This image of the galaxy M51, also known as the "Whirlpool" galaxy, is just one of millions now available to the public from the Sloan Digital Sky Survey (SDSS), an astronomical cataloging project that aims to map one-fourth of the entire sky.

Astronomers used the 2.5-meter (98-inch) SDSS telescope at Apache Point Observatory in New Mexico to observe M51, collecting not just the image of the galaxy but its spectrum too using an attached spectrograph. M51 sits about 30 million light-years away in the constellation Canes Venatici (the Hunting Dogs). One light-year is the distance light travels in a year, about six trillion miles (9.7 trillion kilometers).

Images of about 88 million other astronomical objects, as well as the spectra and redshift for more than 350,000 of them, are now available among the six terabytes of observational data collected by SDSS astronomers since 1998. This is the second time that the some 200 astronomers and 13 institutions that participate in SDSS released their data. In 2003, images of 50 million objects were released along with additional spectrographic data for nearly 200,000 of them.

"Many external researchers are already using the data from earlier public release," said Alex Szalay, an astronomer at Johns Hopkins University and an architect of SDSS data mining tools, in a written statement.

This second data release, or DR2 as its called, contains the last two years of SDSS observations and covers about 3,324 square degrees of the Northern night sky and contains objects that are three million times fainter than those perceptible with the naked eye. A degree is a unit of sky measurement, with one-half of a degree equal to the width of the full moon. The SDSS project is funded through summer 2005 and expects to release more data in the future.