August 29, 2007

Road map of the sky now comes with pictures

Walk out into your backyard, look up, and see the Sloan Digital Sky Survey.

Drawing on 20 terabytes of data gathered over the last eight years, two scientists with the Sloan Digital Sky Survey (SDSS-II) helped develop a tool that allows internet users to "step into their virtual backyard" and view images of hundreds of millions of stars and galaxies.

Named Sky, this new feature of Google Earth provides access to the astronomical images from SDSS-II through the same interface that millions have used to view satellite images of the earth's surface.

The collaboration between SDSS-II and Google was initiated in 2005 by astronomers Andrew Connolly and Ryan Scranton, both then at the University of Pittsburgh. During a subsequent year spent at Google's Pittsburgh Engineering Center, Connolly and Scranton led the development of Sky.

"We were looking at ways to allow easy browsing and visualization of SDSS images," explained Connolly. "When Google Earth came out, we realized it was the perfect solution because it is so intuitive and so powerful. We just had to turn it on its back, so it could look up instead of down."

Just as a Google Earth user can rotate the globe and zoom down to an individual building or street, Scranton explained, "Sky lets you start from a map of the constellations and zoom to the individual pixels of a galaxy image from Sloan or Hubble. It really brings home just how big the Universe is."

The SDSS has imaged about one-quarter of the sky, using a 120-megapixel digital camera on the 2.5-meter telescope of Apache Point Observatory, in New Mexico. To span the remainder, Sky uses shallower images scanned from photographic plates, obtained by other telescopes and provided by the Digital Sky Survey Consortium. Sky also incorporates images from Hubble Space Telescope, covering only a tiny fraction of the sky but providing the spectacular image quality that can only be obtained from space.

"If you live in the suburbs, you can go into your backyard and see a few hundred stars," said University of Washington astronomer Simon Krughoff, another member of the Sky development team. "Now you can go to your virtual backyard and see 100 million stars and 100 million galaxies found by the SDSS."

The SDSS pioneered the use of enormous digital cameras to image large areas of sky at the terapixel resolution needed to reveal the structure of distant galaxies and discover quasars at the far edge of the universe.

"We knew these data would be enormously valuable to professional astronomers," explained SDSS Project Scientist Jim Gunn of Princeton University. "But when we conceived the project in the early 1990s, we had no idea how to make the full resolution pictures available to the world at large. It was just too much data."

"Over the years, we have put a lot of effort into making the SDSS data accessible to the general public and useful for students and teachers at all levels," noted Johns Hopkins University astronomer Alex Szalay, an architect of the SDSS-II database system and its SkyServer web interface ( "Sky is a wonderful way to extend that access because it reaches so many people and makes it so easy to navigate the sky."

Connolly, Scranton and Szalay have collaborated for many years on statistical analyses of the clustering of galaxies in the SDSS, requiring painstaking attention to minute details of the data quality and calibration.

"Digging into the data has its satisfactions," observed Scranton, "especially because these analyses tell you about the history of galaxies and the contents of the Universe. But with Sky, it's great to just step back and look at it all.


  • Andrew Connolly, University of Washington, 412-983-5329,
  • Ryan Scranton, University of Pittsburgh, 412-600-1914,
  • David Weinberg, Scientific Spokesperson, Sloan Digital Sky Survey, 614-406-6243,
  • Gary S. Ruderman, Public Information Officer, Sloan Digital Sky Survey 312-320-4794,