Astronomy Picture of the Day

Explanation: How much dust is in spiral galaxies? Does it block out much of the starlight? Because astronomers rely on an accurate knowledge of galaxy properties to investigate a wide range of problems, like galaxy and quasar evolution and the nature of dark matter, answers to simple questions like this are key. This striking, detailed Hubble Space Telescope image of dust in the outer reaches of a foreground spiral galaxy (left) back lit by an elliptical galaxy offers an elegant approach to providing the answers. As expected, dust lanes in the foreground galaxy seem to be associated with spiral arms. But surprisingly, many dust regions are not completely opaque and the dust is more smoothly distributed than anticipated. This "overlapping" pair of galaxies is cataloged as AM1316-241 and is about 400 million light-years away in the constellation Hydra.