Numbering the stars

After 44 years of research at an observatory in the foothills of the Andes, astronomers are nearing the end of a monumental undertaking: tracking the motion of 100 million stars.

The Yale/San Juan Southern Proper Motion project, led by William van Altena, a professor emeritus of astronomy, has cataloged the proper motion of the celestial objects that light up the sky of the southern hemisphere. But the project’s completion comes at a time when interest in astrometry — the science of measuring the position and motion of stars — is rapidly dwindling in the United States.

“Bill van Altena has dedicated his entire career to doing what many consider a not-so-glamorous but fundamentally important job,” said Terry Girard, a research scientist who has worked with van Altena on the Southern Proper Motion project for the past 23 years. “Unfortunately, this field of study, astrometry, has become less well-supported — in this country at least — at a time when it is becoming more relevant than ever.”


Research for the SPM project began in 1965 at the El Leoncito Observatory (later the Dr. Carlos U. Cesco Observatory) in Argentina with an agreement between Yale and the National University of San Juan, in which the UNSJ agreed to pay the support staff salaries and Yale agreed to pay capital costs and the observer’s salary.

After the death of the first project director, Dirk Brouwer, in 1966, Yale astronomer Adriaan Wesselink led the team until van Altena was recruited in 1974.

Over the next three decades, van Altena’s team, which included an average of six to 10 people at a time, used the observatory’s astrograph to photograph the southern sky.

Girard said the motion of the stars was found by comparing older images of the star fields with more recent ones — measuring the position of a star relative to other stars and to background galaxies and quasars. The resulting measure of the angular motion of the stars was measured in milliseconds of arcs per year — which is why the project took more than four decades.

But though the nature of the research required that the project be conducted over several decades, van Altena and his colleagues in the Department of Astronomy agreed that such a lengthy project is rare in the world of astronomy.

“It has become extremely unusual to conduct research on one project for 40 years,” astronomy professor Daisuke Nagai said. “But some of the important astronomical research, such as measurements of proper motions of stars in our Milky Way Galaxy led by Professor van Altena, do require a lifelong effort.”

A bump in the road

Still, the research should have been finished a decade ago, van Altena said. But when Kodak stopped making the 17-by-17–inch photographic plates used to capture the images of the stars in the mid-1990s, the project struggled to raise the money from the National Science Foundation to purchase the new technology — charge-coupled devices, or CCDs, the chips used in digital cameras to store pictures — they needed to continue photographing the stars.

The team also lost time adjusting to the new CCD and dealing with the added complications that came with its small size. Although the project’s CCD is the largest in Argentina, at 2 ½-by-2 ½ inches, it takes 50 to 60 images from the CCD to equal one photographic plate.

“The new detectors are 100 times faster, but because they are so small, in the end, you have the problem of having to combine all the individual images,” van Altena said. “It increases the complexity enormously. We will get better results, but it’s not the optimum detector for this project.”

A compilation of the data, which van Altena said was the best available of the southern sky, will be made available online early this summer, accompanied by a written description of the project in the Astronomical Journal. The online catalog will show the position, motion, brightness and colors of more than 100 million objects in the Milky Way Galaxy.

The half-dozen astronomers interviewed said the project had significant implications for future research in the field of astronomy.

Other astronomers will be able to use this extensive data to analyze previously unknown, but important, details about stars and galaxies, including the mass distribution of the Milky Way Galaxy and its total mass. Girard said he thought it was an exciting prospect to finally be able to fully determine the orbits of the Milky Way’s satellite systems, such as the Large and Small Magellanic Clouds.

“Professor van Altena and his team’s measurements of the proper motions of many thousands of stars will be a gold mine for researchers that work on the structure of the Milky Way galaxy,” astronomy professor Robert Zinn GRD ’74 said.


Despite having dedicated his entire professional life to the field, van Altena — and other professors in the Department of Astronomy — agreed that astrometry is quickly disappearing as a distinct field of study at Yale and in the United States as a whole, despite the fact that new technology has significantly broadened the horizon of possible astrometric research.

“The long and distinguished history of astrometry at Yale may be coming to an end, or at least winding down,” Richard Larson, a professor of astronomy, said. “Astrometry is no longer regarded by most astronomers as a separate discipline, but as a basic tool in the astronomer’s toolkit.”

This lack of interest, van Altena said, remains largely confined to the United States. He added that the tendency for the U.S. public and funding agencies alike to be swept up by fads — in this case, with an obsession with the origin of the universe and a disdain for the years of observation and analysis required by astrometry — has precipitated the decline in interest.

But he said foreign commitment to astrometry has continued unabated. Europe and Japan are both involved in extensive astrometric research. Most notably, the European Space Agency plans to launch a mission in December 2011 to conduct a census of more than 1,000 million stars in the Milky Way.

Van Altena said he was uncertain about the future of astronomy in the United States, adding that astronomers interested in astrometry would most likely have to go abroad to conduct astrometric research.

“The U.S. is only interested in cosmology — they have completely lost sight of maintaining any kind of balance,” van Altena said. “The NSF has not funded a single grant in astrometry in the past two years.”

Van Altena, who taught the last graduate-level astrometry course in the United States in 2004, has spent recent years trying to generate enthusiasm for his field of study, giving talks and even preparing a course tailored to attract students to astrometry, not only at Yale but at universities across the country.

He is currently in the process of editing a graduate-level introductory textbook on astrometry — the first to be written on the subject — which he said will be sent to the Cambridge University Press by June.

“He deserves much credit for his efforts,” Girard said. “I, as well as others, remain hopeful that he will succeed in maintaining astrometric research at Yale and elsewhere.”