the absolute magnitude of the main sequence turn-off in globular clusters. The oldest globular clusters are 11.5+/-1.3 Gyr old, implying a minimum age of the universe of t_universe > 9.5 Gyr (95% confidence level).
This paper is available here from the astro-ph preprint server.

The RR Lyrae Distance Scale 15 September 1998

The RR Lyrae Distance Scale
P.Popowski and A. Gould
(Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press)

Abstract: We review seven methods of measuring the absolute magnitude M_V of RR Lyrae stars in light of the Hipparcos mission and other recent developments. We focus on identifying possible systematic errors and rank the methods by relative immunity to such errors. For the three most robust methods, statistical parallax, trigonometric parallax, and cluster kinematics, we find M_V (at [Fe/H] = -1.6) of 0.77 +/- 0.13, 0.71 +/- 0.15, 0.67 +/- 0.10. These methods cluster consistently around 0.71 +/- 0.07. We find that Baade-Wesselink and theoretical models both yield a broad range of possible values (0.45-0.70 and 0.45-0.65) due to systematic uncertainties in the temperature scale and input physics. Main-sequence fitting gives a much brighter M_V = 0.45 +/- 0.04 but this may be due to a difference in the metallicity scales of the cluster giants and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67 +/- 0.13 and is potentially very robust, but at present is too new to be fully tested for systematics. If the three most robust methods are combined with Walker's mean measurement for 6 LMC clusters, V_{0,LMC} = 18.98 +/- 0.03 at [Fe/H] = -1.9, then mu_{LMC} = 18.33 +/- 0.08.
This paper is available here from the astro-ph preprint server.

The Distances of the Magellanic Clouds 15 September 1998

The Distances of the Magellanic Cloud, by Alistair R. Walker
(Invited review article to appear in ``Post Hipparcos Cosmic Candles'', F. Caputo & A. Heck (Eds.), Kluwer Academic Publ., Dordrecht, in press)

Abstract: The present status of our knowledge of the distances to the Magellanic Clouds is evaluated from a post-Hipparcos perspective. After a brief summary of the effects of structure, reddening, age and metallicity, the primary distance indicators for the Large Magellanic Cloud are reviewed: The SN 1987A ring, Cepheids, RR Lyraes, Mira variables, and Eclipsing Binaries. Distances derived via these methods are weighted and combined to produce final "best" estimates for the Magellanic Clouds distance moduli.
This paper is available here from the astro-ph preprint server.

International Astronomical Union: 23rd General Assembly, Kyoto

At the 23rd General Assembly of the IAU in Kyoto, Japan (17-30 August 1997) early results from the Hipparcos and Tycho Catalogues were presented at a Joint Discussion session (JD14). Among the topics discussed were the impact on stellar physics of luminosity and age calibrations, primary distance scale indicators, and future prospects for micro-arcsecond astrometry. Details will be published in the IAU `Highlights of Astronomy' (session editor: Catherine Turon, Observatoire de Paris-Meudon).

The Hyades Cluster

The distance, structure, dynamics and age of the Hyades cluster have been studied with Hipparcos data by Perryman et al. (Compressed PostScript or PDF versions available online.) The spatial structure of the cluster is now for the first time mapped in 3 dimensions from directly measured distances (parallaxes) for each individual member of the Hyades. An mpeg movie, prepared by Dr A.G.A. Brown illustrates the 3-d structure of the cluster.

The HIPPARCOS Venice '97 Symposium

In May 1997 a symposium was held in Venice, Italy, to launch The Hipparcos and Tycho Catalogues and to present some early results from the Hipparcos mission. The Proceedings of this symposium were published by ESA as ESA SP-402 in September 1997 and are now available on-line.

The Distances and Evolution of Mira Variables from Hipparcos. (14 February 1997)

The cool, large amplitude, long period variables (Miras) lying at the tip of the Asymptotic Giant Branch (AGB) hold the key to understanding this still puzzling phase of stellar evolution. Although the narrow period-luminosity (PL) relation shown by Miras in the LMC strongly suggests that they should be good distance indicators, it has been difficult to test this idea on stars within our own Galaxy. The first release of Hipparcos trigonometrical parallaxes for 12 of these stars when combined with angular diameters measured interferometrically for some of them yields the quite unexpected result that whilst those with periods less than 400 days are pulsating in the first overtone, two with periods over 400 days are pulsating in the fundamental mode. This is clearly a significant, though not yet properly understood, clue to AGB-star evolution. The Hipparcos parallaxes of the Miras combined with ground-based infrared photometry give a zero-point for the PL relation (overtones) which yields a distance for the LMC in good agreement with the Hipparcos Cepheid scale. (F van Leeuwen, MW Feast, PA Whitelock and B Yudin, MNRAS, 287 , 955-960, 1997: " First Results from Hipparcos Trigonometrical Parallaxes of Mira-Type Variables" )

The Cepheid Distance Scale (14 February 1997)

Since the discovery of the period-luminosity relation for Cepheids at the beginning of this century, the calibration of its zero-point has been a major goal, of primary importance for both galactic and extragalactic distance scales. Hipparcos observations now enable this zero-point to be derived for the first time from the trigonometrical parallaxes of Cepheids themselves. The quality of the Hipparcos data is such that despite the great distances of even the nearest of these stars the zero-point has been established with a distance uncertainty of only five percent. The results indicate the need for an increase in the commonly adopted distance scale by about 10 percent. Calibration of RR Lyrae absolute magnitudes based on the Cepheid scale indicates a relatively young age for globular clusters (about 11 Gyr). An interesting additional result is that the nearest Cepheid (Polaris) is pulsating in the first overtone (M.W. Feast and R.M. Catchpole, MNRAS, 286 , L1 - L5, 1997: " The Cepheid PL Zero-Point from Hipparcos Trigonometric Parallaxes" )

The Guide Star Catalogue for the ISO Observatory (November 1996)

The Hipparcos and Tycho Catalogues were used as the source catalogues for the compilation of the ESA Infrared Space Observatory (ISO) Guide and Calibration Star Catalogue in 1996. This catalogue provides astrometric and photometric data for stars down to magnitude 9.0 all over the sky, and defines several classes of guide star according to multiplicity, variability and isolation criteria. Using this catalogue, the ISO satellite operations have no longer encountered problems with pointing or tracking (some observation time had been lost previously as a result of essentially inaccurate photometric information). Preliminary distances of a subset of Hipparcos stars were also made available to the ISO observatory for instrumental calibration purposes. For further details of the ISO observatory, see the ISO Home Page.

Upper Limits on Masses of Planets Around Nearby Stars (November 1996)

Distances to the recently-discovered planetary system candidates, 47 UMa, 70 Vir and 51 Peg, were determined by M.A.C. Perryman et al. ( "Hipparcos Distances and Mass Limits for the Planetary System Candidates: 47 UMa, 70 Vir, and 51 Peg" , 1996, Astronomy & Astrophysics, 310, L21) based on absolute trigonometric parallaxes from Hipparcos. Standard errors of the parallaxes are in the range 0.66-0.76 milliarcsec, with resulting distances accurate to around 1 per cent. For 70 Vir, the Hipparcos parallax (55.22+/-0.73 milliarcsec) resolves a discrepancy of almost a factor of three in published trigonometric and photometric parallaxes. The residuals of the astrometric parameters were used in combination with the radial velocity data for these orbital systems to place upper limits on the companion masses of between 7-22 and 38-65 Jupiter masses for 47 UMa and 70 Vir respectively, with less stringent limits for 51 Peg. The Hipparcos data therefore provide confirmation of the existence of sub-stellar masses significantly below the brown dwarf limit (of about 0.08 solar masses) surrounding other stars. For the potential of astrometry in the search for planetary systems, see M.A.C. Perryman et al. ( "Searches for Planets Beyond our Solar System: How Astrometry Helps" , 1996, ESA Bulletin 87, 65).

A By-Product is an Elegant Confirmation of General Relativity (November 1996)

The Hipparcos data have been reduced within a relativistic framework, including accounting for gravitational light-bending by the Sun (and Earth). Unlike previous determinations of light-bending, either in the optical or in the radio (see, e.g., M.H. Soffel 1989, Relativity in Astrometry, Springer-Verlag) the regions over which the effect is significant for Hipparcos are no longer restricted to a few solar radii, but extend to most of the celestial sphere. General relativistic light bending amounts to 1.7 arcsec at the solar limb, but is still very significant - about 4 milliarcsec - even at right angles to the solar direction. A value of the PPN light-bending term, gamma, of 0.992+/-0.005 was derived from the NDAC results, and 1.000+/-0.004 from the FAST results (The Hipparcos and Tycho Catalogues, Volume 3, Chapter 16).

Assessment of the Quality of Ground-Based Star Catalogues (September 1996)

A detailed comparison of the Hipparcos Catalogue with positions and proper motions from the FK5 and PPM Catalogues and others can be found in L. Lindegren et al. (1995, Astronomy & Astrophysics, 304, 44). The most rigorous external verifications of the astrometric parameters are given by comparisons with (a) the USNO (optical) Mk III interferometer and VLBI interferometry for positions; (b) the FK5 Catalogue for proper motions; and (c) a variety of possible zero-point determinations for parallaxes as described by F. Arenou et al. (1995, Astronomy & Astrophysics, 304, 52). These investigations indicate that the formal standard errors given in the Hipparcos Catalogue are likely to be very close to the true external errors of the derived astrometric parameters. Detailed comparisons with the positions given in the FK5 Catalogue are provided in The Hipparcos and Tycho Catalogues, Volume 3, Chapter 19.

The Quality of Ground-Based Meridian Circle Instruments (September 1996)

Y. Requieme et al. (1995, Astronomy & Astrophysics, 304, 121) report the re-analysis of meridian circle observations (from the Bordeaux and Carlsberg instruments) using preliminary positions from the Hipparcos Catalogue, compared with reductions made using the FK5 Catalogue. A significant decrease in the residuals is found in both right ascension and declination. The form of the residuals in right ascension as a function of declination differ between the Carlsberg and Bordeaux instruments, and amount to as much as 30 milliarcsec, pointing to the fact that the origin of the differences cannot lie within the Hipparcos Catalogue, but arises rather from small defects in the Bordeaux and Carlsberg instruments or their calibration, masked up until now by the errors in the astrometric reference positions. Different signatures in declination of up to 50 milliarcsec suggest that modelling of refraction may be improved once better reference star positions become widely available.

Photographic Plate Reductions using Hipparcos Positions (September 1996)

A variety of photographic plate reductions have already been carried out using preliminary Hipparcos positions; these confirm the previously-held suspicions that the limited precision of the available reference catalogues has compounded the difficulties of determining the proper choice of plate model. For example, I. Platais et al. (1995, Astronomy & Astrophysics, 304, 141) have undertaken a preliminary analysis of the plates from the Yale/San Juan Southern Proper Motion program, using the preliminary 30-month Hipparcos Catalogue, H30, to provide a reference system with negligible random errors. They have inferred the presence of a consistent magnitude equation and certain significant cubic terms, concluding that the Hipparcos positions offer a very powerful tool for detecting systematic errors in wide-field photographic astrometry. Further details of the reduction of Schmidt plates have been presented by N. Robichon et al. (1995, Astronomy & Astrophysics, 304, 132).

The Hertzsprung-Russell Diagram & Stellar Evolutionary Theories (September 1996)

One of the primary goals of the Hipparcos mission was to furnish high quality trigonometric parallaxes for tens of thousands of stars, in order to refine the detailed structure of the observational Hertzsprung-Russell diagram and to extend the determination of absolute magnitudes to stars significantly more luminous than about Mv="0" mag. With a significant fraction of parallaxes having standard errors below 1 milliarcsec, and systematic errors at 0.1 milliarcsec or better, distance estimates to many tens of thousands of stars in the Hipparcos Catalogue within 100 pc now have an accuracy of better than 10 per cent. A dramatic indication of the quality of the parallaxes is given by the HR diagram constructed from the preliminary 30-month Hipparcos Catalogue, H30. A presentation and discussion of this diagram is given by M.A.C. Perryman et al. (1995, Astronomy & Astrophysics, 304, 69).

Hipparcos Relates the Optical and Radio Emission from SN1987A (September 1996)

Hipparcos positions have been used by J.E. Reynolds et al. (1995, Astronomy & Astrophysics, 304, 116) to allow registration of high-resolution optical and radio images of SN1987A to the 100 milliarcsec level. The significance of the problem is illustrated by the radio-optical overlay published by L. Staveley-Smith et al. (1993, Nature, 366, 136) which shows a 0.5 arcsec displacement between the radio and optical centroids. In a careful succession of reference frame links, Reynolds and colleagues have been able to show that this mis-registration was the result of an inadequate (optical) astrometric reference frame, at least in the vicinity of SN1987A; they are thus able to conclude that the radio emission originates from the interaction between the whole of the expanding shock wave and the surrounding medium.

Ephemerides of Cometary Encounter with Jupiter ... (September 1996)

Preliminary positions from the first iterations of the Hipparcos Catalogue were circulated to groups requiring time-critical access to improved astrometric data. The Hipparcos results were used by ESO observers to improve the prediction of the time of impact between Comet Shoemaker-Levy 9 and Jupiter (West & Hainaut, private communication).

...and Asteroids (September 1996)

Hipparcos star positions were used to assist navigation of the Galileo satellite for its encounter with the asteroid Ida. Details (and a picture of the asteroid) are given in the ESA Bulletin No. 77, page 143; see also W.M. Owen & D.K. Yeomans, 1994, Astronomical Journal, 107, 2295 for background details.