Perseus galaxy cluster (Abell 426), optical image. This is a large cluster containing thousands of galaxies, and it is thought to be one of the largest and most massive objects in the visible universe. The cluster lies around 300 million light years from Earth in the constellation Perseus. The brightest galaxy, at lower left, is the giant elliptical galaxy NGC 1275. As well as being bright at visible wavelengths, the galaxy has an active nucleus, and is a powerful source of radio waves and X-rays. Red filaments can be seen emanating from its nucleus, testament to its activity.