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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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Neutron star bites off more than it can chew

Artist's impression of a neutron star partially devouring a massive clump of matter.

Artist's impression of a neutron star partially devouring a massive clump of matter spat out by its companion star.

ASTRONOMERS HAVE SEEN a faint star flare up at X-ray wavelengths to almost 10,000 times its normal brightness…caused, they think, by the star trying to eat a giant clump of matter.

The flare took place on a neutron star, the collapsed heart of a once much larger star, and part of a binary star system. Only 10 kilometres in diameter, the neutron star is so dense that it generates a strong gravitational field.

The clump of matter was much larger than the neutron star and came from its enormous, blue supergiant companion star.

“This was a huge bullet of gas that the star shot out, and it hit the neutron star…,” says Enrico Bozzo, ISDC Data Centre for Astrophysics, University of Geneva, Switzerland, and team leader of the research.

The flare lasted four hours. The X-rays came from the gas in the clump as it was heated to millions of degrees while being pulled into the neutron star’s intense gravity field.

Because the clump was much bigger than the neutron star, only some of it was swallowed.

An artist's impression of XMM-Newton.

An artist's impression of XMM-Newton.

A lucky observation

The European Space Agency’s XMM-Newton space observatory caught the flare during a scheduled 12.5-hour observation of the system, which is known only by its catalogue number IGR J18410-0535.

But the astronomers were not immediately aware of their catch.

The telescope works through a sequence of observations carefully planned to make the best use of its time, then sends the data to Earth.

It was about 10 days after the observation that Dr Bozzo and his colleagues received the data and quickly realised they had something special. Not only was the telescope pointing in the right direction to see the flare, but the observation had lasted long enough for them to see it from beginning to end.

“I don’t know if there is any way to measure luck, but we were extremely lucky,” says Dr Bozzo. He estimates that an X-ray flare of this magnitude can be expected a few times a year at the most for this particular star system.

The duration of the flare allowed them to estimate the size of the gas clump. It was much larger than the star, probably 16 million kilometres across—that’s about 100 billion times the volume of the Moon, yet it had probably only 1/1,000th of the Moon’s mass.

Adapted from information issued by ESA / AOES Medialab / C. Carreau.

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‘Weird science’ of neutron stars

Cassiopeia A and an artist's impression of a neutron star

Nebula Cassiopeia A, the remains of a massive star that exploded as a supernova. At it's heart is a neutron star (inset, artist's impression), where densities increase from the crust (orange) to the core (red) and finally to the core region where a "superfluid" exists (inner red section).

ASTRONOMERS HAVE GLIMPSED the inner workings of a neutron star and found a unique world where the physics can only be described as “weird.”

A neutron star is the extremely dense, collapsed core left behind from an exploding star, or supernova.

University of Alberta astronomer Craig Heinke’s team found that the neutron star’s core contains a superfluid … a friction-less liquid that could seemingly defy the laws of gravity.

“If you could put some of this superfluid in a jar it would flow up the walls of the container and over the edge,” said Heinke.

Heinke says the core of the neutron star also contains a superconductor, a perfect electrical conductor.

“An electric current in a superconductor never loses energy—it could keep circulating forever.”

Neutron stars contain the densest known matter that is directly observable. One teaspoon of neutron star material weighs six billion tonnes.

“Depending on their composition, superconductors created in laboratories on Earth stop working at anything warmer than -100 to -200 degrees Celsius,” says team member Wynn Ho of the University of Southampton. “In contrast, the incredible densities in neutron stars allow superconductivity at close to a billion degrees Celsius.”

Chandra X-ray telescope

Artist's impression of NASA's Chandra X-ray space telescope.

Cooling down

The discoveries were made when the researchers used NASA’s Chandra X-ray space telescope to investigate a sudden temperature drop on one particular neutron star 11,000 light years from Earth.

Heinke says this neutron star, known as Cassiopeia A, offered the researchers a great opportunity.

“It’s only 330 years old,” said Heinke. “We’ve got ringside seats to studying the life cycle of a neutron star from its collapse to its present, cooling off state.”

The researchers determined that the neutron star’s surface temperature is dropping because its core recently transformed into a superfluid state and is venting off heat in the form of neutrinos … sub-atomic particles that flood through the universe.

Here on Earth our bodies are constantly bombarded by neutrinos from space, with 100 billion neutrinos passing harmlessly though our eyes every second.

They also found that the neutron star’s core is a superconductor … the highest temperature (millions of degrees) superconductor known.

This research helps us to better understand the life cycles of stars, as well as the behaviour of matter at incredibly high densities.

Adapted from information issued by University of Alberta and NASA. Image credits: X-ray, NASA / CXC / UNAM / Ioffe / D. Page, P. Shternin et al.; optical, NASA / STScI; illustration, NASA / CXC / M. Weiss.

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Crushed star puts Einstein to the test

Artist's impression of the J1749 system

Artist's impression of the J1749 system, which comprises a superdense pulsar (a spinning neutron star) and a normal star closely orbiting each other.

  • Stellar pair includes pulsar and a normal star
  • X-rays from the pulsar picked up by space telescope
  • Measurements can test an aspect of Einstein’s theories

A space telescope that “sees” X-rays could be used to test a key prediction of Einstein’s relativity theory.

Scientists using NASA’s Rossi X-ray Timing Explorer (RXTE) have studied a pair of stars that orbit each other so closely that one of them moves in front of the other and causes regular eclipses.

The astronomers can use these eclipses, along with standard physics laws, to estimate the size and mass of one of the stars.

Known collectively as Swift J1749.4-2807—or J1749 for short—one of the objects is a super-dense body called a pulsar, while the other is a normal star. The system is 22,000 light-years from Earth.

Pulsars are spinning neutron stars, the remnant cores left over after a giant star explodes at the end of its life. The matter in a neutron star is so heavily squashed that electrons have been forced into their atoms’ cores and combine with protons to form neutrons, leaving just a huge mass of neutrons.

Neutron stars pack more than the Sun’s mass into a ball just 20 to 25 kilometres across. In fact, their matter is so densely compressed that just one teaspoonful would have a staggering mass of 4,500 million tonnes.

Pulsar is eating its neighbour

Pulsars emit lots of radiation in tight beams, and as they spin they can appear to pulse or flash on and off like lighthouses.

Artist's impression of a pulsar

Artist's impression of a pulsar dragging gas from its companion star.

Astronomers can learn a lot about a pulsar from those flashes, such as how fast it is spinning. The J1749 pulsar spins at 518 times per second!

With J1749, the RXTE satellite spotted three eclipses as well as three pulses of X-rays as the pulsar experienced a series of outbursts.

The X-rays came from hotspots on the pulsar, where gas—sucked (or accreted) from the outer atmosphere of the companion star—had spiralled down and crashed onto the pulsar’s surface. The pulsar is slowly eating its neighbour.

It was these bright X-ray flashes that drew the astronomers’ attention to the J1749 system.

Small variations in the flashes arise from the pulsar’s orbital motion with the companion star, and indicate that the pulsar whizzes around its companion in just 8.8 hours.

The duration of the eclipses have enabled the astronomers to calculate that the companion is about 70% as massive as our Sun, but about 20% bigger than it would normally be for a star of this type—this is because the energy emitted by the pulsar is heating the companion’s outer layers, making them puff out further into space.

“This is the first time we’ve detected X-ray eclipses from a fast pulsar that is also accreting gas,” said Craig Markwardt of NASA’s Goddard Space Flight Centre. “Using this information, we now know the size and mass of the companion star with unprecedented accuracy.”

Artist's impression of RXTE

Artist's impression of the Rossi X-ray Timing Explorer space telescope, which made the observations.

Einstein to the rescue

What the astronomers don’t yet have is an accurate measure of the mass of the pulsar. The standard way to get it would be to use other telescopes to make optical and infrared observations of the companion star’s motion, from which they could work backward mathematically and deduce the pulsar’s mass.

But there is another way. Einstein’s relativity says that massive bodies distort space and slow down time. So what the astronomers hope to do is measure delays in the pulsar flashes as they travel past the companion star, something that RXTE is easily capable of doing.

This will be a good test of Einstein’s theory under extreme stellar conditions.

“High-precision measurements of the X-ray pulses just before and after an eclipse would give us a detailed picture of the entire system,” said Tod Strohmayer, RXTE’s project scientist at Goddard.

But for this, they’ll have to wait for RXTE to spot more X-ray outbursts…so you can be sure they’ll be keeping a close eye on this dynamic stellar duo in the months and years to come.

Story by Jonathan Nally, editor, SpaceInfo.com.au

Images courtesy NASA / GSFC.

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Magnetic star baffles astronomers

Artist’s impression of the magnetar in Westerlund 1

An artist’s impression of the magnetar in the very rich and young star cluster Westerlund 1. Astronomers have demonstrated that this magnetar was formed from a star with at least 40 times as much mass as the Sun. A star as massive as this would have been expected to become a black hole, not a magnetar.

  • Explosion of a star 40 times the mass of the Sun
  • Formed a magnetar instead of a black hole

Astronomers have for the first time demonstrated that a magnetar—an unusual type of neutron star—was formed from the explosion of a star with at least 40 times as much mass as the Sun.

A magnetar is a type of neutron star with an incredibly strong magnetic field — a million billion times stronger than that of the Earth, formed when certain stars undergo supernova explosions.

The result presents great challenges to current ideas of how stars evolve, as a star as massive as this was expected to become a black hole, not a magnetar.

And it raises a fundamental question—just how massive does a star really have to be to become a black hole?

The astronomers studied the extraordinary star cluster Westerlund 1, located 16 000 light-years away, and the closest super star cluster known

Westerlund 1contains hundreds of very massive stars, some shining with a brilliance of almost one million Suns, and some 2,000 times the diameter of the Sun.

“If the Sun were located at the heart of this remarkable cluster, our night sky would be full of hundreds of stars as bright as the full Moon,” says Ben Ritchie, lead author of the paper reporting these results.

The stars all share one thing—they all have the same age, estimated at between 3.5 and 5 million years, as the cluster was formed in a single star-formation episode.

Star cluster Westerlund 1

The young star cluster Westerlund 1 contains hundreds of very massive stars, some shining with a brilliance of almost one million Suns.

Stellar lifespan the key

Westerlund 1 hosts one of the few magnetars known in the Milky Way. As all the stars in Westerlund 1 have the same age, the star that exploded and became the magnetar must have had a shorter lifespan than the surviving stars in the cluster.

“Because the lifespan of a star is directly linked to its mass—the heavier a star, the shorter its life—if we can measure the mass of any one surviving star, we know for sure that the shorter-lived star that became the magnetar must have been even more massive,” says co-author and team leader Simon Clark.

“This is of great significance since there is no accepted theory for how such extremely magnetic objects are formed.”

The astronomers therefore studied the stars that belong to the “eclipsing” double system W13 in Westerlund 1 using the fact that, in such a system, masses can be directly determined from the motions of the stars.

The work shows for the first time that magnetars can evolve from stars so massive they would normally be expect to form black holes.

The previous assumption was that stars with initial masses between about 10 and 25 solar masses would form neutron stars and those above 25 solar masses would produce black holes.

“These stars must get rid of more than nine tenths of their mass before exploding as a supernova, or they would otherwise have created a black hole instead,” says co-author Ignacio Negueruela.

Adapted from information issued by ESO / L. Calçada.

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Dead stars get the chills

Image of Cassiopeia A and an artist's impression of the neutron star

Background: An image of the Cassiopeia A supernova explosion remnant taken by the Chandra X-ray Observatory. Inset: An artist's impression of the neutron star that lives at the heart of Cassiopeia A.

Observations of how the youngest-known neutron star has cooled over the past decade are giving astronomers new insights into the interior of these super-dense dead stars.

Dr Wynn Ho presented the findings at the Royal Astronomical Society (RAS) National Astronomy Meeting in Glasgow last week.

Neutron stars are composed mostly of neutrons crushed together by gravity, compressed to over a million million times the density of lead. They are the dense cores of massive stars that have run out of nuclear fuel and collapsed in supernova explosions.

The Cassiopeia A supernova explosion, likely to have taken place around the year 1680, would have heated the neutron star to temperatures of billions of degrees, from which it has cooled down to a temperature of about two million degrees Celsius.

Dr Ho, of the University of Southampton, and Dr Craig Heinke, of the University of Alberta in Canada, measured the temperature of the neutron star in the Cassiopeia A supernova remnant nebula using data obtained by NASA’s Chandra X-ray Observatory between 2000 and 2009.

An artist's impression of a neutron star

An artist's impression of a neutron star

“This is the first time that astronomers have been able to watch a young neutron star cool steadily over time. Chandra has given us a snapshot of the temperature roughly every two years for the past decade and we have seen the temperature drop during that time by about 3%,” said Dr Ho.

Neutron stars’ cooling cores

Young neutron stars cool through the emission of high-energy neutrinos—particles similar to photons but which do not interact much with normal matter and therefore are very difficult to detect.

Since most of the neutrinos are produced deep inside the star, scientists can use the observed temperature changes to probe what’s going on in the neutron star’s core.

Initially, the core of the neutron star cools much more rapidly than the outer layers. After a few hundred years, equilibrium is reached and the whole interior cools at a uniform rate.

At approximately 330 years old, the Cassiopeia A neutron star is near this cross-over age. If the cooling is only due to neutrino emission, there should be a steady decline in temperature.

However, although Dr Ho and Dr Heinke observed an overall steady trend over the 10-year period, there was a larger change around 2006 that suggests other processes may be active.

“The neutron star may not yet have relaxed into the steady cooling phase, or we could be seeing other processes going on,” said Dr Ho. “We don’t know whether the interior of a neutron star contains more exotic particles, such as quarks, or other states of matter, such as superfluids and superconductors.”

“We hope that with more observations, we will be able to explain what is happening in the interior in much more detail,” said Dr Ho.

Adapted from information issued by NASA / CXC / Southampton / W. Ho et al / NASA / CXC / M.Weiss / MIT / UMass Amherst / M.D. Stage et al.