Did you know some of the brightest sources of light in the sky come from black holes in the centers of galaxies? It sounds a little contradictory, but it’s true! They may not look bright to our eyes, but satellites have spotted oodles of them across the universe.
One of those satellites is our Fermi Gamma-ray Space Telescope. Fermi has found thousands of these kinds of galaxies in the 10 years it’s been operating, and there are many more out there!
Black holes are regions of space that have so much gravity that nothing - not light, not particles, nada - can escape. Most galaxies have supermassive black holes at their centers - these are black holes that are hundreds of thousands to billions of times the mass of our sun - but active galactic nuclei (also called “AGN” for short, or just “active galaxies”) are surrounded by gas and dust that’s constantly falling into the black hole. As the gas and dust fall, they start to spin and form a disk. Because of the friction and other forces at work, the spinning disk starts to heat up.
The disk’s heat gets emitted as light - but not just wavelengths of it that we can see with our eyes. We see light from AGN across the entire electromagnetic spectrum, from the more familiar radio and optical waves through to the more exotic X-rays and gamma rays, which we need special telescopes to spot.
About one in 10 AGN beam out jets of energetic particles, which are traveling almost as fast as light. Scientists are studying these jets to try to understand how black holes - which pull everything in with their huge amounts of gravity - somehow provide the energy needed to propel the particles in these jets.
Many of the ways we tell one type of AGN from another depend on how they’re oriented from our point of view. With radio galaxies, for example, we see the jets from the side as they’re beaming vast amounts of energy into space. Then there’s blazars, which are a type of AGN that have a jet that is pointed almost directly at Earth, which makes the AGN particularly bright.
Our Fermi Gamma-ray Space Telescope has been searching the sky for gamma ray sources for 10 years. More than half (57%) of the sources it has found have been blazars. Gamma rays are useful because they can tell us a lot about how particles accelerate and how they interact with their environment.
So why do we care about AGN? We know that some AGN formed early in the history of the universe. With their enormous power, they almost certainly affected how the universe changed over time. By discovering how AGN work, we can understand better how the universe came to be the way it is now.
A flash of lightning. A roll of thunder. These are normal stormy sights and sounds. But sometimes, up above the clouds, stranger things happen. Our Fermi Gamma-ray Space Telescope has spotted bursts of gamma rays - some of the highest-energy forms of light in the universe - coming from thunderstorms. Gamma rays are usually found coming from objects with crazy extreme physics like neutron stars and black holes.
So why is Fermi seeing them come from thunderstorms?
Thunderstorms form when warm, damp air near the ground starts to rise and encounters colder air. As the warm air rises, moisture condenses into water droplets. The upward-moving water droplets bump into downward-moving ice crystals, stripping off electrons and creating a static charge in the cloud.
The top of the storm becomes positively charged, and the bottom becomes negatively charged, like two ends of a battery. Eventually the opposite charges build enough to overcome the insulating properties of the surrounding air - and zap! You get lightning.
When those electrons run into air molecules, they emit a terrestrial gamma-ray flash, which means that thunderstorms are creating some of the highest energy forms of light in the universe. But that’s not all - thunderstorms can also produce antimatter! Yep, you read that correctly! Sometimes, a gamma ray will run into an atom and produce an electron and a positron, which is an electron’s antimatter opposite!
The Fermi Gamma-ray Space Telescope can spot terrestrial gamma-ray flashes within 500 miles of the location directly below the spacecraft. It does this using an instrument called the Gamma-ray Burst Monitor which is primarily used to watch for spectacular flashes of gamma rays coming from the universe.
There are an estimated 1,800 thunderstorms occurring on Earth at any given moment. Over the 10 years that Fermi has been in space, it has spotted about 5,000 terrestrial gamma-ray flashes. But scientists estimate that there are 1,000 of these flashes every day - we’re just seeing the ones that are within 500 miles of Fermi’s regular orbits, which don’t cover the U.S. or Europe.
The map above shows all the flashes Fermi has seen since 2008. (Notice there’s a blob missing over the lower part of South America. That’s the South Atlantic Anomaly, a portion of the sky where radiation affects spacecraft and causes data glitches.)
Fermi has also spotted terrestrial gamma-ray flashes coming from individual tropical weather systems. The most productive system we’ve seen was Tropical Storm Julio in 2014, which later became a hurricane. It produced four flashes in just 100 minutes!
Black holes, cosmic rays, neutron stars and even new kinds of physics — for 10 years, data from our Fermi Gamma-ray Space Telescope have helped unravel some of the biggest mysteries of the cosmos. And Fermi is far from finished!
On June 11, 2008, at Cape Canaveral in Florida, the countdown started for Fermi, which was called the Gamma-ray Large Area Space Telescope (GLAST) at the time.
The telescope was renamed after launch to honor Enrico Fermi, an Italian-American pioneer in high-energy physics who also helped develop the first nuclear reactor.
The Fermi telescope measures some of the highest energy bursts of light in the universe; watching the sky to help scientists answer all sorts of questions about some of the most powerful objects in the universe.
Its main instrument is the Large Area Telescope (LAT), which can view 20% of the sky at a time and makes a new image of the whole gamma-ray sky every three hours. Fermi’s other instrument is the Gamma-ray Burst Monitor. It sees even more of the sky at lower energies and is designed to detect brief flashes of gamma-rays from the cosmos and Earth.
This sky map below is from 2013 and shows all of the high energy gamma rays observed by the LAT during Fermi’s first five years in space. The bright glowing band along the map’s center is our own Milky Way galaxy!
So what are gamma rays?
Well, they’re a form of light. But light with so much energy and with such short wavelengths that we can’t see them with the naked eye. Gamma rays require a ton of energy to produce — from things like subatomic particles (such as protons) smashing into each other.
Here on Earth, you can get them in nuclear reactors and lightning strikes. Here’s a glimpse of the Seattle skyline if you could pop on a pair of gamma-ray goggles. That purple streak? That’s still the Milky Way, which is consistently the brightest source of gamma rays in our sky.
In space, you find that kind of energy in places like black holes and neutron stars. The raindrop-looking animation below shows a big flare of gamma rays that Fermi spotted coming from something called a blazar, which is a kind of quasar, which is different from a pulsar… actually, let’s back this up a little bit.
One of the sources of gamma rays that Fermi spots are pulsars. Pulsars are a kind of neutron star, which is a kind of star that used to be a lot bigger, but collapsed into something that’s smaller and a lot denser. Pulsars send out beams of gamma rays. But the thing about pulsars is that they rotate.
So Fermi only sees a beam of gamma rays from a pulsar when it’s pointed towards Earth. Kind of like how you only periodically see the beam from a lighthouse. These flashes of light are very regular. You could almost set your watch by them!
Quasars are supermassive black holes surrounded by disks of gas. As the gas falls into the black hole, it releases massive amount of energy, including — you guessed it — gamma rays. Blazars are quasars that send out beams of gamma rays and other forms of light — right in our direction.
When Fermi sees them, it’s basically looking straight down this tunnel of light, almost all the way back to the black hole. This means we can learn about the kinds of conditions in that environment when the rays were emitted. Fermi has found about 5,500 individual sources of gamma rays, and the bulk of them have been blazars, which is pretty nifty.
But gamma rays also have many other sources. We’ve seen them coming from supernovas where stars die and from star factories where stars are born. They’re created in lightning storms here on Earth, and our own Sun can toss them out in solar flares.
Fermi has been looking at the sky for almost 10 years now, and it’s helped scientists advance our understanding of the universe in many ways. And the longer it looks, the more we’ll learn. Discover more about how we’ll be celebrating Fermi’s achievements all year.
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Confused? Don’t be! We get a ton of questions about Fermi and figured we’d take a moment to answer a few of them here.
1. Who was this Fermi guy?
The Fermi telescope was named after Enrico Fermi in recognition of his work on how the tiny particles in space become accelerated by cosmic objects, which is crucial to understanding many of the objects that his namesake satellite studies.
Enrico Fermi was an Italian physicist and Nobel Prize winner (in 1938) who immigrated to the United States to be a professor of physics at Columbia University, later moving to the University of Chicago.
Original image courtesy Argonne National Laboratory
Over the course of his career, Fermi was involved in many scientific endeavors, including the Manhattan Project, quantum theory and nuclear and particle physics. He even engineered the first-ever atomic reactor in an abandoned squash court (squash is the older, English kind of racquetball) at the University of Chicago.
There are a number of other things named after Fermi, too: Fermilab, the Enrico Fermi Nuclear Generating Station, the Enrico Fermi Institute and more. (He’s kind of a big deal in the physics world.)
Fermi even had something to say about aliens! One day at lunch with his buddies, he wondered if extraterrestrial life existed outside our solar system, and if it did, why haven’t we seen it yet? His short conversation with friends sparked decades of research into this idea and has become known as the Fermi Paradox — given the vastness of the universe, there is a high probability that alien civilizations exist out there, so they should have visited us by now.
2. So, does the Fermi telescope look for extraterrestrial life?
Fermi does not look for aliens, extraterrestrial life or anything of the sort! If aliens were to come our way, Fermi would be no help in identifying them, and they might just slip right under Fermi’s nose. Unless, of course, those alien spacecraft were powered by processes that left behind traces of gamma rays.
Fermi detects gamma rays, the highest-energy form of light, which are often produced by events so far away the light can take billions of years to reach Earth. The satellite sees pulsars, active galaxies powered by supermassive black holes and the remnants of exploding stars. These are not your everyday stars, but the heavyweights of the universe.
The LAT sees about one-fifth of the sky at a time and records gamma rays that are millions of times more energetic than visible light. The GBM detects lower-energy emissions, which has helped it identify more than 2,000 gamma-ray bursts – energetic explosions in galaxies extremely far away.
Nope. In movies and comic books, the hero has a tragic backstory and a brush with death, only to rise out of some radioactive accident stronger and more powerful than before. In reality, that much radiation would be lethal.
In fact, as a form of radiation, gamma rays are dangerous for living cells. If you were hit with a huge amount of gamma radiation, it could be deadly — it certainly wouldn’t be the beginning of your superhero career.
5. That sounds bad…does that mean if a gamma-ray burst hit Earth, it would wipe out the planet and destroy us all?
Thankfully, our lovely planet has an amazing protector from gamma radiation: an atmosphere. That is why the Fermi telescope is in orbit; it’s easier to detect gamma rays in space!
Gamma-ray bursts are so far away that they pose no threat to Earth. Fermi sees gamma-ray bursts because the flash of gamma rays they release briefly outshines their entire home galaxies, and can sometimes outshine everything in the gamma-ray sky.
If a habitable planet were too close to one of these explosions, it is possible that the jet emerging from the explosion could wipe out all life on that planet. However, the probability is extremely low that a gamma-ray burst would happen close enough to Earth to cause harm. These events tend to occur in very distant galaxies, so we’re well out of reach.
We hope that this has helped to clear up a few misconceptions about the Fermi Gamma-ray Space Telescope. It’s a fantastic satellite, studying the craziest extragalactic events and looking for clues to unravel the mysteries of our universe!
Every second, every square meter of Earth’s atmosphere is pelted by thousands of high-energy particles traveling at nearly the speed of light. These zippy little assailants are called cosmic rays, and they’ve been puzzling scientists since they were first discovered in the early 1900s. One of the Fermi Gamma-ray Space Telescope’s top priority missions has been to figure out where they come from.
“Cosmic ray” is a bit of a misnomer. Makes you think they’re light, right? But they aren’t light at all! They’re particles that mostly come from outside our solar system — which means they’re some of the only interstellar matter we can study — although the Sun also produces some. Cosmic rays hit our atmosphere and break down into secondary cosmic rays, most of which disperse quickly in the atmosphere, although a few do make it to Earth’s surface.
Cosmic rays are subatomic particles — smaller particles that make up atoms. Most of them (99%) are nuclei of atoms like hydrogen and helium stripped of their electrons. The other 1% are lone electrons. When cosmic rays run into molecules in our atmosphere, they produce secondary cosmic rays, which include even lighter subatomic particles.
So where do cosmic rays come from? We should just be able to track them back to their source, right? Not exactly. Any time they run into a strong magnetic field on their way to Earth, they get deflected and bounce around like a game of cosmic pinball. So there’s no straight line to follow back to the source. Most of the cosmic rays from a single source don’t even make it to Earth for us to measure. They shoot off in a different direction while they’re pin balling.
Photo courtesy of Argonne National Laboratory
In 1949 Enrico Fermi — an Italian-American physicist, pioneer of high-energy physics and Fermi satellite namesake — suggested that cosmic rays might accelerate to their incredible speeds by ricocheting around inside the magnetic fields of interstellar gas clouds. And in 2013, the Fermi satellite showed that the expanding clouds of dust and gas produced by supernovas are a source of cosmic rays.
When a star explodes in a supernova, it produces a shock wave and rapidly expanding debris. Particles trapped by the supernova remnant magnetic field bounce around wildly.
Every now and then, they cross the shock wave and their energy ratchets up another notch. Eventually they become energetic enough to break free of the magnetic field and zip across space at nearly the speed of light — some of the fastest-traveling matter in the universe.
How can we track them back to supernovas when they don’t travel in a straight line, you ask? Very good question! We use something that does travel in a straight line — gamma rays (actual rays of light this time, on the more energetic end of the electromagnetic spectrum).
When the particles get across the shock wave, they interact with non-cosmic-ray particles in clouds of interstellar gas. Cosmic ray electrons produce gamma rays when they pass close to an atomic nucleus. Cosmic ray protons, on the other hand, produce gamma rays when they run into normal protons and produce another particle called a pion (Just hold on! We’re almost there!) which breaks down into two gamma rays.
The proton- and electron-produced gamma rays are slightly different. Fermi data taken over four years showed that most of the gamma rays coming from some supernova remnants have the energy signatures of cosmic ray protons knocking into normal protons. That means supernova remnants really are powerful particle accelerators, creating a lot of the cosmic rays that we see!
One
hundred years ago, on May 29, 1919, astronomers observed a total solar eclipse in
an ambitious effort to test Albert
Einstein’s general theory of relativity by seeing it in action. Essentially, Einstein
thought space and time were intertwined in an infinite “fabric,” like an
outstretched blanket. A massive object such as the Sun bends the spacetime blanket
with its gravity, such that light no longer travels in a straight line as it passes
by the Sun.
This
means the apparent positions of background stars seen close to the Sun in the
sky – including during a solar eclipse – should seem slightly shifted in the
absence of the Sun, because the Sun’s gravity bends light. But until the
eclipse experiment, no one was able to test Einstein’s theory of general
relativity, as no one could see stars near the Sun in the daytime otherwise.
The
world celebrated the results of this eclipse experiment— a victory for
Einstein, and the dawning of a new era of our understanding of the universe.
General
relativity has many important consequences for what we see in the cosmos and
how we make discoveries in deep space today. The same is true for Einstein’s slightly
older theory, special relativity, with its widely celebrated equation E=mc². Here
are 10 things that result from Einstein’s theories of relativity:
1. Universal Speed Limit
Einstein’s
famous equation
E=mc²
contains “c,” the speed of light in a vacuum. Although
light comes in many flavors – from the rainbow of colors humans can see to the
radio waves that transmit spacecraft data – Einstein said all light must obey
the speed limit of 186,000 miles (300,000 kilometers) per second. So, even if
two particles of light carry very different amounts of energy, they will travel
at the same speed.
This
has been shown experimentally in space. In 2009, our Fermi Gamma-ray Space Telescope detected two photons at virtually the same moment, with one carrying a million
times more energy than the other. They both came from a high-energy region near
the collision of two neutron stars about 7 billion years ago. A neutron star is
the highly dense remnant of a star that has exploded. While other theories
posited that space-time itself has a “foamy” texture that might slow down more energetic
particles, Fermi’s observations found in favor of Einstein.
2. Strong Lensing
Just
like the Sun bends the light from distant stars that pass close to it, a
massive object like a galaxy distorts the light from another object that is
much farther away. In some cases, this phenomenon can actually help us unveil
new galaxies. We say that the closer object acts like a “lens,” acting like a
telescope that reveals the more distant object. Entire clusters of galaxies can
be lensed and act as lenses, too.
When
the lensing object appears close enough to the more distant object in the sky,
we actually see multiple images of that faraway object. In 1979, scientists
first observed a double image of a quasar, a very bright object at the center
of a galaxy that involves a supermassive black hole feeding off a disk of
inflowing gas. These apparent copies of the distant object change in brightness
if the original object is changing, but not all at once, because of how space
itself is bent by the foreground object’s gravity.
Sometimes,
when a distant celestial object is precisely aligned with another object, we
see light bent into an “Einstein ring” or arc. In this image from our Hubble Space
Telescope,
the sweeping arc of light represents a distant galaxy that has been lensed,
forming a “smiley face” with other galaxies.
3. Weak Lensing
When
a massive object acts as a lens for a farther object, but the objects are not specially
aligned with respect to our view, only one image of the distant object is
projected. This happens much more often. The closer object’s gravity makes the
background object look larger and more stretched than it really is. This is
called “weak lensing.”
Weak
lensing is very important for studying some of the biggest mysteries of the
universe: dark matter and dark energy. Dark matter is an invisible material
that only interacts with regular matter through gravity, and holds together
entire galaxies and groups of galaxies like a cosmic glue. Dark energy behaves like
the opposite of gravity, making objects recede from each other. Three upcoming
observatories – Our Wide Field Infrared
Survey Telescope,
WFIRST, mission, the European-led Euclid space mission with NASA participation,
and the ground-based Large Synoptic Survey Telescope
— will be key players in this effort. By surveying distortions of weakly lensed
galaxies across the universe, scientists can characterize the effects of these persistently
puzzling phenomena.
Gravitational
lensing in general will also enable NASA’s James Webb Space telescope to look
for some of the very first stars and galaxies of the universe.
4. Microlensing
So
far, we’ve been talking about giant objects acting like magnifying lenses for
other giant objects. But stars can also “lens” other stars, including stars
that have planets around them. When light from a background star gets “lensed”
by a closer star in the foreground, there is an increase in the background
star’s brightness. If that foreground star also has a planet orbiting it, then
telescopes can detect an extra bump in the background star’s light, caused by
the orbiting planet. This technique for finding exoplanets, which are planets
around stars other than our own, is called “microlensing.”
Our Spitzer Space Telescope, in collaboration with ground-based
observatories, found an “iceball” planet through microlensing. While
microlensing has so far found less than 100 confirmed planets, WFIRST could find more than 1,000 new
exoplanets using this technique.
5. Black Holes
The
very existence of black holes, extremely dense objects from which no light can escape, is a prediction
of general relativity. They represent the most extreme distortions of the
fabric of space-time, and are especially famous for how their immense gravity affects
light in weird ways that only Einstein’s theory could explain.
In
2019 the Event Horizon Telescope international collaboration, supported by the
National Science Foundation and other partners, unveiled the first image of a black hole’s event
horizon,
the border that defines a black hole’s “point of no return” for nearby
material. NASA’s Chandra
X-ray Observatory, Nuclear
Spectroscopic Telescope Array (NuSTAR),
Neil Gehrels Swift Observatory, and Fermi Gamma-ray Space Telescope all looked
at the same black hole in a coordinated effort, and researchers are still
analyzing the results.
6. Relativistic Jets
This
Spitzer image shows the galaxy Messier 87 (M87) in infrared light, which has a
supermassive black hole at its center. Around the black hole is a disk of
extremely hot gas, as well as two jets of material shooting out in opposite
directions. One of the jets, visible on the right of the image, is pointing
almost exactly toward Earth. Its enhanced brightness is due to the emission of light
from particles traveling toward the observer at near the speed of light, an
effect called “relativistic beaming.” By contrast, the other jet is invisible
at all wavelengths because it is traveling away from the observer near the
speed of light. The details of how such jets work are still mysterious, and
scientists will continue studying black holes for more clues.
7. A Gravitational Vortex
Speaking
of black holes, their gravity is so intense that they make infalling material
“wobble” around them. Like a spoon stirring honey, where honey is the space
around a black hole, the black hole’s distortion of space has a wobbling effect
on material orbiting the black hole. Until recently, this was only theoretical.
But in 2016, an international team of scientists using European Space Agency’s XMM-Newton and our Nuclear Spectroscopic Telescope Array (NUSTAR) announced they had observed the signature of wobbling
matter for the first time. Scientists will continue studying these odd effects
of black holes to further probe Einstein’s ideas firsthand.
Incidentally,
this wobbling of material around a black hole is similar to how Einstein
explained Mercury’s odd orbit. As the closest planet to the Sun, Mercury feels
the most gravitational tug from the Sun, and so its orbit’s orientation is
slowly rotating around the Sun, creating a wobble.
8. Gravitational Waves
Ripples
through space-time called gravitational waves were hypothesized by Einstein
about 100 years ago, but not actually observed until recently. In 2016, an
international collaboration of astronomers working with the Laser Interferometer
Gravitational-Wave Observatory (LIGO)
detectors announced a landmark discovery: This enormous experiment detected the
subtle signal of gravitational waves that had been traveling for 1.3 billion
years after two black holes merged in a cataclysmic event. This opened a brand
new door in an area of science called multi-messenger astronomy, in which both
gravitational waves and light can be studied.
For example,
our telescopes collaborated to measure light from two neutron stars
merging after LIGO detected gravitational wave signals from the event, as
announced in 2017. Given that gravitational waves from this event were detected
mere 1.7 seconds before gamma rays from the merger, after both traveled 140
million light-years, scientists concluded Einstein was right about something
else: gravitational waves and light waves travel at the same speed.
9. The Sun Delaying Radio Signals
Planetary
exploration spacecraft have also shown Einstein to be right about general
relativity. Because spacecraft communicate with Earth using light, in the form
of radio waves, they present great opportunities to see whether the gravity of
a massive object like the Sun changes light’s path.
In
1970, our Jet Propulsion Laboratory announced that Mariner VI and VII,
which completed flybys of Mars in 1969, had conducted experiments using radio
signals — and also agreed with Einstein. Using NASA’s
Deep Space Network (DSN), the two Mariners took several hundred radio measurements for
this purpose. Researchers measured the time it took for radio signals to travel
from the DSN dish in Goldstone, California, to the spacecraft and back. As
Einstein would have predicted, there was a delay in the total roundtrip time
because of the Sun’s gravity. For Mariner VI, the maximum delay was 204
microseconds, which, while far less than a single second, aligned almost
exactly with what Einstein’s theory would anticipate.
In
1979, the Viking landers performed an even more accurate experiment along these
lines. Then, in 2003 a group of scientists used NASA’s Cassini Spacecraft to repeat these kinds of
radio science experiments with 50 times greater precision than Viking. It’s
clear that Einstein’s theory has held up!
10. Proof from Orbiting
Earth
In
2004, we launched a spacecraft called Gravity
Probe B
specifically designed to watch Einstein’s theory play out in the orbit of
Earth. The theory goes that Earth, a rotating body, should be pulling the
fabric of space-time around it as it spins, in addition to distorting light with
its gravity.
The
spacecraft had four gyroscopes and pointed at the star IM Pegasi while orbiting
Earth over the poles. In this experiment, if Einstein had been wrong, these
gyroscopes would have always pointed in the same direction. But in 2011,
scientists announced they had observed tiny changes in the gyroscopes’
directions as a consequence of Earth, because of its gravity, dragging space-time
around it.
BONUS: Your GPS! Speaking of time delays, the
GPS (global positioning system) on your phone or in your car relies on Einstein’s
theories for accuracy. In order to know where you are, you need a receiver –
like your phone, a ground station and a network of satellites orbiting Earth to
send and receive signals. But according to general relativity, because of
Earth’s gravity curving spacetime, satellites experience time moving slightly
faster than on Earth. At the same time, special relativity would say time moves
slower for objects that move much faster than others.
When
scientists worked out the net effect of these forces, they found that the
satellites’ clocks would always be a tiny bit ahead of clocks on Earth. While
the difference per day is a matter of millionths of a second, that change
really adds up. If GPS didn’t have relativity built into its technology, your
phone would guide you miles out of your way!
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Gamma-ray bursts are the brightest, most violent explosions in the universe, but they can be surprisingly tricky to detect. Our eyes can’t see them because they are tuned to just a limited portion of the types of light that exist, but thanks to technology, we can even see the highest-energy form of light in the cosmos — gamma rays.
So how did we discover gamma-ray bursts?
Accidentally!
We didn’t actually develop gamma-ray detectors to peer at the universe — we were keeping an eye on our neighbors! During the Cold War, the United States and the former Soviet Union both signed the Nuclear Test Ban Treaty of 1963 that stated neither nation would test nuclear weapons in space. Just one week later, the US launched the first Vela satellite to ensure the treaty wasn’t being violated. What they saw instead were gamma-ray events happening out in the cosmos!
Things Going Bump in the Cosmos
Each of these gamma-ray events, dubbed “gamma-ray bursts” or GRBs, lasted such a short time that information was very difficult to gather. For decades their origins, locations and causes remained a cosmic mystery, but in recent years we’ve been able to figure out a lot about GRBs. They come in two flavors: short-duration (less than two seconds) and long-duration (two seconds or more). Short and long bursts seem to be caused by different cosmic events, but the end result is thought to be the birth of a black hole.
Short GRBs are created by binary neutron star mergers. Neutron stars are the superdense leftover cores of really massive stars that have gone supernova. When two of them crash together (long after they’ve gone supernova) the collision releases a spectacular amount of energy before producing a black hole. Astronomers suspect something similar may occur in a merger between a neutron star and an already-existing black hole.
Long GRBs account for most of the bursts we see and can be created when an extremely massive star goes supernova and launches jets of material at nearly the speed of light (though not every supernova will produce a GRB). They can last just a few seconds or several minutes, though some extremely long GRBs have been known to last for hours!
A Gamma-Ray Burst a Day Sends Waves of Light Our Way!
Our Fermi Gamma-ray Space Telescope detects a GRB nearly every day, but there are actually many more happening — we just can’t see them! In a GRB, the gamma rays are shot out in a narrow beam. We have to be lined up just right in order to detect them, because not all bursts are beamed toward us — when we see one it’s because we’re looking right down the barrel of the gamma-ray gun. Scientists estimate that there are at least 50 times more GRBs happening each day than we detect!
So what’s left after a GRB — just a solitary black hole? Since GRBs usually last only a matter of seconds, it’s very difficult to study them in-depth. Fortunately, each one leaves an afterglow that can last for hours or even years in extreme cases. Afterglows are created when the GRB jets run into material surrounding the star. Because that material slows the jets down, we see lower-energy light, like X-rays and radio waves, that can take a while to fade. Afterglows are so important in helping us understand more about GRBs that our Neil Gehrels Swift Observatory was specifically designed to study them!
Last fall, we had the opportunity to learn even more from a gamma-ray burst than usual! From 130 million light-years away, Fermi witnessed a pair of neutron stars collide, creating a spectacular short GRB. What made this burst extra special was the fact that ground-based gravitational wave detectors LIGO and Virgo caught the same event, linking light and gravitational waves to the same source for the first time ever!
For over 10 years now, Fermi has been exploring the gamma-ray universe. Thanks to Fermi, scientists are learning more about the fundamental physics of the cosmos, from dark matter to the nature of space-time and beyond. Discover more about how we’ll be celebrating Fermi’s achievements all year!
The Fermi Gamma-ray Space Telescope has been observing some of the most extreme objects and events in the universe — from supermassive black holes to merging neutron stars and thunderstorms — for 10 years. Fermi studies the cosmos using gamma rays, the highest-energy form of light, and has discovered thousands of new phenomena for scientists.
In 2016, Fermi showed the Moon is brighter in gamma rays than the Sun. Because the Moon doesn’t have a magnetic field, the surface is constantly pelted from all directions by cosmic rays. These produce gamma rays when they run into other particles, causing a full-Moon gamma-ray glow.
Many galaxies, including our own, have black holes at their centers. In active galaxies, dust and gas fall into and “feed” the black hole, releasing light and heat. In 2015 for the first time, scientists using Fermi data found hints that a galaxy called PG 1553+113 has a years-long gamma-ray emission cycle. They’re not sure what causes this cycle, but one exciting possibility is that the galaxy has a second supermassive black hole that causes periodic changes in what the first is eating.
Gamma-ray bursts are the most luminous explosions in the universe. In 2013, Fermi spotted the brightest burst it’s seen so far in the constellation Leo. In the first three seconds alone, the burst, called GRB 130427A, was brighter than any other burst seen before it. This record has yet to be shattered.
We can directly observe only 20 percent of the matter in the universe. The rest is invisible to telescopes and is called dark matter — and we’re not quite sure what it is. In 2012, Fermi helped place new limits on the properties of dark matter, essentially narrowing the field of possible particles that can describe what dark matter is.
‘Superflares’ in the Crab Nebula
The Crab Nebula supernova remnant is one of the most-studied targets in the sky — we’ve been looking at it for almost a thousand years! In 2011, Fermi saw it erupt in a flare five times more powerful than any previously seen from the object. Scientists calculate the electrons in this eruption are 100 times more energetic than what we can achieve with particle accelerators on Earth.
Neutron stars have magnetic fields trillions of times stronger than Earth’s. Magnetars are neutron stars with magnetic fields 1,000 times stronger still. In 2009, Fermi saw a storm of gamma-ray bursts from a magnetar called SGR J1550-5418, which scientists think were related to seismic waves rippling across its surface.
A Dark Pulsar
We observe many pulsars using radio waves, visible light or X-rays. In 2008, Fermi found the first gamma-ray only pulsar in a supernova remnant called CTA 1. We think that the “beam” of gamma rays we see from CTA 1 is much wider than the beam of other types of light from that pulsar. Those other beams never sweep across our vision — only the gamma-rays.
A simulated image of NASA’s Nancy Grace Roman Space Telescope’s future observations toward the center of our galaxy, spanning less than 1 percent of the total area of Roman’s Galactic Bulge Time-Domain Survey. The simulated stars were drawn from the Besançon Galactic Model.
Exploring the Changing Universe with the Roman Space Telescope
The view from your backyard might paint the universe as an unchanging realm, where only twinkling stars and nearby objects, like satellites and meteors, stray from the apparent constancy. But stargazing through NASA’s upcoming Nancy Grace Roman Space Telescope will offer a front row seat to a dazzling display of cosmic fireworks sparkling across the sky.
Illustration of the Chandra telescope in orbit around Earth. Credit: NASA/CXC & J. Vaughan
On July 23, 1999, the space shuttle Columbia launched into orbit carrying NASA’s Chandra X-ray Observatory. August 26 marked 25 years since Chandra released its first images.
These were the first of more than 25,000 observations Chandra has taken. This year, as NASA celebrates the 25th anniversary of this telescope and the incredible data it has provided, we’re taking a peek at some of its most memorable moments.
Dolphins x Astronauts: The collab we didn’t know we needed
A pod of curious dolphins added extra meaning and porpoise to the recovery of Crew-9′s SpaceX Dragon capsule and its four explorers shortly after splashdown.
Inside the capsule were astronauts Nick Hague, Suni Williams, Butch Wilmore, and cosmonaut Aleksandr Gorbunov, who splashed down off the coast of Florida at 5:57pm ET (2127 UTC) on March 18, 2025, concluding their scientific mission to the International Space Station.
See Crew-9 return from deorbit to splashdown in this video. (The dolphins appear at 1:33:56.)
