When my colleagues and I embarked on solving a century-old cosmic mystery, we stumbled upon an unexpected celestial laboratory in Terzan 5 a dense star cluster currently racing through our galaxy at breakneck speed. This stellar oddity has allowed us to study the behavior of cosmic rays, high-energy particles that have puzzled astronomers since their discovery in 1912.
Cosmic rays are particles like protons and electrons that move at nearly the speed of light, zipping through space. But these particles don’t travel in straight lines. Instead, they get tossed around by the galaxy’s magnetic fields, making it difficult to figure out where they come from.
Terzan 5 has become a crucial piece of the puzzle. This star cluster is home to many rapidly rotating, highly magnetized stars called millisecond pulsars, which accelerate cosmic rays to incredible speeds. As these cosmic rays travel through space, they collide with photons (particles of light) and create gamma rays—high-energy radiation that travels straight through space, unaffected by magnetic fields.
But something strange happened when we observed the gamma rays from Terzan 5. They didn’t seem to come directly from the star cluster. Instead, they appeared to be displaced by about 30 light-years, a mystery that had remained unsolved since 2011.
After much research, we discovered that Terzan 5 is currently plunging through the galaxy at hundreds of kilometers per second. This high-speed journey has created a sort of “tail” of magnetic fields, much like a comet’s tail. The cosmic rays from Terzan 5 travel along this magnetic tail, and while we can’t see the gamma rays they produce right away, the magnetic fields eventually cause the cosmic rays to change direction. After about 30 years, these rays finally point toward Earth, explaining why the gamma rays appear to come from a different location.
Our findings, published in Nature Astronomy, mark the first time we’ve been able to measure how quickly magnetic fields can change the direction of cosmic rays. This brings us one step closer to understanding the mysterious radiation discovered by physicist Victor Hess more than a century ago.
