Terzan 5, a densely packed star cluster, is currently hurtling through our galaxy at extraordinary speeds, acting as an unexpected laboratory for astrophysicists. This stellar formation, described as a “Galactic Comet,” is providing new insights into a mystery that has puzzled scientists for over a century: the behavior of cosmic rays.
Unraveling the mystery of cosmic rays
Cosmic rays, high-energy particles that zip through space at nearly the speed of light, have confounded astronomers since their discovery in 1912. These particles, which include atomic nuclei and elementary particles such as protons and electrons, are deflected by the magnetic fields they encounter in interstellar space, making it difficult to trace their origins. “Our observations of radiation produced by Terzan 5’s cosmic rays have allowed us to measure, for the first time, how quickly these particles change direction due to fluctuations in interstellar magnetic fields,” stated the research team, whose findings were published in Nature Astronomy.
A century of curiosity
The story of cosmic rays begins with Austrian-American physicist Victor Hess, who in 1912 measured radiation levels in a high-altitude balloon and discovered they were significantly higher than on the ground, even during a solar eclipse. This groundbreaking discovery led to the understanding that the radiation had to be coming from space. Despite more than a century of study, many questions about rays remain unanswered. One of the key challenges has been understanding how quickly cosmic rays change direction when influenced by the magnetic fields in space. This is where Terzan 5 plays a crucial role.
Terzan 5: A unique cosmic ray generator
Terzan 5, a globular cluster located near the centre of our galaxy, contains a significant number of millisecond pulsars—rapidly rotating, highly magnetized stars that are capable of accelerating cosmic rays to extreme velocities. However, these cosmic rays do not reach Earth directly due to the constantly fluctuating magnetic fields that deflect them. Instead, the presence of cosmic rays is detected indirectly through gamma rays, high-energy uncharged particles that are produced when cosmic rays collide with photons of starlight. Unlike cosmic rays, gamma rays are not deflected by magnetic fields and can travel straight to Earth.
The mystery of the misaligned gamma rays
Interestingly, in the case of Terzan 5, the gamma rays do not align perfectly with the positions of the stars in the cluster. Instead, they appear to originate from a region about 30 light-years away, where there is no obvious source. This displacement first noticed in 2011, remained unexplained until a recent study provided a groundbreaking hypothesis.
As Terzan 5 plunges through the galaxy at hundreds of kilometers per second, it creates a tail of magnetic fields, akin to a comet’s tail streaming through the solar wind. Cosmic rays generated by the cluster initially travel along this magnetic tail. However, due to magnetic fluctuations, the directions of these cosmic rays gradually change, eventually pointing some of them toward Earth. This process takes approximately 30 years, which explains why the gamma rays appear to come from a region displaced from the cluster itself.
A new tool for understanding the Universe
The findings from Terzan 5 represent a significant advance in our understanding of cosmic rays and interstellar magnetic fields. By measuring how long it takes for magnetic fluctuations to alter the direction of cosmic rays, scientists can now test theories about the behavior of these magnetic fields and their origins. This research marks a crucial step forward in solving the century-old mystery of cosmic rays, bringing us closer to understanding the enigmatic radiation that has intrigued scientists since Victor Hess’s historic discovery over 100 years ago.