Astronomers have used NASA’s Chandra X-ray Observatory to break down the cause of a remarkable formation of radio waves they have nicknamed the “Treble Clef” because its distinct shape strongly resembles the musical symbol.

The researchers discovered the Treble Clef is likely caused by an outburst from a supermassive black hole, followed by a collision of two galaxy clusters. A paper describing the findings, led by Andrea Botteon of the National Institute for Astrophysics, or INAF, in Italy, published Wednesday in the Astronomy & Astrophysics journal .

An explanation for the Treble Clef has been particularly difficult to find because the formation lies almost directly behind the plane of the Milky Way, meaning that stars, gas, and dust from our home galaxy largely block the view from Earth. However, by using telescopes that can peer through the intervening material, a team of researchers was able to identify what is responsible for this uncommon structure.

NASA’s Chandra and the German Extended RÖntgen Survey with an Imaging Telescope Array (eROSITA) X-ray telescope found diffuse, hot gas at the location of the Treble Clef, showing that a galaxy cluster hosts the object. The researchers also determined the galaxy cluster is, in fact, two clusters in the process of merging. Galaxy clusters are some of the largest structures in the universe, containing hundreds of individual galaxies and huge reservoirs of superheated gas.

This new composite image of the Treble Clef and its cluster shows the multimillion-degree gas detected in X-rays by Chandra and eROSITA in blue, with some of the Chandra data derived from a special technique the authors used to highlight important features in the Chandra data. Radio data shown in red from the LOw Frequency ARray (LOFAR), a large network of radio telescopes in Europe reveals the doppelganger of the musical notation. The final piece of the combined image comes from infrared data obtained by the United Kingdom Infrared Telescope, appearing mostly white.

The authors concluded that the Treble Clef likely formed in a two-stage process. First, an earlier outburst from a supermassive black hole in the galaxy cluster’s center drove clouds of charged particles and magnetic fields away at close to the speed of light. This led to a type of cosmic crescendo and initially produced bright radio waves that later faded as the particles lost energy. Next, the collision between the two clusters compressed or reenergized the cloud, which became brighter again in radio waves. Shock waves — similar to the sonic booms from supersonic jets — from the collision may have played a role in this reenergizing process.

To reveal evidence for the cluster collision in the Chandra data, the authors used a special technique that examined how much the cluster’s hot gas deviates from a symmetrical shape. Most of the hot gas is symmetrical and shaped approximately like an oval. The authors removed this symmetrical oval shape from the original X-ray image. The remaining X-ray emission in the residual image shows t is brightest on the left and right, and faintest in the center. The new composite image combines both the original X-ray and the residual X-ray images.

The brighter X-rays in the residual image, toward the left and right of the cluster’s center, imply that the collision between the two clusters likely occurred along the left-right direction as seen in the image. The researchers think the direct collision between the two clusters occurred a few hundred million years ago, and the clusters are now moving away from each other.