But we humans are clever little animals and have managed to build instruments that can see the light we cannot. One of these is NASA’s Fermi Gamma-ray Space Telescope, an observatory that hangs out in low Earth orbit and monitors the sky for gamma rays, the highest energy light in the universe.
Fermi constantly monitors the entire sky, observing gamma-ray sources and how they change over time, giving astronomers a map of the different producers of gamma-rays that we can detect. This data is compiled into a catalog that researchers can use to investigate the production of gamma radiation.
The animation represents a year of fluctuating gamma radiation from 1,525 sources, represented by pulsating purple circles, collected between February 2022 and February 2023, with each image representing three days of observations. The larger the circle, the brighter the gamma radiation.
The yellow circle, meanwhile, represents the Sun’s apparent path across the sky during that time period.
“We were inspired to put together this database by astronomers who study galaxies and wanted to compare visible-light and gamma-ray curves over long time scales,” said astrophysicist Daniel Kocevski of NASA’s Marshall Space Flight Center in Huntsville.
“We got requests to process one object at a time. Now the scientific community has access to all the analyzed data for the entire catalog.”
Most of the twinkling lights you see come from a type of galaxy known as blazars. These are a subset of quasar galaxies. A quasar is a galaxy with an extremely active core, meaning that the supermassive black hole is sucking up material at an enormous rate. This material is heated by the extreme activity around the black hole so that it blasts across space. Quasars emit the brightest light in the universe.
Some of these quasars have jets of plasma sent from the galactic core. When the black hole is born, some of the material swirling around it is redirected and accelerated along the magnetic field lines outside the event horizon. When it reaches the poles, this material is sent into space at high speeds, often approaching the speed of light in a vacuum.
A blazar is a quasar whose jet is directed towards or nearly towards Earth. Because of this orientation, the light appears even brighter over the entire spectrum. Blazars are known sources of gamma radiation, but their light fluctuates on fairly short timescales; their fluctuations can help astronomers study how these giants feed.
Combined with other data, they can also help answer questions about the universe. For example, detections of neutrinos by observatories such as IceCube in Antarctica have only recently been traced back to blazar galaxies.
Blazars represent over 90 percent of the gamma-ray sources in the new addition to the Fermi gamma-ray catalog. The other objects that emit gamma rays include a type of neutron star called pulsars, the lasered remnants of material left over from supernova explosions, and binary systems such as binary neutron stars.
And there’s the gamma-ray glow from the plane of the Milky Way galaxy, represented in the animation by a splotchy orange band stretching across the center of the image. There lighter color represents a more brilliant glow.
The long-term observations will hopefully provide a deeper insight into some of the phenomena associated with gamma ray sources. For example, tracking a neutrino to a brighter period of a blazar’s activity can help narrow down the processes that produce these mysterious particles.
“Having the historical light curve database,” says astrophysicist Michela Negro of the University of Maryland, Baltimore County, and NASA’s Goddard Space Flight Center, “can lead to new multimessenger insights into past events.”
And we get a hint of how we might see the universe if we had alien eyes.
The recently updated catalog is freely available from The Astrophysical Journal Supplement Series.