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The Crab Nebula

WHEN A MASSIVE STAR EXPLODES at the end of its life, the shattered remains become known as a supernova remnant. The one shown here is called the Crab Nebula.

This is a composite view produced with data from two telescopes: the Herschel Space Observatory and the Hubble Space Telescope. Herschel is a European Space Agency (ESA) mission with important NASA contributions, and Hubble is a NASA mission with important ESA contributions.

A wispy and filamentary cloud of gas and dust, the Crab Nebula is the remnant of a supernova explosion that was observed by Chinese astronomers in the year 1054.

The Crab Nebula

A new view of the Crab Nebula, a supernova remnant, using data gathered by the Herschel Space Observatory and the Hubble Space Telescope.

The image combines Hubble’s view of the nebula at visible wavelengths, obtained using three different filters sensitive to the emission from oxygen and sulphur ions (both shown here in blue). Herschel’s far-infrared image (shown here in red) reveals the emission from dust in the nebula.

While studying the dust content of the Crab Nebula with Herschel, a team of astronomers have detected emission lines from argon hydride, a molecular ion containing the noble gas argon. This is the first detection of a noble-gas based compound in space.

At the heart of the nebula is the Crab Pulsar, a rapidly spinning neutron star that emits a beam of radio waves. As the pulsar spins, the beam sweeps across the field of view as seen from Earth (a pure fluke, as it could have been pointed in any other direction).

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Words and image adapted from information issued by ESA / Herschel / PACS / MESS Key Programme Supernova Remnant  Team; NASA, ESA and Allison Loll / Jeff Hester (Arizona State University).

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Take a tour of the Crab Nebula

THE CRAB NEBULA IS ONE OF THE BRIGHTEST sources of high-energy radiation in the sky. Little wonder—it’s the expanding remains of an exploded star, a supernova seen in 1054.

Scientists have used virtually every telescope at their disposal, including NASA’s Chandra X-ray Observatory, to study the Crab.

The supernova left behind a magnetised neutron star—a pulsar. It’s about the size of Washington DC, but it spins 30 times per second. Each rotation sweeps a lighthouse-like beam past us, creating a pulse of electromagnetic energy detectable across the spectrum.

The pulsar in the Crab Nebula is among the brightest sources of high-energy gamma rays. Recently, NASA’s Fermi Gamma Ray Observatory and Italy’s AGILE Satellite detected strong gamma-ray flares from the Crab, including a series of “superflares” in April 2011.

To help pinpoint the location of these flares, astronomers enlisted Chandra space telescope.

With its keen X-ray eyes, Chandra saw lots of activity, but none of it seems correlated with the superflare. This hints that whatever is causing the flares is happening with about a third of a light-year from the pulsar. And rapid changes in the rise and fall of gamma rays imply that the emission region is very small, comparable in size to our Solar System.

The Chandra observations will likely help scientists to home in on an explanation of the gamma-ray flares one day.

Even after a thousand years, the heart of this shattered star still offers scientists glimpses of staggering energies and cutting edge science.

Adapted from information issued by Harvard-Smithsonian Centre for Astrophysics. Still image courtesy (X-ray) NASA / CXC / SAO / F.Seward, (optical) NASA / ESA / ASU / J.Hester & A.Loll, (infrared) NASA / JPL-Caltech / Univ. Minn. / R.Gehrz.

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Happy birthday, Crab Nebula!

Crab Nebula

The Crab Nebula is a supernova remnant, the aftermath of a titanic stellar explosion that was seen on Earth in the year 1054 CE.

ALMOST A THOUSAND YEARS AGO, on July 4 in the year 1054 CE, astronomers in China and the Middle East noticed a bright new star in the night sky. It appeared in the constellation Taurus, and remained visible for roughly two years.

They couldn’t have known what it really was—a supernova, a titanic stellar explosion that occurred when a massive star reached the end of its life. The explosion was matched by an implosion, which crushed the star’s core and produced a neutron star—made of matter so dense that the atoms’ electrons were forced into their nuclei and combined with the protons to form more neutrons.

The density is so high, that a teaspoon of neutron star material has as much mass as 900 Great Pyramids.

And the neutron star is spinning, making it a pulsar…so called because the natural radiation it emits is sent out along beams, which sweep across our field of view like a lighthouse, seeming to pulse on and off.

So much for the implosion. The explosion hurled enormous quantities of gas into space, eventually forming the glowing cloud, or supernova remnant, we see today. On early photographs, which didn’t show much detail, the nebula looked very crab-like, hence its name.

The distance to the Crab Nebula is a bit uncertain, but it’s probably around 6,300 light-years away. And the cloud is expanding at a speed of 1,500 kilometres per second!

Download a 1280 x 1280 wallpaper image of the Crab Nebula here.

Story by Jonathan Nally. Image courtesy NASA, ESA and Allison Loll/Jeff Hester (Arizona State University). Image acknowledgement: Davide De Martin (ESA/Hubble).

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The Case of the Cosmic Crab

A NEW MOVIE FROM NASA’S Chandra X-ray Observatory shows a sequence of images of the Crab Nebula, taken over an interval of seven months and showing dramatic variations.

The Crab Nebula is one of the most famous objects in the sky. It is the remnant cloud from a supernova (exploding star) that was seen by astronomers in China and other countries in the year 1054.

At the centre of the nebula is a pulsar, a rapidly spinning neutron star. It has a mass greater than our Sun but is only tens of kilometres wide, and is spinning at the rate of 30 times per second.

The pulsar’s spin is gradually slowing down, and as it does so large amounts of energy are injected into its surroundings. In particular, a high-speed wind of matter and anti-matter particles ploughs into the surrounding nebula, creating a shock wave that forms the expanding ‘ring’ seen in the movie.

In addition, ‘jets’ shooting out from the poles of the pulsar spew X-ray emitting matter and antimatter particles in a direction at right angles to the ring.

The goal of the latest Chandra observations was to pinpoint the location of remarkable flares spotted by NASA’s Fermi Gamma Ray Observatory satellite and Italy’s AGILE satellite.

A strong gamma-ray flare was detected from the Crab in September 2010, followed by an even stronger series of “superflares” in April 2011. The gamma-ray satellites were not able to locate the source of the flares within the nebula, but it was hoped that Chandra, with its high-resolution images, would.

Scientists have put together a short sequence of the images taken by Chandra, showing the remarkable changes in the nebula:

Chandra began observing the Crab on monthly intervals beginning six days after the discovery of the gamma-ray flare in September 2010. This established a baseline of seven images before the superflare was seen last month.

What was unexpected, though, was that nothing significant showed up in the Chandra observations as compared with the Fermi observations. Astronomers are now trying to figure out why that is so.

One possible explanation is that the gamma-ray flares picked up by Fermi happened very close to the pulsar, in which case they would have been missed by Chandra, because the Crab pulsar is so bright that the detectors are in essence “overexposed” so variations from that region cannot be observed. (Note that in the movie an artificial source of constant brightness is included to show the position of the pulsar.)

Adapted from information issued by CXC. Crab Nebula image courtesy (X-ray) NASA / CXC / SAO / F. Seward; (optical) NASA / ESA / ASU / J. Hester & A. Loll; (infrared) NASA / JPL-Caltech / Univ. Minn. / R. Gehrz.

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Crab’s candle starts to flicker

  • Crab Nebula is 6,500 light-years from Earth
  • It is the remains of an exploded star (a supernova)
  • Now shown to unexpectedly vary its energy output

DATA FROM SEVERAL NASA satellites has astonished astronomers by revealing unexpected changes in X-ray emission from the Crab Nebula, once thought to be the steadiest high-energy source in the sky.

“For 40 years, most astronomers regarded the Crab as a standard candle,” said Colleen Wilson-Hodge, an astrophysicist at NASA’s Marshall Space Flight Centre, who presented the findings recently at the American Astronomical Society meeting in Seattle.

“Now, for the first time, we’re clearly seeing how much our candle flickers.”

The Crab Nebula is the wreckage of an exploded star whose light reached Earth in 1054. Located 6,500 light-years away, it is one of the most studied objects in the sky.

At the heart of the expanding gas cloud lies what’s left of the original star’s core, a superdense neutron star that spins 30 times a second. All of the Crab’s high-energy emissions are thought to be the result of physical processes that tap into this rapid spin.

For decades, astronomers have regarded the Crab’s X-ray emissions as so stable that they’ve used it to calibrate space-borne instruments. They also customarily describe the emissions of other high-energy sources in “millicrabs,” a unit derived from the nebula’s output.

Crab Nebula

This view of the Crab Nebula comes from the Hubble Space Telescope and spans 12 light-years. The supernova remnant, located 6,500 light-years away, is among the best-studied objects in the sky. Image courtesy NASA / ESA / ASU / J. Hester.

“The Crab Nebula is a cornerstone of high-energy astrophysics,” said team member Mike Cherry at Louisiana State University (LSU), “and this study shows us that our foundation is slightly askew.”

Satellite tag teams

The story unfolded when Cherry and Gary Case, also at LSU, first noticed the Crab’s dimming in observations by the Gamma-ray Burst Monitor (GBM) aboard NASA’s Fermi Gamma-ray Space Telescope.

The team then analysed GBM observations of the object from August 2008 to July 2010 and found an unexpected but steady decline of several percent at four different “hard” X-ray energies.

With the Crab’s apparent constancy well established, the scientists needed to prove that the fadeout was real and was not an instrumental problem associated with the GBM.

“If only one satellite instrument had reported this, no one would have believed it,” Wilson-Hodge said.

Graph showing multi-wavelength observations of the Crab Nebula

Data from four satellites show that the Crab Nebula's energy output has varied. Powerful gamma-ray flares (pink vertical lines) have been detected as well. Graph courtesy NASA Goddard Space Flight Centre.

So the team amassed data from the fleet of sensitive X-ray observatories now in orbit—NASA’s Rossi X-Ray Timing Explorer (RXTE) and Swift satellites and the European Space Agency’s International Gamma-Ray Astrophysics Laboratory (INTEGRAL).

The results confirm a real intensity decline of about 7 percent at certain energy ranges. They also show that the Crab has brightened and faded by as much as 3.5 percent a year since 1999.

The scientists say that astronomers will need to find new ways to calibrate instruments in flight and to explore the possible effects of the inconstant Crab on past findings.

Showing some flare

Fermi’s other instrument, the Large Area Telescope, has detected unprecedented gamma-ray flares from the Crab, showing that it is also surprisingly variable at much higher energies.

The nebula’s power comes from the central neutron star, which is also a pulsar that emits fast, regular radio and X-ray pulses. This pulsed emission exhibits no changes associated with the decline, so it cannot be the source.

Instead, researchers suspect that the long-term changes probably occur in the nebula’s central light-year, but observations with future telescopes will be needed to know for sure.

Adapted from information issued by NASA MSFC.

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