Fermi - Blog Posts

Cosmic Piece of Pi!

Did you know that pi is involved nearly anywhere you look? We’re not talking about your favorite pastry! Pi (also written as the Greek letter 𝞹, or the number 3.14159...) is an irrational number, which means it can’t be written as a simple fraction like ½. It is the ratio of a circle’s circumference (the distance around its edge) to its diameter (the distance across it) and will always be the same number, regardless of the circle’s size. Here are some places you can find pi in the universe around us!

Our Transiting Exoplanet Survey Satellite, TESS, watches slices of the sky in its hunt for worlds outside our solar system — how many exoplanets are in its night-sky pie? Last July, TESS scientists created a mosaic of 208 images of the southern sky. At that time, it contained 29 confirmed and 1,000 possible exoplanets, and we’re still studying the data to find more. Since this awe-inspiring image is of the southern hemisphere (or half of a 3D circle), there will always be pi! Every slice contains something delicious for scientists to study.

Pi recently played a crucial role in new discoveries about Alpha Draconis, a well-studied pair of stars. After discovering these stars regularly eclipse each other, pi helped scientists learn more about them. Scientists detected the eclipses while monitoring the brightness of Alpha Draconis for periodic dips that could’ve been caused by planets passing between the star and us. Instead of a planet, though, researchers found that its smaller partner in crime was passing in between us and the larger star for about six hours at a time! 💫

Pi comes in handy as we learn more about these two stars. Knowing the percentage of the decrease in Alpha Draconis’ light and the formula for the area of a circle (A=𝞹r2 — or area equals pi times the square of the circle's radius), scientists can predict the sizes of both stars.  Because stars typically orbit in an elliptical (or oval) shape, pi also helps scientists use the detection of these eclipses to figure out the orbits of the two stars!

So far we’ve seen pi in many places! But it's also interesting to look at where pi can't be found! We mentioned earlier that many orbit calculations involve pi … but not every one does! Pi does not factor into calculations of hyperbolic orbits — orbits that aren't complete, or don't return to where they started — the same way that it does with elliptical orbits! This is most commonly seen with comets. While many comets orbit normally in our solar system, some oddballs just pass through, like the interstellar ‘Oumuamua that zipped passed us in 2017. ☄️

Perhaps the most popular place you may find pi is in the shape of a typical pie! While NASA’s Fermi Gamma-ray Space Telescope studies gamma-rays, and not blueberries, we think this cool Fermi pie is worth sharing for Pi Day!

Find more ways scientists look up at the night sky and use pi here. And now, don’t be irrational, and go have some pi(e)! 🥧

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Five Record-Setting Gamma-ray Bursts!

For 10 years, our Fermi Gamma-ray Space Telescope has scanned the sky for gamma-ray bursts (GRBs), the universe’s most luminous explosions!

Most GRBs occur when some types of massive stars run out of fuel and collapse to create new black holes. Others happen when two neutron stars, superdense remnants of stellar explosions, merge. Both kinds of cataclysmic events create jets of particles that move near the speed of light.

A new catalog of the highest-energy blasts provides scientists with fresh insights into how they work. Below are five record-setting events from the catalog that have helped scientists learn more about GRBs:

1. Super-short burst in Boötes!

The short burst 081102B, which occurred in the constellation Boötes on Nov. 2, 2008, is the briefest LAT-detected GRB, lasting just one-tenth of a second!

2. Long-lived burst!

Long-lived burst 160623A, spotted on June 23, 2016, in the constellation Cygnus, kept shining for almost 10 hours at LAT energies — the longest burst in the catalog.

For both long and short bursts, the high-energy gamma-ray emission lasts longer than the low-energy emission and happens later.

3. Highest energy gamma-rays!

The highest-energy individual gamma ray detected by Fermi’s LAT reached 94 billion electron volts (GeV) and traveled 3.8 billion light-years from the constellation Leo. It was emitted by 130427A, which also holds the record for the most gamma rays — 17 — with energies above 10 GeV.

4. In a constellation far, far away!

The farthest known GRB occurred 12.2 billion light-years away in the constellation Carina. Called 080916C, researchers calculate the explosion contained the power of 9,000 supernovae.

5. Probing the physics of our cosmos!

The known distance to 090510 helped test Einstein’s theory that the fabric of space-time is smooth and continuous. Fermi detected both a high-energy and a low-energy gamma ray at nearly the same instant. Having traveled the same distance in the same amount of time, they showed that all light, no matter its energy, moves at the same speed through the vacuum of space.

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5 of Your Fermi Gamma-ray Space Telescope Questions Answered

The Fermi Gamma-ray Space Telescope is a satellite in low-Earth orbit that detects gamma rays from exotic objects like black holes, neutron stars and fast-moving jets of hot gas. For 11 years Fermi has seen some of the highest-energy bursts of light in the universe and is helping scientists understand where gamma rays come from.

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?

No. Although both are named after Enrico Fermi, the Fermi telescope and the Fermi Paradox have nothing to do with one another.

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. 

3. Does the telescope shoot gamma rays?

No. Fermi DETECTS gamma rays using its two instruments, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM).

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.

The highest-energy gamma ray from a gamma-ray burst was detected by Fermi’s LAT, and traveled 3.8 billion light-years to reach us from the constellation Leo.

4. Will gamma rays turn me into a superhero?

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!

Now that you know the basics, you probably want to learn more! Follow the Fermi Gamma-ray Space Telescope on Twitter (@NASAFermi) or Facebook (@nasafermi), and check out more awesome stuff on our Fermi webpage.

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Blowing Bubbles in the Gamma-ray Sky

Did you know our Milky Way galaxy is blowing bubbles? Two of them, each 25,000 light-years tall! They extend above and below the disk of the galaxy, like the two halves of an hourglass. We can’t see them with our own eyes because they’re only apparent in gamma-ray light, the highest-energy light in the universe.

We didn’t even know these humongous structures were smack in the middle of our galaxy until 2010. Scientists found them when they analyzed the first two years of data from NASA’s Fermi Gamma-ray Space Telescope. They dubbed them the “Fermi bubbles” and found that in addition to being really big and spread out, they seem to have well-defined edges. The bubbles’ shape and the light they give off led scientists to think they were created by a rapid release of energy. But by what? And when?

One possible explanation is that they could be leftovers from the last big meal eaten by the supermassive black hole at the center of our galaxy. This monster is more than 4 million times the mass of our own Sun. Scientists think it may have slurped up a big cloud of hydrogen between 6 and 9 million years ago and then burped jets of hot gas that we see in gamma rays and X-rays.

Another possible explanation is that the bubbles could be the remains of star formation. There are massive clusters of stars at very the center of the Milky Way — sometimes the stars are so closely packed they’re a million times more dense than in the outer suburb of the galaxy where we live. If there was a burst of star formation in this area a few million years ago, it could have created the surge of gas needed to in turn create the Fermi bubbles.

It took us until 2010 to see these Fermi bubbles because the sky is filled with a fog of other gamma rays that can obscure our view. This fog is created when particles moving near light speed bump into gas, dust, and light in the Milky Way. These collisions produce gamma rays, and scientists had to factor out the fog to unveil the bubbles.

Scientists continue to study the possible causes of these massive bubbles using the 10 years of data Fermi has collected so far. Fermi has also made many other exciting discoveries — like the the collision of superdense neutron stars and the nature of space-time. Learn more about Fermi and how we’ve been celebrating its first decade in space.

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Things That Go Bump in the Gamma Rays

Some people watch scary movies because they like being startled. A bad guy jumps out from around a corner! A monster emerges from the shadows! Scientists experience surprises all the time, but they’re usually more excited than scared. Sometimes theories foreshadow new findings — like when there’s a dramatic swell in the movie soundtrack — but often, discoveries are truly unexpected. 

Scientists working with the Fermi Gamma-Ray Space Telescope have been jumping to study mysterious bumps in the gamma rays for a decade now. Gamma rays are the highest-energy form of light. Invisible to human eyes, they’re created by some of the most powerful and unusual events and objects in the universe. In celebration of Halloween, here are a few creepy gamma-ray findings from Fermi’s catalog.

Stellar Graveyards

If you were to walk through a cemetery at night, you’d expect to trip over headstones or grave markers. Maybe you’d worry about running into a ghost. If you could explore the stellar gravesite created when a star explodes as a supernova, you’d find a cloud of debris expanding into interstellar space. Some of the chemical elements in that debris, like gold and platinum, go on to create new stars and planets! Fermi found that supernova remnants IC 443 and W44 also accelerate mysterious cosmic rays, high-energy particles moving at nearly the speed of light. As the shockwave of the supernova expands, particles escape its magnetic field and interact with non-cosmic-ray particles to produce gamma rays. 

Ghost Particles

But the sources of cosmic rays aren’t the only particle mysteries Fermi studies. Just this July, Fermi teamed up with the IceCube Neutrino Observatory in Antarctica to discover the first source of neutrinos outside our galactic neighborhood. Neutrinos are particles that weigh almost nothing and rarely interact with anything. Around a trillion of them pass through you every second, ghost-like, without you noticing and then continue on their way. (But don’t worry, like a friendly ghost, they don’t harm you!) Fermi traced the neutrino IceCube detected back to a supermassive black hole in a distant galaxy. By the time it reached Earth, it had traveled for 3.7 billion years at almost the speed of light!

Black Widow Pulsars

Black widows and redbacks are species of spiders with a reputation for devouring their partners. Astronomers have discovered two types of star systems that behave in a similar way. Sometimes when a star explodes as a supernova, it collapses back into a rapidly spinning, incredibly dense star called a pulsar. If there’s a lighter star nearby, it can get stuck in a close orbit with the pulsar, which blasts it with gamma rays, magnetic fields and intense winds of energetic particles. All these combine to blow clouds of material off the low-mass star. Eventually, the pulsar can eat away at its companion entirely.

Dark Matter

What’s scarier than a good unsolved mystery? Dark matter is a little-understood substance that makes up most of the matter in the universe. The stuff that we can see — stars, people, haunted houses, candy — is made up of normal matter. But our surveys of the cosmos tell us there’s not enough normal matter to keep things working the way they do. There must be another type of matter out there holding everything together. One of Fermi’s jobs is to help scientists narrow down the search for dark matter. Last year, researchers noticed that most of the gamma rays coming from the Andromeda galaxy are confined to its center instead of being spread throughout. One possible explanation is that accumulated dark matter at the center of the galaxy is emitting gamma rays!

Fermi has helped us learn a lot about the gamma-ray universe over the last 10 years. Learn more about its accomplishments and the other mysteries it’s working to solve. What other surprises are waiting out among the stars?

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Gamma-ray Bursts: Black Hole Birth Announcements

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!

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Pick Your Favorite Findings From Fermi’s First Decade

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.

Here are a few of our favorite Fermi discoveries, pick your favorite in the first round of our “Fermi Science Playoff.” 

Colliding Neutron Stars

In 2017, Fermi detected a gamma ray burst at nearly the same moment ground observatories detected gravitational waves from two merging neutron stars. This was the first time light and ripples in space-time were detected from the same source.

The Sun and Moon in Gamma Rays

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.

Record Rare from a Blazar

The supermassive black hole at the center of the galaxy 3C 279 weighs a billion times the mass of our Sun. In June 2015, this blazar became the brightest gamma-ray source in the sky due to a record-setting flare.

The First Gamma-Ray Pulsar in Another Galaxy

In 2015, for the first time, Fermi discovered a gamma-ray pulsar, a kind of rapidly spinning superdense star, in a galaxy outside our own. The object, located on the outskirts of the Tarantula Nebula, also set the record for the most luminous gamma-ray pulsar we’ve seen so far.

A Gamma-Ray Cycle in Another Galaxy

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 Rays from Novae

A nova is a fairly common, short-lived kind of explosion on the surface of a white dwarf, a type of compact star not much larger than Earth. In 2014, Fermi observed several novae and found that they almost always produce gamma-rays, giving scientists a new type of source to explore further with the telescope.

A Record-Setting Cosmic Blast

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.

Cosmic Rays from Supernova Leftovers

Cosmic rays are particles that travel across the cosmos at nearly the speed of light. They are hard to track back to their source because they veer off course every time they encounter a magnetic field. In 2013, Fermi showed that these particles reach their incredible speed in the shockwaves of supernova remains — a theory proposed in 1949 by the satellite’s namesake, the Italian-American physicist Enrico Fermi.

Discovery of a Transformer Pulsar

In 2013, the pulsar in a binary star system called AY Sextanis switched from radio emissions to high-energy gamma rays. Scientists think the change reflects erratic interaction between the two stars in the binary.

Gamma-Ray Measurement of a Gravitational Lens

A gravitational lens is a kind of natural cosmic telescope that occurs when a massive object in space bends and amplifies light from another, more distant object. In 2012, Fermi used gamma rays to observe a spiral galaxy 4.03 billion light-years away bending light coming from a source 4.35 billion light-years away.

New Limits on Dark Matter

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.

Thunderstorms Hurling Antimatter into Space

Terrestrial gamma-ray flashes are created by thunderstorms. In 2011, Fermi scientists announced the satellite had detected beams of antimatter above thunderstorms, which they think are a byproduct of gamma-ray flashes.

Giant Gamma-Ray Bubbles in the Milky Way

Using data from Fermi in 2010, scientists discovered a pair of “bubbles” emerging from above and below the Milky Way. These enormous bubbles are half the length of the Milky Way and were probably created by our galaxy’s supermassive black hole only a few million years ago.

Hint of Starquakes in a Magnetar

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.

Have a favorite Fermi discovery or want to learn more? Cast your vote in the first of four rounds of the Fermi Science Playoff to help rank Fermi’s findings. Or follow along as we celebrate the mission all year.

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What are the Universe’s Most Powerful Particle Accelerators?

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 aren't dangerous to those of us who spend our lives within Earth's atmosphere. But if you spend a lot of time in orbit or are thinking about traveling to Mars, you need to plan how to protect yourself from the radiation caused by cosmic rays.

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.

Most cosmic rays reach the same amount of energy a small particle accelerator could produce. But some zoom through the cosmos at energies 40 million times higher than particles created by the world’s most powerful man-made accelerator, the Large Hadron Collider. (Lightning is also a pretty good particle accelerator).

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!

There are still other cosmic ray sources on the table — like active galactic nuclei — and Fermi continues to look for them. Learn more about what Fermi’s discovered over the last 10 years and how we’re celebrating its accomplishments.

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What's Made in a Thunderstorm and Faster Than Lightning? Gamma Rays!

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.

Scientists suspect that lightning reconfigures the cloud's electrical field. In some cases this allows electrons to rush toward the upper part of the storm at nearly the speed of light. That makes thunderstorms the most powerful natural particle accelerators on Earth!

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!

Learn more about what Fermi's discovered about gamma rays over the last 10 years and how we're celebrating its accomplishments.

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The Universe's Brightest Lights Have Some Dark Origins

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.

Fermi's helped us learn a lot about the gamma-ray universe over the last 10 years. Learn more about Fermi and how we're celebrating its accomplishments all year.

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Meet Fermi: Our Eyes on the Gamma-Ray Sky

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. 

Fermi has had many other things named after him, like Fermi’s Paradox, the Fermi National Accelerator Laboratory, the Enrico Fermi Nuclear Generating Station, the Enrico Fermi Institute, and the synthetic element fermium.

Photo courtesy of Argonne National Laboratory

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. 

Gamma rays were in the news last year because of something Fermi spotted at almost the same time as the National Science Foundation (NSF)’s Laser Interferometer Gravitational-Wave Observatory (LIGO) and European Gravitational Observatory’s Virgo on August 17, 2017. Fermi, LIGO, Virgo, and numerous other observatories spotted the merger of two neutron stars. It was the first time that gravitational waves and light were confirmed to come from the same source.

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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