Astronomers have detected a hot spot about the size of a football field on a city-sized star that is 500 light-years away.

The star is called Geminga. It is a burnt out shell known as a neutron star, the result of a massive star erupting long ago in a supernova explosion. Geminga contains about one and a half times the mass of the Sun. The material is in the form of neutrons -- able to huddle together more tightly than other matter -- into a sphere about 12.4 miles (20 kilometers) across.

Like all neutron stars, Geminga has an enormous magnetic field, hundreds of billions of times stronger than Earth's. And like other neutron stars, Geminga rotates rapidly -- four times per second. The combination of magnetism and rotation induces strong electric fields.

"Such electric fields are natural particles accelerator," Patrizia Caraveo of IASF-CNR in Italy explained in an e-mail interview.

Accelerated particles are forced to radiate away their energy, producing high-energy gamma-ray photons. A photon feels the magnetic field and materializes as an electron-positron pair, which feels the magnetic field and produce a gamma ray photon, in a cycle repeated two or three times, Caraveo said.

Geminga it barrels through space, a result of being shot like a cannonball from the ancient supernova event. It was already known to sport a tail of material that shines bright in X-rays. But what is the tail made of? If the electrons are funneled into space to create the X-ray tail, then theory holds that the antimatter positrons ought to crash back onto the star to create a hot spot.

With the discovery of the hot spot, that mechanism seems to be confirmed, Caraveo and her colleagues say.

The new discovery was made with the European Space Agency's orbiting XMM-Newton X-ray observatory. It is detailed in the July 16 issue of the journal Science.

The hot spot was not directly imaged. It and its size are inferred by noting changes in the X-ray emissions as the star rotates, Caraveo said. It is about 3.6 million degrees Fahrenheit (2 million Celsius), while the rest of the star's surface is about one-fourth as hot.

X-rays are high-energy light, part of the same electromagnetic spectrum as visible light and lowly radio waves. Gamma rays are the highest form of energy on this spectrum. Geminga is the second brightest source of constant gamma rays known. So the finding provides insight into how X-rays and gamma rays can be related to a single process.

"Putting together the gamma-ray behavior and the newly found X-ray one, we hope to be able to better understand the emission mechanisms on neutron stars, a fascinating subject which is still little known," Caraveo said.