English: Images from NASA's Hubble Space Telescope have provided a dramatic glimpse of a large and massive galaxy under assembly by the merging of smaller, lighter galaxies. Astrophysicists believe that this is the way galaxies grew in the young universe. Now, Hubble observations of the radio galaxy MRC 1138-262, nicknamed the "Spiderweb Galaxy" show dozens of star-forming satellite galaxies as individual clumpy features in the process of merging. A radio galaxy emits more of its energy in the form of long-wavelength radiation (radio wavelengths) than at visible light wavelengths. Because the galaxy is 10.6 billion light-years away, astronomers are seeing it as it looked in the universe's early formative years, only 3 billion years after the Big Bang.

A striking feature of the Spiderweb Galaxy is the presence of several faint, small linear galaxies within the merging structure. The complexity and clumpiness agree with predictions of hierarchical galaxy formation models. Hierarchical structure formation is the scenario in which galaxies and clusters are assembled "from the bottom up," with small building blocks merging to form the larger structures. It also supports the assumption that distant powerful radio galaxies represent the merging of smaller star systems to create the giant galaxies seen at the centers of galaxy clusters in our own cosmic neighborhood. The Hubble provides a unique real-world example for simulations of forming dominant cluster galaxies.

The Spiderweb Galaxy is located in the southern constellation of Hydra (the water snake), and is one of the most massive galaxies known.