Although it may seem that there is nothing but empty space among the stars in a galaxy, the areas are actually filled with a thin gas called the interstellar medium. Similarly, the gas in the empty areas among galaxies in a cluster is known as the intercluster medium. In this false color image of the region around Cygnus A, thin gas extends more than 3 million light years from the galaxyΓÇÖs center. Because the gas is very hotΓÇö100 billion degrees KelvinΓÇöit is most easily visible through x-ray telescopes. The small white bar indicates a length of 5 arc minutes. The huge radio lobes, by comparison, measure only 2 arc minutes end to end and would fit within the brightest area in this image.
