Sloan Digital Sky Survey, often abbreviated SDSS, is a project designed to map the night sky, creating a census of the stars, galaxies, and other objects in a vast region of the universe. Professional astronomers and students use SDSS data for research projects in every field of astronomy. A dedicated telescope conducts the survey from Apache Point Observatory near Sunspot, New Mexico. The SDSS telescope features a primary mirror 2.5 meters (8.2 feet) in diameter. The telescope takes images of the night sky and records the spectra (singular, spectrum) of objects that appear in the images. An object’s spectrum is a measure of the amount of electromagnetic energy the object gives off at each wavelength. The spectra that the SDSS records cover a range slightly greater than that of visible light. The survey has cataloged hundreds of millions of objects since the project began operating in 1998. It is scheduled to continue through 2025.
The SDSS telescope is recording the spectra of over 1 million stars, galaxies, and quasars (extremely luminous objects at the center of some distant galaxies). By analyzing these spectra, astronomers can measure each object’s cosmological redshift, a stretching in the wavelengths of light the object gives off caused by the expansion of the universe. Knowing a faraway object’s cosmological redshift enables astronomers to determine the object’s distance from Earth. As a result, the survey is producing the most comprehensive three-dimensional map of the distribution of galaxies ever made. Astronomers can compare this map to various models to test theories about the universe’s contents and development. The SDSS data have confirmed, for example, that most of the universe consists of dark energy, a little-understood form of energy that apparently makes the universe’s expansion speed up.
The survey has also mapped the distribution of stars in our own galaxy, the Milky Way. The results showed that stars are spread in an uneven, “lumpy” fashion. This lumpiness appears to result from the Milky Way colliding with and absorbing small galaxies over the last few billion years.
The SDSS data have revealed many previously unknown objects. The survey discovered the most distant quasars known, whose light has taken about 13 billion years to reach Earth. In the Milky Way, the survey has uncovered cool, dim objects called brown dwarfs. A brown dwarf has a greater mass (amount of matter) than a planet but less mass than a star.
The SDSS data have also provided evidence for the existence of dark matter, the invisible substance that makes up most of the matter in the universe. The survey has identified distortions in the appearance of some distant galaxies. The gravitational pull of dark matter in nearby galaxies bends the light from these distant ones, causing their shapes to appear distorted.
A partnership of academic institutions called the Astrophysical Research Consortium manages the Sloan Digital Sky Survey. The project takes its name from the Alfred P. Sloan Foundation, a nonprofit organization that provided major funding for the survey.
