After turning millions of Internet users into virtual explorers of the world with Google Earth, the Internet search giant is now hoping to turn many of them into virtual stargazers.



Google is unveiling within Google Earth today a new service called Sky that will allow users to view the skies as seen from Earth. Like Google Earth, Sky will let users fly around and zoom in, exposing increasingly detailed imagery of some 100 million stars and 200 million galaxies.



“You will be able to browse into the sky like never before,” said Carol Christian, an astronomer with the Space Telescope Science Institute, a nonprofit academic consortium that supports the Hubble Space Telescope.



While other programs allow users to explore the skies, they typically combine a mix of representations of stars and galaxies that are overlaid with photographs, Ms. Christian said. “These are really the images of the sky. Everything is real.”



The Sky imagery was stitched together from more than one million photographs from scientific and academic sources, including the Sloan Digital Sky Survey, the Palomar Observatory at the California Institute of Technology and the NASA-financed Hubble.



Google said that it developed the project strictly because some of its engineers were interested in it, and that it had no plans to make money from it for now.



“It’s merely about getting new kinds of information out there for the public,” said Chikai Ohazama, a Google Earth project manager.



As with Google Earth, individual users and organizations will be able to overlay photographs, annotations and other kinds of data on top of Sky’s basic images and make them available to others as layers — called mash-ups.



Sky already has layers showing various constellations, a user’s guide to galaxies, the position of planets two months into the future and animations of lunar positions.



A “backyard astronomy” layer highlights stars, galaxies and nebulae that are visible to the naked eye, with binoculars or with small telescopes.



“I think it will certainly be a great educational venue,” said S. George Djorgovski, a Caltech astronomy professor.



“As the Google Earth example has shown, people are extremely ingenious in coming up with mash-ups and inventing other uses for it.”



Professor Djorgovski has developed a mash-up depicting events like cosmic explosions. “This was a simple way to convey there is a dynamic aspect to the universe,” he said.



Microsoft has a research project called the World Wide Telescope that offers similar capabilities to Sky. The project was once headed by Jim Gray, the veteran Microsoft researcher who disappeared this year after a sailing trip off San Francisco Bay.



To get Sky, users will have to download the latest version of Google Earth.

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