WASHINGTON (AP) - Astronomers have narrowed down the location of about half of the mysterious sources of gamma rays that radiate from the Milky Way galaxy, the celestial home of the solar system, according to a study released Wednesday. Gamma rays are invisible to the eye, but are the most powerful form of light, far exceeding visible light, ultraviolet radiation and X-rays. Gamma rays are about 100 million times more powerful than visible light. Astronomers have found 170 sources of gamma rays in the Milky Way, but have been unable to link the sources with objects, such as stars, that emit other forms of light. In a new study appearing this week in the journal Nature, astronomers at NASA's Goddard Space Flight Center said they have found that half of the l70 gamma ray sources lie in a narrow band along the central plane of the Milky Way and may originate from known objects that shine too faintly to be otherwise identified.