Dark clouds of interstellar dust stand out in the picture because they absorb the light of background stars. Most of the clouds lie in the plane of the galaxy, forming the dark band, about 500 light-years thick, that appears to cut the galaxy in two from upper right to lower left. A similar effect can be seen in our own sky. If one views the Milky Way by going well away from city lights, dust clouds in the disk of our own galaxy appear to split the glowing band of the Milky Way in two. By studying the color and the amount of light absorbed by these distant clouds in NGC 4013, astronomers can estimate the amount of matter in them.
Individual clouds contain as much as one million times the amount of mass in our Sun.
Dark interstellar clouds are believed to be where new stars are formed. Later, when the dust disperses, the young stars become visible as clusters of blue stars. NGC 4013 shows several examples of these stellar kindergartens near the center of the image, lying in front of the dark band along the galaxy's equator. The extremely bright star near the upper left corner, however, is merely a nearby foreground star belonging to our own Milky Way, which happens to lie in the line of sight to NGC 4013. Author Thomas J. Kelly details how the Apollo Lunar Module was developed in this Smithsonian History of Aviation and Spaceflight Series book.