Hipparcos and Tycho catalogs
The Hipparcos and Tycho Catalogues are two astrometric star catalogues that have played a pivotal role in astronomy since their publication (ESA, 1997).
The Hipparcos Catalogue contains highly accurate astrometry for some 118000 stars all over the sky: their positions, their yearly proper motions, and their parallaxes; all with milliarcsecond precision. The positions are important for establishing a system of reference points on the sky, the proper motions are important for understanding the motions of the stars among each other, and the parallaxes form the basis for the distance scale of the Milky Way galaxy, which is then the basis for knowing the distance scale of the whole universe. The Hipparcos Catalogue is not only the most accurate star catalogue, but has represented the largest accuracy leap ever achieved in the history of astrometry.
For a definition of proper motion, parallax, and arcsecond, please see the article on astrometry.
The Tycho Catalogue from 1997 of one million stars has been superseded by the Tycho-2 Catalogue from 2000, which extends Hipparcos to more than 2.5 million stars. It is less precise than Hipparcos, and it does not contain parallaxes, but it gives positions and proper motions, as well as the colours of the stars.
The two catalogues contain the results of the European Space Agency (ESA), mission HIPPARCOS, i.e. HIgh Precision PARrallax COllecting Satellite. The name of the mission is an acronym, but also alludes to the Greek astronomer Hipparchos. Without invoking any acronyms, The Tycho Catalogue is simply named after the Danish astronomer Tycho Brahe.
The project involved several hundred people from dozens of scientific institutions and industrial contractors. The scientific part of this enormous effort was lead by M.A.C. Perryman, ESA project scientist; E. Høg, University of Copenhagen; J. Kovalevsky, Côte d'Azur Observatory; L. Lindegren, Lund Observatory; and C. Turon, Paris Observatory. Many more made very significant contributions, but it is beyond the scope of the present article to go into details.
A fundamental problem in astronomy is to measure distances in the universe. For stars in our neighbourhood, distances can be determined due to the very slight variation, the parallax, in the direction to the star when seen from different points of the orbit of the Earth around the Sun. ESA launched the Hipparcos satellite in 1989 with the specific purpose of obtaining very accurate astrometry, in particular parallaxes. The selection of 118000 stars among proposals submitted by astronomers from all over the world was a complex task, partly based on the importance of the stars for improving our understanding of our Milky Way galaxy, and partly to obtain a uniform grid of reference stars on the sky. Several iterations and a large number of ground-based observations were necessary to obtain a satisfactory list of stars, published as the Hipparcos Input Catalogue. The final selection includes all stars we can see with the naked eye (some 5000) and is even complete to stars a few times fainter, while the rest have more specific purposes. It was for example important to include stars of many different types, because only when accurate distances are known, do we also know the amount of energy a certain kind of star is emitting. Without the distance, we only know the amount of energy we receive at the Earth, but not if the star is distant and luminous or if it is closer and less luminous. This is essential information for understanding the evolution of the stars. It was also important to include stars belonging to various groups and clusters, in order to find the distances to such relatively nearby clusters. A star cluster consists of stars of the same age and same chemical composition, but different masses. Studying star clusters therefore gives important information on stellar evolution. When we understand the properties of nearby clusters, we can use our knowledge to infer the distances to a more distant cluster only from the colours and brightnesses of the stars in that cluster.
Distances from Hipparcos
Hipparcos managed to reach accuracies around 1 milliarcsecond for most stars. This means distances accurate to better than 20% for stars within about 200 parsec, and better than 10% for stars within about 100 parsec (about 300 light years). This is enough to serve as a stepping stone to larger distances, but it is important to realise that it is only a stepping stone. The distance to the centre of the Milky Way is more than 8000 parsec, and the nearest major galaxy, the Andromeda galaxy, is roughly 80 times further away than that. The central regions of our own galaxy, not to mention other galaxies, are therefore out of reach for Hipparcos, and their distances can only be determined by indirect methods.
Principles of Hipparcos
Why from space
It is a golden rule of space science that you only do things from space that it is practically impossible to do from ground. Astronomers have been doing astrometry for thousands of years, so why go into space? The thing is that by the mid 20th century, the limiting factor for progress in astrometry was the jittering motion of the star images that is caused by the atmosphere. Going outside the atmosphere would suddenly remove this problem and also remove any gravitational bending of instrument parts. Proper motions can be measured from ground, but you need a lot of patience because the motions are very small. Highly precise parallaxes can in some cases be measured from ground, but doing something like the Hipparcos catalogue from ground is literally impossible.
How was it done
To do an accurate mapping of the sky, you need to measure large angles with high precision. From ground this was typically done with highly specialised telescopes equipped with scales, principally meridian circles, allowing you to read the angle turned between two observations. To achieve something similar from space, a new and rather elegant solution was employed. Hipparcos had not just one telescope, but two, looking in directions 58 degrees apart. The satellite was constantly rotating in such a way that the two telescopes were seeing the same belt of the sky during a rotation period. The focal plane contained an accurately manufactured grid, and the observation consisted in timing the appearances and disappearances of the star image as it passed this grid. The focal plane was common for the two telescopes, and the detector was constantly switching its attention between the several stars from the observing list that were visible at the same time. At any one time, Hipparcos was therefore in fact measuring distances not only between neighbouring stars, but also between stars in fields of the sky separated about 58 degrees. The rotation axis was kept at a fixed angle to the Sun, and slowly describing circles around the Sun. In this way all the stars on the observing list were observed many times throughout a little more than three years. They were observed at many different times of the year, and with many different orientations of the axis, and each time with different areas of the sky in the other field of view. A careful mathematical analysis of these several million observations then gave the astrometric parameters for the stars. In the early 1990s this was far from being a trivial task, and a large collaboration of astronomers all over Europe worked hard for many years to obtain the Hipparcos catalogue.
From the observations alone, you can determine the positions and motions of the stars with respect to one another, but not fix the common orientation and rotation of the whole set of stars. For this purpose Hipparcos observed a number of stars with radio emission, where accurate astrometry had been obtained from the ground with radio telescopes. At radio wavelengths, the atmosphere is less troublesome, and when combining simultaneous observations with radio telescopes thousands of kilometres apart, accuracies even better than Hipparcos can be achieved.
Later improvements to Hipparcos
Being good does not mean being perfect, and several minor improvements to the Hipparcos analysis have been published. As Hipparcos only observed stars from a fixed list, the stellar contents cannot be increased, but various small imperfections in the data analysis can be amended. An important work is the analysis primarily of slight variations in the rotation speed of the spacecraft, carried out by F. van Leeuwen (2007) at University of Cambridge. He thereby managed to improve the accuracy of the Hipparcos results, especially for the brightest stars. The effort of one man armed with a laptop, but of course building on the foundations of an enormous collaboration.
The Tycho project
The star mapper
The Hipparcos focal plane was equipped with a system, called the Star Mapper, for determining the orientation of the instruments at any moment during the observations. The orientation had to be known with sufficient accuracy to do the star measurements correctly, as they depended on switching the detector to follow a very small area of the sky where the star was expected to be found. Later, during the data analysis on ground, the orientation could be determined far more accurately, from the Hipparcos observations. The Star Mapper was a system of slits of different orientation and mutual distances in an otherwise opaque plate, and behind the slits a light detector. A star image passing the slits would then give a signal with a telltale pattern, revealing the precise location in the focal plane where this star was crossing. Knowing in advance the position of the star on the sky, the pointing of the satellite was easily derived with the required precision of about one arcsecond. In practice, stars from both telescopes had to be used.
In the first plans, the Star Mapper only had this role of a guide, but it was decided during the mission planning to increase the telemetry rate sufficiently to send the full set of Star Mapper data to ground. In addition, the Star Mapper was upgraded to register light in two different wavelength bands, allowing the colour of the stars to be measured. This data set was then used to make the Tycho Catalogue of, initially, one million stars. It was not as accurate as the Hipparcos Catalogue, but better than similar catalogue made from ground, and an important extension of the project, with the additional advantage of being directly tied to the coordinate system defined by Hipparcos.
Later improvements to Tycho
Already during the mission, plans were laid for combining the Tycho positions with the century old Astrographic Catalogue in order to determine the motions of this set of stars accurately. Preparing the Astrographic Catalogue for this was an enormous task as many thousand pages of tables had to be key punched and the data analysed in detail. This work was undertaken by Sternberg Astronomical Institute in Moscow, which was not directly involved in the Hipparcos mission otherwise. At the same time a similar project was in progress at the United States Naval Observatory, and due to an unfortunate lack of communication and coordination, two slightly different catalogues were eventually published: The ACT (Astrographic Catalogue and Tycho) and the TRC (Tycho Reference Catalogue).
Another plan laid during the mission was to analyse the Tycho data with more advanced methods, which were coming within reach due to the rapid progress in still faster and still cheaper computing power. It was again foreseen to publish a catalogue purely based on space data and without proper motions, called Tycho2, and later include ground based work in a compiled catalogue, TRC2, with proper motions primarily based on the Astrographic Catalogue. The prospect of having a Tycho2, a TRC2, as well as perhaps an ACT2, was not encouraging, and efforts were instead joined. The result was the Tycho-2 catalogue of 2.5 million stars. The final catalogue contains both the positions derived solely from the Tycho data, and also mean positions and proper motions based on the inclusion of 143 ground based catalogues.
A common way to measure the impact of a scientific paper is to see how often it is quoted in other scientific papers. If a paper is quoted often, it is surely important (the reverse, on the other hand, need not be true). When the journal Astronomy and Astrophysics celebrated its 40th anniversary in 2009, it published a special issue reprinting the 40 papers through the years with the highest number of citations. Both the letter announcing the completion of the Hipparcos catalogue and the letter announcing Tycho-2 were included in that issue. Also the public astronomical data bases are constantly interrogated for Hipparcos and Tycho data.
The future of space astrometry
Hipparcos was the first space astrometry mission, but it will certainly not be the last. Presently two missions are approaching launch, the Japanese nano Jasmine, and the European Gaia. Planned missions in the United States include JMAPS and SIM Lite.
Nano Jasmine, to be launched in August 2011, is a proof of principle mission for the much more ambitious Jasmine mission. It will use infrared detectors, but has telescopes of only 5 cm aperture, and will therefore only observe the brightest stars.
Next for launch is Gaia, scheduled for November 2012. It will observe the brightest more than 1000 million stars, with accuracies of 10 microarcseconds for the brighter sources, decreasing to about 300 microarcseconds (0.3 milliarcseconds) at the faint end. The brightest about 5000 stars will not be observed as they will saturate the detectors, and nano Jasmine will therefore form an important complement to Gaia.
- Fabricius, Claus et al. (2002). The Tycho double star catalogue A&A 384: 180-189. 
- Høg, Erik et al. (2000). The Tycho-2 catalogue of the 2.5 million brightest stars A&A 355: L27-L30. 
- Høg, Erik et al. (2000). Construction and verification of the Tycho-2 Catalogue A&A 357: 367-386. 
- Kovalevsky, Jean et al. (1997). The Hipparcos Catalogue as a realisation of the extragalactic reference system A&A 323: 620-633. 
- Lindegren, Lennart et al. (1997). Double star data in the Hipparcos Catalogue A&A 323: L53-L56. 
- Perryman, Michael A C et al. (1997). The Hipparcos Catalogue A&A 323: L49-L52. 
- Perryman, Michael A C et al. (1997). The Hipparcos and Tycho Catalogues SP-1200, vols 1-17 ESA Publications Division, Noordwijk.
- Turon, Catherine et al. (1992). The Hipparcos Input Catalogue. I- Star selection A&A 258: 74-81. 
- Turon, Catherine et al. (1992). The Hipparcos Input Catalogue SP-1136, vols 1-7 ESA Publications Division, Noordwijk.
- van Leeuwen, Floor (2007). Hipparcos, the New Reduction of the Raw Data Springer, Heidelberg.
- Perryman, M (2009). Astronomical Applications of Astrometry Cambridge University Press, Cambridge. ISBN 978-0-521-51489-7.
- Perryman, M (2010). The Making of History's Greatest Star Map Springer, Heidelberg. ISBN 978-3-642-11601-8
- The ESA Gaia website
- The ESA HIPPARCOS website
- The ESA Tycho-2 page
- Hipparcos download, new reduction
- Tycho-2 download