Sky map captures cosmic star glow.

The faint glow of a new sky map paints a portrait of starlight over the history of the universe. The picture primarily depicts emissions from cool, relatively lightweight stars, which constitute the majority of stars in the cosmos.

Recorded at a near-infrared wavelength of 3.5 micrometers ([micro]m), the map fills a gap in measurements of the light that different stellar populations contribute to the overall brightness of the sky, says Eli Dwek of NASA's Goddard Space Flight Center in Greenbelt, Md. Previous maps, recorded at shorter wavelengths, depict emissions from the relatively small number of hot, massive stars, which radiate most of their light in the ultraviolet and quickly die out.

Dwek and Richard G. Arendt of Raytheon STX at the Goddard Space Flight Center constructed the new map using data gathered by the Cosmic Background Explorer satellite. They describe their study in the Nov. 20 Astrophysical Journal Letters.

To estimate the amount of starlight from outside our galaxy, the researchers had to subtract two local sources of infrared emission: dust and stars within the Milky Way. Light scattered by solar system dust had already been removed. Dwek says the amount of extragalactic starlight they deduce is slightly higher than had been predicted by models using shorter-wavelength observations.

The new detection thus provides a more accurate estimate "of the total amount of energy that was released into the universe by starlight and not absorbed by dust," Dwek asserts. Astrophysicists believe that stars emit much less light at wavelengths longer than 3.5 [micro]m, he notes. Combined with previous studies (SN: 1/10/98, p. 20), the new finding suggests that dust absorbs at least half of all starlight, he says.

The total amount of light emitted by stars and the fraction absorbed by dust trace the formation and structure of galaxies, says Dwek. Researchers continue to argue about the distances between Earth and most of the stars that contribute to the sky background. In a paper accepted for future publication in Astrophysical Journal Letters, Martin Harwit, an astrophysicist in Washington, D.C., contends that most of the starlight comes from galaxies residing no more than about 8 billion light-years away.