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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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Giant ring of black holes

Composite image of Arp 147

This image of galaxy pair Arp 147 combines X-ray and visible light data to reveal the presence of black holes. Gas being sucked into the black holes emits X-rays (shown as pink).

  • Arp 147 is a pair of galaxies that collided, triggering a wave of star formation
  • Many of the new stars exploded as supernovae, some forming black holes.
  • Ring of these black holes can be seen in Chandra X-ray Observatory images

JUST IN TIME FOR VALENTINE’S DAY comes a new image of a ring — not of jewels, but of black holes.

The image above shows Arp 147, a pair of interacting galaxies located about 430 million light-years from Earth. The image is a combination of X-rays from the NASA’s Chandra X-ray Observatory (pink) and optical data from the Hubble Space Telescope (red, green, blue).

Arp 147 contains the remnant of a spiral galaxy (right) that collided with the elliptical galaxy on the left. This collision produced an expanding wave of star formation that shows up as a blue ring containing in abundance of massive young stars. These stars race through their evolution in a few million years or less and explode as supernovae, leaving behind neutron stars and black holes.

A fraction of the neutron stars and black holes will have companion stars, and may become bright X-ray sources as they pull in gas from their companions.

The nine X-ray sources scattered around the ring in Arp 147 are so bright that they must be black holes, with masses that are likely 10 to 20 times that of the Sun.

An X-ray source is also detected in the nucleus of the red galaxy on the left and may be powered by a poorly fed supermassive black hole. This source is not obvious in the composite image but can easily be seen in the X-ray image.

Other objects unrelated to Arp 147 are also visible—a foreground star in the lower left of the image and a background quasar as the pink source above and to the left of the red galaxy.

Infrared observations with NASA’s Spitzer Space Telescope and ultraviolet observations with NASA’s Galaxy Evolution Explorer (GALEX) have enabled estimates to be made of the rate of star formation in the ring.

These estimates, combined with the use of models for the evolution of binary stars have enabled scientists to conclude that the most intense star formation may have ended some 15 million years ago, in Earth’s time frame.

The results were published in the October 1, 2010, issue of The Astrophysical Journal. The scientists involved were Saul Rappaport and Alan Levine from the Massachusetts Institute of Technology, David Pooley from Eureka Scientific, and Benjamin Steinhorn, also from MIT.

Adapted from information issued by the Chandra X-ray Centre. Image credits: X-ray, NASA / CXC / MIT / S. Rappaport et al; optical, NASA / STScI.

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Stellar cannibal hides it age well

Artist's impression of the star BP Piscium

Artist's impression of the star BP Piscium. Astronomers think it has eaten another star, with the leftover material forming a surrounding gas and dust cloud, within which new planets might form. Some of the leftover stuff is being shot out in "jets" from near the poles of the star.

  • Young looking star is actually probably quite old
  • Seems to have recently devoured a companion star
  • Leftover crumbs could be forming into planets

An astronomer may have caught a cannibalistic star in the act of devouring a companion and making a new generation of planets from the resulting cloud of leftover crumbs.

Using data from NASA’s Chandra X-ray Observatory, Joel Kastner, professor at Rochester Institute of Technology (RIT), has found evidence that a star in the constellation of Pisces—called BP Piscium, or BP Psc for short—is not the young star it appears to be, but is more likely a one billion-year-old red giant that has gobbled up a star or planet in its vicinity.

The star’s extreme properties have puzzled astronomers since Kastner and Ben Zuckerman, professor at the University of California, Los Angeles, first looked at BP Psc 15 years ago. The star is about 1,000 light-years from Earth.

False-colour image of BP Piscium

False-colour image of BP Piscium, put together using X-ray and optical wavelength observations. The two jets blasting out of the star are several light-years long.

Conflicting characteristics have caused confusion as to whether the star is young or old.

Kastner attributes the star’s potentially deceptive youthful appearance to two things: an surrounding cloud of gas and dust that resembles the sort that forms planets around young stars; and prominent “jets” extending from the poles of the star that eject material at high velocity.

A typical young star sucks in material from the surrounding cloud, incorporating about 90 percent of the material and spitting out the rest through jets or geysers shooting out in opposite directions.

Kastner and his colleagues were doubtful about the youth of the star. For one thing, the star is isolated, whereas most young stars form in clusters.

“As hard as people have looked, they have not been able to find [another] young star near BP Psc,” says Kastner, a professor in RIT’s Chester F. Carlson Centre for Imaging Science. “That was one of several things that made Ben [Zuckerman] and me suspect that it wasn’t actually young.”

Second, this enigmatic star in the Pisces constellation lacks the large abundance of lithium on its surface that is typical of young stars. Older stars lose their lithium in nuclear reactions when mixing and churning folds the gases into the centre of the star. According to Kastner, other key spectral features involving the star’s radius and surface gravity also point to the star’s advanced age.

Stellar cannibalism

Kastner is ready to close the debate with data obtained from the Chandra X-ray Observatory.

“The last piece of evidence, which, to me, is the nail in the coffin that BP Psc is old rather than young, is that its rate of X-ray production is very similar to old, yet rapidly spinning, giant stars that have surface temperatures similar to BP Psc,” Kastner says.

If BP Psc were a young star, it would emit X-rays in the hundreds, even up to a few thousand, in a day’s observing time with Chandra, Kastner notes. Instead, it is a weak X-ray source.

Artist's impression of the Chandra X-ray Observatory.

Artist's impression of NASA’s Chandra X-ray Observatory.

“We stared at BP Psc for one day with Chandra and only detected about 18 X-rays,” Kastner says. “We could almost name them.”

The rate of X-rays coming from the star are in keeping with a class of rapidly rotating old stars having similar temperature to BP Psc, Kastner says. This class is thought to be the result of one star swallowing another close companion star.

“Our working speculation is that we are observing the star right at the point at which it has swallowed its companion and hence formed a [surrounding cloud from the leftover bits],” Kastner says. “Some of the material that used to be its companion has fallen onto the star and some has been shot out at high speeds, and that’s what we’re seeing.”

The enigmatic star is likely about a billion years old and just entering the red giant stage in its life cycle in which it swells to digest its star or planet companion.

“It could be a small star or a large planet,” Kastner says. “We don’t know which it could be, but we’re very interested in finding out.”

“In order to understand the extrasolar planets that are now being discovered by the dozen, we need to figure out how planets might be forming and therefore where we should go look for them,” Kastner says. “I think this object is especially interesting because it gives us a good shot at finding young planets around an old star.”

Image credits: (X-ray) NASA / CXC / RIT / J. Kastner et al; (optical) UCO / Lick / STScI / M. Perrin et al; (illustration) CXC / M. Weiss.

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Galactic super-volcano in action

HST image of M87

This Hubble Space Telescope image shows the central core and accompanying outflowing "jet" of the giant elliptical galaxy M87. In the centre of the galaxy there lurks a supermassive black hole.

A galactic “super-volcano” in the massive galaxy known as M87 is erupting and blasting gas outwards, as witnessed by NASA’s Chandra X-ray Observatory and the US National Science Foundation’s (NSF) Very Large Array (VLA) of radio telescopes.

The cosmic volcano is being driven by a giant black hole in the galaxy’s centre and preventing hundreds of millions of new stars from forming.

At a distance of about 50 million light-years, M87 is relatively close to Earth and lies at the centre of the Virgo cluster, which contains thousands of galaxies.

M87’s location, coupled with long observations over Chandra’s lifetime, has made it an excellent subject for investigations of how a massive black hole impacts its environment.

Core of the galaxy M87

This is the core of the galaxy M87, seen at X-ray and radio wavelengths. A huge black hole, hiding in the middle, is ejecting energetic particles that push gas outwards. That gas would ordinarily form millions of new stars, so the black hole's activity is acting like a brake on star formation.

“Our results show in great detail that supermassive black holes have a surprisingly good control over the evolution of the galaxies in which they live,” said Norbert Werner of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University and the SLAC National Accelerator Laboratory, who led one of two papers describing the study.

“And it doesn’t stop there. The black hole’s reach extends ever farther into the entire cluster, similar to how one small volcano can affect practically an entire hemisphere on Earth.”

The space around M87 is filled with hot gas glowing in X-ray light, which has been detected by Chandra. As this gas cools, it should fall in toward the M87’s centre where it could continue to cool even faster and form new stars.

However, radio observations with the Very Large Array suggest that in M87 jets of very energetic particles produced by the black hole interrupt this process. These jets lift up the relatively cool gas near the centre of the galaxy and produce shock waves in the galaxy’s “atmosphere” because of their supersonic speed.

In M87, the plumes of cooler gas being lifted upwards contain as much mass as all of the gas contained within 12,000 light-years of the centre of the galaxy cluster.

This shows the black hole-powered volcano is very efficient at blasting the galaxy free of the gas that would otherwise cool down and form stars.

The eruption in M87 that lifted up the cooler gas must have occurred about 150 million years earlier, but a smaller eruption only about 11 million years earlier produced the shock wave.

Adapted from information issued by Chandra X-ray Centre.

Images courtesy Tod R. Lauer, Sandra M. Faber / NASA / X-ray (NASA / CXC / KIPAC / N. Werner, E. Million et al); radio (NRAO / AUI / NSF / F. Owen)

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Black hole blowing bubbles

An artist's impression of the black hole

A black hole has been found with huge "jets" of particles "squirting" out in opposite directions. In this artist's impression, the black hole is in the middle of the white whirlpool at lower right; at upper left is a normal star, whose gas is being sucked in by the black hole. A gas bubble being inflated the black hole's jets, is 1,000 light-years across and too big to show in this image.

  • Black hole squirting out jets of particles
  • Moving at almost 1 million km per hour
  • Inflating a bubble in the surrounding gas

Combining observations made with ESO’s Very Large Telescope and NASA’s Chandra X-ray telescope, astronomers have uncovered the most powerful pair of “jets” ever seen from a stellar black hole.

This phenomenon, also known as a microquasar, blows a huge bubble of hot interstellar gas, 1,000 light-years across.

“We have been astonished by how much energy is injected into the gas by the black hole,” says Manfred Pakull.

“This black hole is just a few solar masses, but is a real miniature version of the most powerful quasars and radio galaxies, which contain black holes with masses of a few million times that of the Sun.”

Black holes are known to release a prodigious amount of energy when they swallow matter.

The energy doesn’t actually come out of the black hole itself, but rather is emitted from the gas that is spiralling in towards the black hole.

Artist's impression of the Chandra X-ray Observatory satellite

Artist's impression of the Chandra X-ray Observatory satellite, which made some of the observations

Black hole “bubbles” in space

It was thought that most of the energy came out in the form of radiation, predominantly X-rays.

However, the new findings show that some black holes can release at least as much energy, and perhaps much more, in the form of focused beams or jets of fast moving particles instead of radiation.

The fast particles slam into the surrounding interstellar gas, heating it and triggering an expansion. The inflating bubble contains a mixture of hot gas and ultra-fast particles at different temperatures.

Observations in several energy bands (optical, radio, X-rays) help astronomers calculate the total rate at which the black hole is heating its surroundings.

The astronomers could see the spots where the jets smash into the interstellar gas located around the black hole. Their observations show that the bubble of hot gas is inflating at a speed of almost one million kilometres per hour.

“The length of the jets in NGC 7793 is amazing, compared to the size of the black hole from which they are launched,” says Robert Soria. “If the black hole were shrunk to the size of a soccer ball, each jet would extend from the Earth to beyond the orbit of Pluto.”

Artist's impression of a jet coming out of a black hole

Many black holes emit jets of particles

More waiting to be found

This research will help astronomers understand the similarity between small black holes formed from exploded stars and the supermassive black holes at the centres of galaxies.

Very powerful jets have been seen from supermassive black holes, but are thought to be less frequent in the smaller microquasar variety. The new discovery suggests that many of them may simply have gone unnoticed so far.

The gas-blowing black hole is located 12 million light-years away, in the outskirts of the spiral galaxy NGC 7793. From the size and expansion velocity of the bubble the astronomers have found that the jet activity must have been ongoing for at least 200,000 years.

The discovery is reported this week in the journal Nature.

Adapted from information issued by ESO / L. Calçada.

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Telescope sees shocking blue “bullet”

Supernova remnant cloud N49 in the Large Magellanic Cloud galaxy.

Supernova remnant cloud N49 in the Large Magellanic Cloud galaxy. The blue blob in the lower right corners is being ejected at a speed of 8 million km per hour.

  • Supernova remnant in a neighbouring galaxy
  • X-ray data sees a “bullet-shaped” object shooting out
  • “Bullet” is travelling at 8 million km per hour

This beautiful composite image shows N49, the aftermath of a supernova explosion in the Large Magellanic Cloud (one of the Milky Way’s neighbour galaxies).

A new, long observation from NASA’s Chandra X-ray Observatory, shown as blue colours, reveals evidence for a “bullet-shaped” object being blown out of a debris field left over from an exploded star.

In order to detect this bullet—which can be seen as the blue “blob” in the lower right hand corner of the image—a team of researchers led by Sangwook Park of Penn State University used Chandra to observe N49 for over 30 hours. The bullet is rich in silicon, sulphur and neon.

The detection of this bullet shows that the explosion that destroyed the star was highly asymmetric.

The bullet is travelling at a high speed of about 8 million kilometres per hour away from a bright point source in the upper left part of N49. This bright source may be a so-called soft gamma ray repeater (SGR), a source that emits bursts of gamma rays and X-rays.

Artist's impression of the Chandra X-ray Observatory

Artist's impression of the Chandra X-ray Observatory.

Supernova shockwave

A leading explanation for SGRs is that they are neutron stars with extremely powerful magnetic fields. Since neutron stars are often created in supernova explosions, an association between SGRs and supernova remnants is not unexpected. This case is strengthened by the apparent alignment between the bullet’s path and the bright X-ray source.

However, the new Chandra data also shows that the bright source is more obscured by gas than expected if it really lies inside the supernova remnant. In other words, it is possible that the bright X-ray source actually lies beyond the remnant and is projected along the line of sight.

Another possible bullet is located on the opposite side of the remnant, but it is harder to see in the image because it overlaps with the bright emission—described below—from the shock-cloud interaction.

Optical data from the Hubble Space Telescope (yellow and purple colouring) shows bright filaments where the shock wave generated by the supernova is crashing into the densest regions of nearby clouds of cool, molecular gas.

Using the new Chandra data, the age of N49—as it appears in the image—is thought to be about 5,000 years, and the energy of the explosion is estimated to have been about twice that of an average supernova. These preliminary results suggest that the original explosion was caused by the collapse of a massive star.

Adapted from information issued by Chandra X-ray Centre.

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.