This composite radio light image and rendition of our galaxy as seen in visible light shows enigmatic "high-velocity clouds" of gas high above the plane of the Milky Way which rain gas into the galaxy, seeding it with the stuff of stars.
The cloud outlined, and possibly others too, is now known to have low heavy element content and to be raining down onto the Milky Way disk, seeding it with material for star birth. Identifying this infalling gas helps in solving a long-standing mystery of galactic evolution by revealing a source of the low-metallicity gas required to explain the observed chemical composition of stars near the Sun.
In this all-sky projection, the edge-on plane of our galaxy appears as a white horizontal strip. The false-color orange-yellow "clouds" are regions containing neutral hydrogen, which glows in 21-centimeter radiation. Hubble Space Telescope's spectrograph was aimed at one of the clouds (encircled) to measure its detailed composition and velocity.
This discovery is based on a combination of data from NASA's Hubble Space Telescope, three radio telescopes (at Effelsberg in Germany, and Dwingeloo and Westerbork in the Netherlands), the William Herschel Telescope on the island of La Palma and the Wisconsin H-alpha Mapper at NOAO's Kitt Peak Observatory.
Photo Credit: Image composite by Ingrid Kallick of Possible Designs, Madison Wisconsin. The background Milky Way image is a drawing made at Lund Observatory. High-velocity clouds are from the survey done at Dwingeloo Observatory (Hulsbosch & Wakker, 1988).