Radio map at neutral hydrogen (21cm) wavelength of spiral galaxy M81 & its companion galaxy NGC 3077 (left). The photograph is colour coded for the intensity of the gas: intense sources are yellow, with less intense sources in red & blue. The gas is strongly concentrated in the spiral arms where it is forming into young stars. The central core of the galaxy shows a depletion of the gas, which has been exhausted by older star formations. The image also reveals two complicated bridges of hydrogen connecting M81 (right) to NGC 3077 (left).