Gamma Rays: The Strongest Radiation in the Universe

gamma-ray sky
This is what the gamma-ray sky looks like as seen by NASA's Fermi telescope. All the bright sources are emitting gamma rays at strengths greater than 1 GeV (giga-electron-volt). Credit: NASA/DOE/Fermi LAT Collaboration

Everyone has heard of the electromagnetic spectrum. It's a collection of all wavelengths and frequencies of light, from radio and microwave to ultraviolet and gamma. The light we see is called the "visible" part of the spectrum. The rest of the frequencies and waves are invisible to our eyes, but detectable using special instruments. 

Gamma rays are the most energetic part of the spectrum. They have the shortest wavelengths and highest frequencies.

These characteristics make them extremely hazardous to life, but they also tell astronomers a lot about the objects that emit them in the universe. Gamma-rays do occur on Earth, created when cosmic rays hit our atmosphere and interact with the gas molecules. They're also a by-product of the decay of radioactive elements, particularly in nuclear explosions and in nuclear reactors.

Gamma rays aren't always a deadly threat: in medicine, they're used to treat cancer (among other things). However, there are cosmic sources of these killer photons, and for the longest time, they remained a mystery to astronomers. They stayed that way until telescopes were built that could detect and study these high-energy emissions.

Cosmic Sources of Gamma Rays

Today, we know much more about this radiation and where it comes from in the universe. Astronomers detect these rays from extremely energetic activities and objects such as supernova explosions, neutron stars, and black hole interactions.

These are difficult to study because of the high energies involved, they are sometimes very bright in "visible" light, and the fact that our atmosphere protects us from most gamma rays. To "see" these activities properly, astronomers send specialized instruments to space, so they can "see" the gamma rays from high above Earth's protective blanket of air.

NASA's orbiting Swift satellite and the Fermi Gamma-ray Telescope are among the instruments astronomers currently use to detect and study this radiation.

Gamma-ray Bursts

Over the past few decades, astronomers have detected extremely strong bursts of gamma rays from various points in the sky. By "long", astronomers mean only a few seconds to a few minutes. However, their distances, ranging from millions to billions of light-years away, indicate that these objects and events must be very bright in order to be seen from across the universe. 

The so-called "gamma-ray bursts" are the most energetic and brightest events ever recorded. They can send out prodigious amounts of energy in just a few seconds—more than the Sun will release throughout its entire existence. Until very recently, astronomers could only speculate about what caused such massive explosions. However, recent observations have helped them track down the sources of these events. For example, the Swift satellite detected a gamma-ray burst that came from the birth of a black hole that lay more than 12 billion light-years away from Earth. That is very early in the universe's history. 

There are shorter bursts, less than two seconds long, which were really a mystery for years.

Eventually astronomers linked these events to activities called "kilonovae", which occur when two neutron stars or a neutron star or a black hole merge together. At the moment of the merger, they give off short bursts of gamma-rays. They can also emit gravitational waves.

The History of Gamma-ray Astronomy

Gamma-ray astronomy had its start during the Cold War. Gamma-ray bursts (GRBs) were first detected in the 1960s by the Vela fleet of satellites. At first, people were worried that they were signs of a nuclear attack. Over the next decades, astronomers began searching out the sources of these mysterious pinpoint explosions by searching for optical light (visible light) signals and in ultraviolet, x-ray, and signals. The launch of the Compton Gamma Ray Observatory in 1991 took the search for cosmic sources of gamma rays to new heights.

Its observations showed that GRBs occur throughout the universe and not necessarily inside our own Milky Way Galaxy.

Since that time, the BeppoSAX observatory, launched by the Italian Space Agency, as well as the High Energy Transient Explorer (launched by NASA) have been used to detect GRBs. The European Space Agency's INTEGRAL mission joined the hunt in 2002. More recently, the Fermi Gamma-ray Telescope has surveyed the sky and charted gamma-ray emitters. 

The need for fast detection of GRBs is key to searching out the high-energy events that cause them. For one thing, the very short-burst events die out very quickly, making it difficult to figure out the source. X-satellites can pick up the hunt (since there's usually a related x-ray flare). To help astronomers quickly zero in on a GRB source, the Gamma Ray Bursts Coordinates Network immediately sends out notifications to scientists and institutions involved in studying these outbursts. That way, they can immediately plan follow-up observations using ground-based and space-based optical, radio and X-ray observatories.

As astronomers study more of these outbursts, they'll gain a better understanding of the very energetic activities that cause them. The universe is filled with sources of GRBs, so what they learn will also tell us more about the high-energy cosmos. 

Fast Facts

  • Gamma rays are the most energetic type of radiation known. They are given off by very energetic objects and processes in the universe. 
  • Gamma rays can also be created in the lab, and this type of radiation is used in some medical applications.
  • Gamma-ray astronomy is done with orbiting satellites that can detect them without interference from Earth's atmosphere.