As my colleagues and I set out to solve a centuries-old cosmic mystery, we discovered an unexpected celestial laboratory in Terzan 5, a dense star cluster currently hurtling through our galaxy at breakneck speed.

This stellar curiosity has allowed us to study the behavior of cosmic rays – high-energy particles whose irregular trajectories through space have puzzled astronomers since their discovery in 1912.

By observing the cosmic rays produced by Terzan 5, we have achieved a scientific first: we have measured how quickly these particles change direction due to fluctuations in the interstellar magnetic field. Our research was published in Natural astronomy.

Fast radiation from space

Nobody expected cosmic radiation. When radioactivity was first discovered in the 1890s, scientists thought that all sources of radiation were on Earth.

But in 1912, Austrian-American physicist Victor Hess measured ambient radiation in a high-altitude balloon and found that even during a solar eclipse, when the sun was obscured, it was much higher than at ground level. This meant that the radiation had to come from space.

Today we know the mysterious radiation that Hess discovered as cosmic rays: atomic nuclei and elementary particles such as protons and electrons that have somehow been accelerated to nearly the speed of light. These particles race through interstellar space, and thanks to their high energy, a small portion of them can penetrate the upper atmosphere, as Hess discovered.

However, it is not easy to say where they come from. Cosmic rays are charged particles, which means that their direction of movement changes when they encounter a magnetic field.

The static picture of the cosmic radiation cosmos

The magnetic deflection effect is the basic technology for old cathode ray tube monitors and televisions. They use it to direct electrons onto the screen and thus create an image. Interstellar space is full of magnetic fields, and these fields are constantly fluctuating, deflecting cosmic rays in random directions – much like a broken cathode ray tube in an old television that only shows noise.

Instead of cosmic radiation coming to us directly from its source like light, it is distributed almost evenly throughout the galaxy. Here on Earth, we see that it comes almost evenly from all directions.

Although we now understand this general picture, we lack most of the details. The uniformity of cosmic rays across the sky tells us that the direction of cosmic rays is changing randomly, but we have no good way of measuring how quickly this process occurs.

The actual cause of the magnetic fluctuations is also unknown to us. At least we didn’t know this until now.

Terzan 5 and the shifted gamma rays

This is where Terzan 5 comes in. This star cluster is a prolific producer of cosmic rays because it contains a large population of rapidly rotating, incredibly dense and magnetized stars called millisecond pulsars, which accelerate cosmic rays to extremely high speeds.

Because of the fluctuating magnetic fields, these cosmic rays do not reach the Earth. However, we can see a telltale sign of their presence: some of the cosmic rays collide with photons of starlight and convert them into energetic, uncharged particles called gamma rays.

Gamma rays travel in the same direction as the cosmic rays that produced them. Unlike cosmic rays, however, gamma rays are not deflected by magnetic fields. They can travel in a straight line and reach the Earth.

Because of this effect, we often see gamma rays coming from powerful sources of cosmic rays. But in Terzan 5, for some reason, the gamma rays don’t exactly match the positions of the stars. Instead, they seem to come from a region about 30 light-years away where there is no obvious source.

A “comet” of galactic proportions

This shift was an unexplained curiosity since its discovery in 2011 until we found an explanation.

Terzan 5 is located near the center of our galaxy today, but this is not always the case. The star cluster actually moves in a very wide orbit that keeps it far from the plane of the galaxy most of the time.

It’s just that it’s hurtling through the galaxy right now. Because this fall is happening at hundreds of kilometers per second, the cluster is cloaked in a mantle of magnetic fields, like the tail of a comet hurtling through the solar wind.

The cosmic rays emitted by the cluster initially travel along the tail. We cannot see the gamma rays produced by these cosmic rays because the tail is not aimed directly at us – these gamma rays are emitted along the tail and away from us.

This is where the magnetic fluctuations come in. If the cosmic rays stayed exactly aligned with the tail, we would never see them, but because of the magnetic fluctuations, their directions begin to change.

Eventually, some of them will point at us and produce visible gamma rays. However, this takes about 30 years, which is why the gamma rays do not appear to be coming from the cluster itself.

By the time enough of them are aimed at us and their gamma rays are bright enough to be visible, they have already traveled 30 light-years in the cluster’s magnetic tail.

Cosmic radiation and interstellar magnetic fields

Thanks to Terzan 5, we have now been able to measure for the first time how long it takes for magnetic fluctuations to change the direction of cosmic rays. We can use this information to test theories about how interstellar magnetic fields work and what causes their fluctuations.

This brings us a big step closer to understanding the mysterious radiation from space that Hess discovered over 100 years ago.

This article is republished from The Conversation under a Creative Commons license. Read the original article.