New York: Yale astrophysicist Kevin Schawinski and colleagues at Oxford never envisioned the strange object amateur astronomer Hanny van Arkel found in archived images of the night sky.

The Dutch school teacher, volunteer in the Galaxy Zoo project, co-founded by Schawinski that allows public participation in astronomy research online, discovered a mysterious object some are calling a 'cosmic ghost'.

When she posted the image that quickly became known as "Hanny's Voorwerp" (Dutch for object) on the Galaxy Zoo forum, astronomers soon realised Arkel might have found a new category of astronomical object.

"At first, we had no idea what it was. It could have been in our solar system, or at the edge of the universe," said Schawinski, a member and co-founder of the Galaxy Zoo team.

Scientists working with telescopes worldwide and with satellites were asked to take a look at the mysterious Voorwerp. "What we saw was really a mystery," said Schawinski.

"The Voorwerp didn't contain any stars." Rather, it was made entirely of gas so hot - about 10,000 Celsius - that the astronomers felt it had to be illuminated by something powerful. They will soon use the Hubble Space Telescope to get a closer look.

Since there was no obvious source at hand in the Voorwerp itself, the team looked to find the source of illumination around the Voorwerp, and turned to the nearby galaxy IC 2497.

"We think that in the recent past the galaxy IC 2497 hosted an enormously bright quasar," Schawinski explained. "Because of the vast scale of the galaxy and the Voorwerp, light from that past still lights up the nearby Voorwerp even though the quasar shut down sometime in the past 100,000 years, and the galaxy's black hole itself has gone quiet."

"From the point of view of the Voorwerp, the galaxy looks as bright as it would have before the black hole turned off - it's this light echo that has been frozen in time for us to observe," said Chris Lintott, a co-organiser of Galaxy Zoo at Oxford University in Britain.

Quasars are very unusual, highly luminous objects, powered by super-massive black holes, and most are extremely distant. "The strange 'Hanny's Voorwerp' looks like it could be the nearest example of a luminous quasar," said C. Megan Urry, Israel Munson professor of the physics department in Yale, who was not involved in the research.

"This discovery really shows how citizen science has come of age in the Internet world," commented Bill Keel of the University of Alabama, a galaxyzoo.org team member. "Hanny's attentiveness alerted us not only to a peculiar object, but to a window into the cosmic past which might have eluded us for a long time otherwise. IANS