The fourth edition of "The General Catalogue of Trigonometric Stellar Parallaxes," recently published by the Yale University Observatory, is not exactly the type of light reading most people would take to the beach. Astrophysicists who are trying to unravel the mysteries of stellar evolution and the size and age of the universe, however, will probably find the catalogue, which lists the precise measurements to 8,112 stars, to be compelling reading.
Considered a standard reference book, the expanded catalogue marks the continuation of a Yale tradition of surveying star locations that dates back to 1924. That was the year astronomer Frank Schlesinger published the first parallax catalogue in which opposite points on the earth's annual orbit around the sun were used for measuring star distances.
The catalogue's fourth edition has 27 percent more stars than the third edition, published in 1963, according to its editor, astronomy professor William F. van Altena. He and retired senior research astronomer Dorrit Hoffleit began the project 18 years ago and were recently joined by John T. Lee, Yale associate in research astronomy. Funded by the National Science Foundation, the catalogue represents the measurements from about 500,000 photographic plates made in the last century and incorporates data from observations dating back to 1855.
"This is extremely important, time-consuming work that is at the basis of all we know," said Sabatino Sofia, chair of Yale's astronomy department. "It's solid knowledge, not glamorous knowledge."
Finding a star's parallax requires noting a single star's position on two nights six months apart. During that time, the earth revolves to the opposite side of the sun, a distance of 149,600,000 kilometers, and the star appears to have shifted in relation to background stars. The same effect can be achieved by viewing your index finger at arm's length by closing first one eye and then the other, causing the finger to appear to shift in relation to the wall behind it. The closer the object, the greater the shift --parallax. By using the opposite sides of the earth's orbit as points of a triangle, with the third point being a distant star, and using a basic mathematical formula, the distance to the star can be calculated.
The method works only for stars within a distance of about 1,000 light years, beyond which point the parallax shift is too small to measure. However, stars in the parallax catalogue can be used as yardsticks to gauge distances to stars at the fringes of the universe, based on their relative brightness compared to parallax stars, explains Professor van Altena.
"From that information, it may be indirectly possible to calculate the rate of expansion of the universe, how long it has been expanding, and whether that expansion rate is slowing," he said. "We also give data on how quickly two stars are orbiting around each other in a binary star system and miscellaneous information to aid in determining the reliability of the data."
The accuracy of distance measurements for the newly added stars is considerably better than in previous editions, he adds. "For each star, we average several different measurements made over a period of years to increase the accuracy. We also list equatorial coordinates, brightness measurements, spectral data that correlates with temperatures and atmospheric pressures, and cross identifications with other catalogues."
Printed copies of the two-volume catalogue are available from the Yale University Observatory, and the computer-readable version will be available soon from the Astronomical Data Center --ADC at Goddard Space Flight Center in Maryland, and from the Strasbourg Stellar Data Center --CDS in France. In 1991 the ADC issued a CD ROM disk that contained a preliminary version of the catalogue.