Terzan 5. (ESA/Hubble/Wikimedia Commons/CC BY 4.0)
A dense star cluster known as Terzan 5, speeding through our galaxy, has provided scientists with an unprecedented opportunity to unravel a century-old cosmic enigma. This fascinating discovery brings us closer to understanding the nature of cosmic rays—high-energy particles that have puzzled astronomers since their discovery over a hundred years ago.
Cosmic Rays: The Century-Old Mystery
Cosmic rays, discovered in 1912 by Austrian-American physicist Victor Hess, are high-energy particles that travel through space at nearly the speed of light. Comprising atomic nuclei and elementary particles like protons and electrons, these rays originate from deep within our galaxy, yet pinpointing their sources has been one of astronomy’s greatest challenges. The reason for this mystery lies in their behavior: as charged particles, cosmic rays are susceptible to the magnetic fields that permeate space. These fields constantly fluctuate, causing the rays to deflect and scatter in random directions, creating a nearly uniform spread of cosmic rays across the sky.
This “staticky” view has long hindered scientists from identifying the cosmic rays’ origin points. While we’ve known for decades that cosmic rays interact with magnetic fields, we’ve been unable to measure how quickly these particles change direction—until now.
Terzan 5: A Celestial Laboratory
Enter Terzan 5, a globular star cluster near the center of the Milky Way. The cluster contains an unusually high number of millisecond pulsars—rapidly rotating, highly magnetized neutron stars that accelerate particles to immense speeds. These pulsars are potent sources of cosmic rays. Yet, what makes Terzan 5 so unique is its current trajectory: it’s plunging through our galaxy at hundreds of kilometers per second. This motion creates a comet-like tail of magnetic fields that envelopes the cluster, offering scientists an ideal natural laboratory to study cosmic rays and their interactions with interstellar magnetic fields.
The Mystery of the Displaced Gamma Rays
For over a decade, astronomers have observed a curious phenomenon involving Terzan 5. Gamma rays—high-energy light created when cosmic rays collide with photons of starlight—have been detected, but they appear to be coming from a region about 30 light-years away from the cluster itself, where there seems to be no source. This offset has been a puzzle since 2011, with no clear explanation—until now.
The globular cluster Terzan 5 (centre) is shown in visible light, overlaid with gamma ray intensity. The gamma ray source is centre below and to the right of Terzan 5. A zoomed-in version of the central region is shown in the upper left. (ESO/Digitized Sky Survey 2/F. Ferraro)
As Terzan 5 speeds through the galaxy, it creates a magnetic tail that stretches out behind it, much like a comet’s tail formed by the solar wind. The cosmic rays within the cluster travel along this magnetic tail, but due to the tail’s orientation, they don’t initially point toward Earth. However, the ever-changing magnetic fields cause the cosmic rays’ directions to slowly shift. Over the course of 30 years, some of these rays finally align with Earth, allowing the gamma rays they produce to be detected.
Measuring Magnetic Fluctuations for the First Time
This delay in detection provides a unique window into the behavior of cosmic rays and interstellar magnetic fields. For the first time, scientists have been able to measure the time it takes for magnetic fluctuations to change the direction of cosmic rays. This groundbreaking data helps refine our understanding of how magnetic fields operate on galactic scales, shedding light on the processes that have long obscured the origins of cosmic rays.
A Major Step Forward
Terzan 5’s magnetic tail offers more than just a solution to the mystery of displaced gamma rays—it represents a major leap forward in our understanding of cosmic rays and interstellar magnetic fields. By studying this speeding star cluster, scientists are now able to test theories about how magnetic fields fluctuate and influence the behavior of these energetic particles.
Ultimately, this discovery brings us one step closer to solving one of astronomy’s longest-standing mysteries: the origin of cosmic rays. Thanks to Terzan 5 and its magnetic tail, we are beginning to decode the complex interactions that shape our universe’s energetic landscape—an endeavor that started more than 100 years ago with Victor Hess’s pioneering discovery.
This article emphasizes the depth of our current understanding of cosmic rays while highlighting how new discoveries continue to drive the field forward. Terzan 5’s comet-like trail of magnetic fields could pave the way for further breakthroughs in the study of cosmic rays and the fundamental forces at play in our galaxy.
