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The Energetic Universe

An exploding supernova can outshine an entire galaxy. Black holes devour matter and generate enough light to be seen across the universe. Violent processes far away can propel cosmic ray protons to Earth, carrying as much energy as a major-league baseball. High-energy astrophysics research is dedicated to studying the extreme phenomena the cosmos has to offer, using the tools of astronomy and particle physics.

Astronomy at the Extremes

In 1991, the Fly’s Eye experiment in Utah detected a cosmic ray from somewhere in deep space. This single subatomic particle carried 40 million times the highest energy that can be made by particle accelerators on Earth, a figure so staggering that researchers called it the “Oh-My-God particle”.

Since then, cosmic ray astronomers have detected a few other particles with similar extreme energies. While ultra-high-energy cosmic rays don’t hit detectors on Earth very often, the universe contains many things capable of immense destruction. High-energy astrophysics studies these objects and their output, whether it’s gamma rays or X-rays, particles moving extremely close to the speed of light, or explosions bright enough to be seen from billions of light-years away.

  • Ordinary stars like the Sun produce high-energy particles, though on a small scale in cosmic terms. Magnetic processes around the Sun accelerate electrically-charged particles, speeding them out into the Solar System where we call them the solar wind. The same processes also make gamma rays. Earth’s magnetic field protects us from most of the solar wind, and our atmosphere shields us from gamma rays, but these outbursts profoundly affect asteroids, comets, and other Solar System bodies. They also can disrupt communications and could be harmful for astronauts traveling to Mars.

  • When stars much more massive than the Sun reach the end of their lives, they explode as supernovas. These are some of the brightest, most extreme phenomena in the universe. At their peak luminosity, supernovas can outshine whole galaxies and be visible from billions of light-years away. They emit all manner of high-energy radiation, along with particles of many types and gravitational waves. Supernovas are also the source of many of the atoms that make planets, including Earth.

  • Supernova remnants are themselves the source of high energy cosmic rays, gamma rays, and X-rays. Powerful processes within the material left behind after a supernova explosion accelerate electrically charged particles to high energies, while emitting X-rays and gamma rays.

  • White dwarfs — the remnants of stars like the Sun — can also go supernova if they collide with each other or gain enough extra mass to become unstable. These Type Ia supernovas explode in very similar ways, so cosmologists use them to track the expansion of the universe.

  • Colliding neutron stars produce gravitational waves detectable by LIGO and other observatories. These collisions also produce a lot of high-energy light as the neutron stars destroy each other. Like supernovas, these collisions produce new elements, including gold.

  • Most large galaxies harbor a supermassive black hole millions or billions of times the mass of the Sun. When these black holes feed on gas or stars, they channel some of the material into jets, which blast back into the host galaxy. The light emitted from the hot jets makes these “active” black holes some of the brightest single objects in the universe, where they go by names like quasars and blazars. The “Oh-My-God” particle and other high-energy cosmic rays may have been accelerated by the extreme environment near one of these black holes.

  • Many other systems produce high-energy light and particles, including newborn stars, collisions between galaxies or galaxy clusters, and other transient astronomical events.

The Cassiopeia A supernova remnant as seen by NASA's Chandra X-ray observatory. Supernovas are among the most energetic phenomena in the cosmos, often outshining entire galaxies at their peaks.

Credit: NASA/CXC/SAO