Space bubbles

Two giant gas bubbles - each one 25,000 light-years wide - discovered in our galaxy are baffling astronomers.

The two vast structures, stretching to the north and to the south of the centre of the Milky Way, are so big that a beam of light, travelling at 186,282 miles per second, would take 50,000 years to get from the edge of one to the edge of the other.

The previously unseen bubbles were discovered by astronomer Doug Finkbeiner, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, using NASA's Fermi Gamma-Ray Telescope.

He admitted yesterday: "We don't fully understand their nature or origin."

They span more than half the visible sky, from the constellation of Virgo to the constellation of Grus, and are thought to be millions of years old.

They were not noticed before because they were lost in a fog of gamma radiation across the skies

Astronomers' best guess is that the bubbles were created by an eruption from a supersized black hole at the centre of our galaxy.

Mr Finkbeiner and his team discovered the bubbles by processing publicly available data from Fermi's Large Area Telescope.

The space telescope, launched in 2008, is the most powerful detector of gamma rays, which are the most energetic form of light.

Scientist David Spergel, of Princeton University, New Jersey, said: In other galaxies, we see that starbursts can drive enormous gas outflows.

Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics.