Survey releases latest catalog of astronomical data

By Steven Schultz

Princeton NJ -- The Sloan Digital Sky Survey, a consortium of astronomers with leadership at Princeton, has released to the public one of the largest catalogs of astronomical data ever produced.

The data, published in the form of a searchable electronic database, contains images and measurements of 88 million galaxies, stars and quasars and covers 8 percent of the entire sky. The database, known as data release 2 or DR2, is the second major release from the Sloan survey, which unveiled a smaller catalog in 2003.


M51, the ''whirlpool'' galaxy in the constellation Canes Venatici, is one of 88 million galaxies in the SDSS database.

The new release is a cumulative catalog of data collected since the survey began in 1998. The SDSS consortium, which includes more than 200 astronomers at 13 institutions, plans to map up to a quarter of the entire sky and to determine the position and brightness of several hundred million celestial objects.

SDSS data already have led to important discoveries about the age, history and structure of the universe as well as insights into subjects ranging from the formation of galaxies to the number and size of local asteroids. However, there is much more science to be done, said Michael Strauss, a Princeton professor of astrophysics and the scientific spokesperson for the survey consortium.

"Getting DR2 out to the broader astronomical community and to the general public will allow these data to be analyzed for projects limited only by the imagination and ingenuity of the user," Strauss said. The data is available through the SDSS Web site: <>.

"Even with the hundreds of people involved in SDSS, there is much more science than we can possibly do," Strauss added. "We are keeping the students here at Princeton very busy. A number of Ph.D. theses, senior theses and junior papers have used these data, but we feel we're just getting started."

Among other Princeton scientists currently involved in SDSS are James Gunn, who designed the digital camera on which the survey is based, Jill Knapp, Robert Lupton, Zeljko Ivezic and David Schlegel. SDSS is unique in its combination of breadth and sensitivity, said Strauss. The SDSS telescope at Apache Point Observatory in New Mexico records objects that are 3 million times fainter than the human eye can see, and also is capable of measuring the spectra from many objects at once.

One area of study arising from the SDSS data concerns the nature of quasars, which are extremely bright objects caused by black holes. The survey data thus far include 50,000 quasars, five times more than were known before the survey began. Most galaxies have supermassive black holes in their centers. As material falls into the black hole, it heats up tremendously and can shine as a quasar before disappearing. Princeton researchers are using the SDSS to understand why some black holes shine as quasars while others do not.

Other research subjects were largely unanticipated before SDSS and have become major areas of study. For example, the survey revealed a number of unusual types of stars as well as a surprising history of collisions between our Milky Way galaxy and others.

The consortium currently has funding through July 2005 and is looking for additional support to carry the work forward through 2008. Given what SDSS has taught astronomers so far, the original goals for the survey were "woefully incomplete, wonderfully incomplete," said Strauss. "One reason that this survey is so great is that we keep thinking of new projects that had not occurred to us before."


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