High-energy gamma rays emitted by exploding stars fly in the face of a long-standing theory holding that such nova explosions shouldn't generate enough power to create such radiation, astrophysicists say.

Novae are short-lived and rapid increases in the luminosity of an otherwise ordinary star, the result of runaway thermonuclear blasts on the surface of a white dwarf star in a binary system, fueled by mass being pulled off a companion star.

A nova explosion is equal to around 100,000 times the energy our sun emits over the course of a year. Unlike supernovas, they do not result in the destruction of stars undergoing the explosions on their surface.

Although bright in optical wavelengths, novae had not been considered sources for high-energy gamma rays because they've not been assumed capable of accelerating particles to the required energies to create gamma rays, the most energetic form of light, researchers said.

However, several such stellar eruptions observed by NASA's Fermi Gamma-ray Space Telescope strongly suggest the relatively common nova will produce high-energy gamma rays routinely, they say.

"There's a saying that one is a fluke, two is a coincidence, and three is a class, and we're now at four novae and counting with Fermi," says Teddy Cheung, an astrophysicist at the U.S. Naval Research Laboratory in Washington.

The first was observed in 2010, then two more in 2012 and one in 2013, says Chueng, first author of a study reporting the novae published in the journal Science.

The observation establish classical novae phenomenon as a distinct class of gamma-ray sources, the study says.

Between 20 and 50 novae occur annually within our Milky Way Galaxy, the researchers say, although most go undetected because their brightening visible light becomes obscured by intervening interstellar dust and their gamma rays lose energy with distance.

The gamma-ray novae detected by Fermi all occurred relatively nearby in our galaxy, between 9,000 and 15,000 light years distance.

"This was a completely unexpected discovery and we still don't understand the cause," says computational astrophysicist Sumner Starrfield of Arizona State University. "No one suspected novae were violent enough to be emitting at these very high energies.

"However, it now seems possible that a signification fraction -- near 100 percent -- of novae are gamma ray sources."

Where the energy comes from to make classical novae gamma ray emitters is still unknown, he says, but the research confirms that they are not as unusual a phenomenon as originally thought.

"We are puzzled and ongoing studies of possible causes are underway via computer modeling of the events," says Starrfield. "This exciting discovery is telling us something important about the explosions of classical novae but we don't, as yet, know what it means."