UK Astronomers Help To Rechart The Heavens
Almost 300 scientists from all over the world will be gathering in Venice this week to present startling new evidence which will revolutionise our understanding of the scale of the heavens. Among those offering new insights into the size of the universe and the distances of stars and galaxies will be astronomers from a number of UK observatories and universities.
The exciting new results include the discovery that many so-called 'local' stars and galaxies are not where we thought they were at all. Some are nearer than previously believed, while others are much further away. Similar evidence will also be presented for some of the moons of Jupiter and Saturn. In addition, astronomers will provide confirmation that the universe is perhaps 5-10% larger and older than suggested by earlier measurements.
The Venice '97 Symposium will take place from 13-16 May. It has been called to bring together the results from Europe's unique astrometry mission, Hipparcos, and coincides with the publication of the first accurate censuses of the stars in our Galaxy. The Hipparcos Catalogue contains information on the positions of 118,218 stars while the Tycho Catalogue gives rather less precise information on a further 1,038,332 stars.
A major surprise involves a catalogue of about 3000 'nearby' stars which were all thought to lie within some 75 light years of the Sun. Hipparcos data indicate that some 20 to 30 percent of these stars are much further away than this limit, putting in doubt some of the other indirect ways currently used to calibrate stellar distances. This result may have serious implications for our understanding of the galactic distance scale.
Andrew Murray, formerly of the RGO, and collaborators made a similar selection of stars thought to be within 250 light years. New results to be presented in Venice indicate that only about half of the original group are actually within this distance. A discussion of the dynamics of these stars will be presented by Dr. Margaret Penston of RGO and Dr. James Binney of Oxford University.
In addition, a group led by Dr. Michael Perryman of ESTEC / Leiden Observatory has obtained very significant improvements in our knowledge of the nearby Hyades star cluster. A distance estimate almost 10 times more accurate than that obtained from the ground is now available, and various uncertainties in the earlier ground-based determinations can now be eliminated.
Closer to home, Hipparcos also measured the positions of two moons of Jupiter, Europa and Callisto, and Titan, the largest moon of Saturn, as well as some 40 minor planets. The measurements of the moons analyzed by Dr. Leslie Morrison of RGO and others show that small discrepancies between available ephemerides (predicted positions) and observations as made by the Carlsberg Meridian Circle are confirmed by the Hipparcos data. These results will be of crucial importance in the planning of interplanetary space missions. Similar observations by the Carlsberg telescope were used to direct the Galileo spacecraft close to the asteroids Ida and Dactyl en route to Jupiter.
Some of the results have already been presented in preliminary form at an RAS meeting in London last February. Updates on these will be given at the Venice Symposium. They include the first parallax-based calibration of the positions of Cepheid variable stars by Dr. Michael Feast (University of Cape Town) and Dr. Robin Catchpole (Royal Greenwich Observatory), and of Mira variables by Drs. Floor van Leeuwen (RGO), Michael Feast and Patricia Whitelock (SAAO). These studies have already shown that two neighbouring galaxies known as the Magellanic Clouds seem to be some 5 percent further away than was previously thought - 171,000 light years for the Large Magellanic Cloud instead of 163,000. By extrapolation, the data also suggest that the universe is between 10 and 12 billion years old - about 5 percent older (and larger) than previously estimated.
Cepheids and Miras are important stepping stones on the extra- galactic distance scale because there is a direct relationship between the rate of their fluctuation and the brightness of these natural 'candles' - a so-called period-luminosity relationship. The brightness of the stars will obviously diminish with distance. So, once the distances of a few Cepheids are known, the distances of all the others can be accurately calculated.
Further research by Dr. van Leeuwen has produced one of the most unexpected results: the finding that the Pleiades cluster lies some 15 percent closer to the Sun than had been assumed over the past 50 years. This means that stars in this cluster have a relatively low luminosity for their temperature. The reasons behind this phenomenon are still very unclear, and the only explanation currently available, a much increased helium abundance for all stars in this cluster, appears rather unlikely. Measurements from spectra of early type stars in this cluster will be needed to clear up the mystery.
The Hipparcos parallax data will also provide the first serious possibility to obtain more detailed calibrations of stellar luminosities. The understanding of how the brightness of stars is affected by chemical composition, stellar rotation and age will no longer have to rely almost entirely on theoretical models. This will help defining much better boundaries for the models of stellar structure and stellar evolution.
Data Reductions and the role of the UK
The data reductions for Hipparcos were the responsibility of two independent data reduction consortia, NDAC (based in Sweden, Denmark and the UK) and FAST (based in France, Italy, Germany and the Netherlands). Both consortia had the task of providing catalogues of fully reduced data within 2 years of the end of the mission.
As a member of the NDAC consortium, the Royal Greenwich Observatory (RGO) had the responsibility for the first photon count analysis, the attitude reconstruction and the photometric analysis. The RGO group developed the software for this from 1982 until final implementation 1991-1993. In addition, they carried out the final merging of the NDAC and FAST photometric reduction results, and the variability analysis for all 120,000 stars that were measured.
The UK was also involved with the preparation of the pre-flight Hipparcos Input Catalogue (RGO and the Institute of Astronomy, Cambridge) and the final transformation of the Hipparcos reference frame to the radio reference frame (provided by the Merlin radio telescope array). One instrument that made particularly important contributions at this stage was the Carlsberg Meridian Circle, operated on La Palma by Denmark, the UK and Spain.
Notes1. THE HIPPARCOS MISSION Hipparcos is unlike any other space mission in two main respects. Firstly, all of the objects to be studied were decided upon before launch, with all programme stars in the 'Input Catalogue' being observed at regular intervals through the mission. Secondly, the entire, complex data stream was reduced independently by two separate consortia of scientists.
The mission started badly with the failure of the Apogee Boost Motor which placed the satellite in the wrong orbit. This was a highly elliptical path, requiring much greater input from ground control, additional ground stations and adaptations of some of the data reduction software. The data return was significantly better than expected due to the efforts of the operation teams at ESOC and ESTEC and to the data reduction consortia.
Hipparcos was built by the European Space Agency. It was launched in August 1989 and remained operational until March 1993. The aim of the mission was to measure very accurately the positions of more than 100,000 stars and some solar system objects.
2. ASTROMETRY: PARALLAX AND PROPER MOTION One of the key methods was to measure the apparent shift in location of each star against the celestial background as seen from the Earth (and Hipparcos) at different times of year. Just as a finger held in front of one's face seems to shift position as first one eye and then the other is opened and closed, so stars also seem to shift position when seen from different viewpoints. This is known as parallax.
If we know the distance between both eyes and the angle from each eye to the finger, we can calculate the finger's position by triangulation. Similarly, since we know the distance between the Earth and the Sun (93 million miles or 150 million km) and the parallax angles for each star, we can work out the distances to the stars.
Another important yardstick can also be determined from Hipparcos data - the proper motions of the stars. The proper motion is the real movement of the star across the sky and is a partial reflection of its motion through space relative to the Sun. Usually, the further the star, the smaller its proper motion.
Astrometric measures are of fundamental importance in many areas of astronomical study. For example, they allow astronomers to learn about the structure of our Milky Way galaxy, and to calculate the distance scale of the universe.
3. IMPORTANCE OF ASTROMETRY FROM SPACE Astrometric measurements are usually very hard to make since their accuracy is severely limited by the Earth's atmosphere. Another problem is that stellar parallaxes are very small since the stars are so far away. From the ground the most accurate measurements have been to a precision of about 8 milli-arcseconds. These problems were overcome when Hipparcos went into orbit. The satellite succeeded in fixing stellar positions to within 1 - 2 milliarcseconds (equivalent to measuring the height of a child standing on the moon). As a result, the number of stars for which a direct measurement of distance is known has dramatically increased.
4. THE VENICE SYMPOSIUM The Venice '97 Symposium is co-sponsored by Matra Marconi Space and Alenia Spazio, the industrial architects of the Hipparcos satellite, and will be held at Fondazione Cini on the island of San Giorgio under the auspices of the city of Venice.
During the Symposium, participants may be contacted by calling (+39) 41528-9900.
Outside Symposium dates, UK contacts are:
A daily summary of the Symposium presentations will be posted on the Internet at:
or under the What's New section of ESA's science pages at:
Dr Jacqueline Mitton, RAS Public Relations Officer
Phone: Cambridge ((0)1223) 564914
FAX: Cambridge ((0)1223) 572892
Peter Bond, Space Science Advisor,
Phone: Cranleigh ((0)1483) 268672
Fax: Cranleigh ((0)1483) 274047