NASA’s Astronomy Picture of the Day for October 19, 2026 is a cosmic coincidence worth a second look: a glowing, curling radio structure that looks remarkably like a musical treble clef. Its real story is about a supermassive black hole, a collision between two galaxy clusters, and a type of radio source astronomers call a “radio phoenix”.

Composite image of the Treble Clef radio source: a filamentary, treble-clef-shaped structure in red radio emission inside a blue cloud of hot X-ray gas from two merging galaxy clusters, with infrared stars in white.
The Treble Clef: radio emission in red, X-rays in blue, infrared in white. Image credit: X-ray: NASA/CXC/INAF-IRA/A. Botteon et al.; Infrared: UKIDSS-GPS; Radio: ASTRON/LOFAR; Image processing: NASA/CXC/SAO/N. Wolk. Via NASA APOD (Authors and editors: Robert Nemiroff, Jerry Bonnell, Cecilia Chirenti, Keighley Rockcliffe).

What is the Treble Clef?

The Treble Clef is a faint, filamentary radio source catalogued as VLSS J0318.9+5755. According to NASA’s APOD, it stretches over 2 million light-years. For comparison, that is roughly the distance from the Milky Way to the Andromeda Galaxy. Its resemblance to a musical symbol is an accident of geometry. NASA’s Chandra team describes it as pareidolia, the human tendency to see familiar shapes in random data, and stresses there is no scientific link to the musical notation.

The object sits almost directly behind the plane of our own galaxy, in a region astronomers call the Zone of Avoidance, where the stars, gas and dust of the Milky Way block most visible light. That is why it stayed a puzzle for so long. The source was examined in 2004 but its nature remained unknown, and the research paper notes it was originally believed to be a pulsar before being suggested to belong to a heavily obscured galaxy cluster.

Seeing through the Milky Way

The solution came from observing at wavelengths that pass through the obscuring material. X-ray data from NASA’s Chandra X-ray Observatory and the eROSITA telescope revealed diffuse, multimillion-degree gas at the location of the Treble Clef. That hot gas is the signature of a galaxy cluster, and the data showed it is in fact two clusters in the process of merging. Galaxy clusters are among the largest structures in the universe, containing hundreds of galaxies and vast reservoirs of superheated gas.

The radio data come from LOFAR, a large network of radio telescopes in Europe. The study also used follow-up observations from the uGMRT and the LOFAR LBA Sky Survey. In the composite image, radio emission is shown in red, X-rays in blue, and infrared data from the United Kingdom Infrared Telescope in mostly white.

A radio phoenix in two acts

The authors conclude that the Treble Clef is a radio phoenix, an old cloud of radio-emitting particles that has been brought back to life. NASA’s summary describes a likely two-stage process:

  1. The outburst. A supermassive black hole at the center of a galaxy in the cluster expelled clouds of charged particles and magnetic fields at close to the speed of light. These particles initially shone brightly in radio waves, then faded as they lost energy.
  2. The collision. When the two galaxy clusters collided, the fossil cloud was compressed or reenergized and became bright in radio waves again. Shock waves from the merger, similar in principle to the sonic booms of supersonic jets, may have contributed.

The researchers estimate that the direct collision happened a few hundred million years ago, and that the two clusters are now moving apart. In the paper, the authors also speculate that gas motions generated by the merger shape the structure’s complex filaments, and that the same motions may have produced a candidate radio halo tentatively seen at the cluster center.

How they found the collision in the X-ray data

Most of the cluster’s hot gas forms a roughly symmetrical oval. To reveal what the collision had disturbed, the team subtracted that smooth oval from the Chandra image and examined the residual emission. The leftover X-rays were brightest to the left and right of the center and faintest in the middle, which suggests the clusters collided along that left-right direction. The final composite image combines both the original and the residual X-ray data.

Why it matters

The paper reports an extremely steep radio spectrum, with a spectral index above 4 between 400 and 650 MHz. In plain terms, the source gets rapidly brighter toward lower radio frequencies, a hallmark of old, energy-depleted particles. Combined with its filamentary shape, this makes it an ideal target for future very-low-frequency observations with the planned LOFAR2.0 upgrade. As APOD puts it, unusual objects like this give astronomers clues about how whole galaxy clusters interact and how the universe evolves.

A note on certainty: the explanation is a well-supported interpretation, not a closed case. APOD calls it a hypothesis, NASA’s Chandra release says “likely”, and the paper itself labels some elements, such as the radio halo and the role of gas motions, as speculative or tentative.

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