Image: Crab Nebula
NASA / ESA and Jeff Hester (Ariz
This image of the Crab Nebula, taken by the Hubble Space Telescope, reveals the intricate epitaph of a long-dead star. Scientists had long considered the Crab to be a steady celestial light source.
updated 1/6/2011 4:30:58 PM ET 2011-01-06T21:30:58

One of the most well-known celestial objects still has some tricks up its sleeve, according to a new discovery of surprising gamma-ray flares coming from the famous Crab Nebula.

The Crab, long-considered such a steady celestial light that it was used to calibrate other sources, has now had three flare-ups where it brightened significantly in the gamma-ray range for a few days, astronomers report.

"Our belief of a stable Crab got smashed completely — now we have to think again," said Marco Tavani, an astronomer at the INAF-IASF (Istituto Nazionale di Astrofisica-Istituto di Astrofisica Spaziale e Fisica Cosmica) in Rome. Tavani was lead author of one of two papers announcing the discovery of the flares in the Jan. 7 issue of the journal Science.

Tavani's team used the Italian Space Agency's AGILE satellite to observe flares in October 2007 and  September 2010. Another team, led by Stefan Funk and Rolf Buehler at the SLAC National Accelerator Laboratory at Stanford University, observed the September 2010 flare as well, along with one in February 2009, using NASA's Fermi Gamma-ray Space Telescope.

"We had always thought we understand essentially what is going on in the Crab Nebula, but obviously these flares we didn't expect," Funk told

The Crab Nebula is actually the burial ground of a long-dead star. Its photogenic layers of colorful gas are the sloughed-off remains of the star's body, ejected before it collapsed in on itself to create a dense hulk  called a neutron star.

The particular neutron star at the heart of the Crab Nebula is called a pulsar, because it emits a continuous beam of radiation like a lighthouse that appears to pulse when it crosses Earth's line of sight.

Yet why this nebula is emitting these strange flares is not known. These flares, which each lasted a few days, are different from gamma-ray bursts, which are much shorter explosions of light sometimes created when a massive star dies.

"It's still a real mystery what is the ultimate cause," Tavani said.

The basic idea is that the pulsar releases a stream of charged particles that get accelerated — though by what means remains open to debate. When the particles — mostly electrons and their positively charged siblings, positrons — hit the nebula surrounding the pulsar, they interact with the nebula's magnetic field, causing them to release a type of light known as synchrotron radiation, which is mostly in the form of gamma rays.

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These are the first such gamma-ray flares to be seen from any nebula.

"It's essentially telling us something new about how particles are accelerated in astrophysical objects, in particular in these nebulae," Funk said.

This acceleration is about 1,000 times more energetic than the largest man-made particle accelerators, including Fermilab's Tevatron in Illinois and CERN's Large Hadron Collider in Geneva, Tavani said.

The cosmic particle accelerator in the Crab is speeding up those electrons and positrons to nearly the speed of light. As a result, the gamma rays they release are of higher energies than any others ever before seen from astrophysical sources.

"The ultimate goal of the studies is to really understand the process of particle acceleration," Tavani said. "This would be really great for models and theories that can address how particles are accelerated to these very large energies."

There are still many unanswered questions about the mechanism at work in the Crab Nebula, including why it flares up only periodically.

"It's very exciting and it's very important to continue to study it," Tavani said.

The researchers hope to catch the Crab Nebula in the act of flaring up in the future, as well as to possibly observe the phenomenon in other nebulas.

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Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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