Volume rendering of 3-D simulation of a pulsar's formation. Credit: Image courtesy of North Carolina State UniversityPulsars are rapidly rotating neutron stars formed in supernova explosions, which occur when a massive star reaches the end of its life and explodes. The remaining matter is compressed into a dense, rapidly spinning mass – a neutron star, or pulsar – so-called because scientists first discovered them due to their regularly timed radio emissions.
Pulsars spin very rapidly – 20 or more times per second. Scientists have assumed that the spin was caused by the conservation of angular momentum from a star that was spinning before it exploded.
“Think about figure skaters,” Blondin says. “They start a spin with their arms and legs farther out from the body, and increase their rotation speed when they pull their limbs in more tightly. That’s what the conservation of angular momentum is – the idea that if you take a large object with a slight rotation and compress it down, the rotation speed will increase.”
However, scientists had no idea if the stars that were producing the pulsars were even spinning to begin with. Blondin and his colleague decided to create a computer model of a supernova explosion using the new Cray X1E supercomputer at the National Center for Computational Sciences, the only computer with enough processing power to accomplish the task. The resultant model demonstrated that a pulsar’s spin doesn’t have anything to do with whether or not the star that created it was spinning; instead, the spin is created by the explosion itself.
“We modeled the shockwave, which starts deep inside the core of the star and then moves outward,” Blondin says. “We discovered that as the shockwave gains both the momentum and the energy needed to blow outward and create the explosion, it starts spiraling all on its own, which starts the neutron star at the center of the star spinning in the opposite direction. None of the previous two-dimensional modeling of supernova explosions had picked up on this phenomena.”
Dr. John Blondin, professor of physics in NC State’s College of Physical and Mathematical Sciences, along with colleague Anthony Mezzacappa at the Oak Ridge National Laboratory.
Their findings are published in the Jan. 4 edition of the journal Nature._______________________________________________________
Latest Spin on Neutron Stars from Louise Riofrio
Scientist Discovers New Explanation For Pulsar's Spin from Science Daily
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Sunlight heats ice on surface of comet McNaught_______________________________________________________
The unique images reveal three clear jets of gas, which are seen to spiral away from the nucleus as it rotates, like a Catherine Wheel firework.
"These jets are produced when sunlight heats ices on the surface of the comet, causing them to evaporate into space and create 'geyser' like jets of gas and small dust particles, which stretch over 13,000 km into space - greater than the diameter of the Earth - despite the fact that the nucleus of the comet is probably less than 25 km in diameter,"
By comparing images like this taken at different times, astronomers should be able to calculate how fast the nucleus rotates from the changing pattern of jets.
Other images also reveal that while the gas forms spiral jets, the large dust particles released from the comet follow a different pattern, as they are thrown off the comet's surface on the brightly lit side towards the Sun, producing a bright fan, which is then blown back by the pressure of sunlight itself.
Unique Observations Of Comet McNaught Reveal Sprinkling NucleusComet McNaught. Image courtesy of European Southern Observatory_______________________________________________________
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Labels: Astro Physics, Comets, ESO, Pulsars
This effect resembles the so-called caustic pattern of light ripples observed when looking at a sandy bottom as light is focused by the wave crests and valleys.
In the gravitational lens here, the yellow cluster galaxies act as wave crests whereas the banana-shaped arcs are the caustic patterns created by the focusing power of the matter cluster. Cluster Abell 2667 is a massive luminous galaxy cluster located 3.2 billion light-years from Earth.
There are many galaxies of different shapes and sizes around us today. Roughly half are gas-poor elliptical-shaped galaxies with little new star formation activity, and half are gas-rich spiral and irregular galaxies with high star formation activity. Observations have shown that gas-poor galaxies are most often found near the centre of crowded galaxy clusters, whereas spirals spend most of their lifetime in solitude.
The mystery, gleaned from deep observations of the Universe, is that when the Universe was half its present age only one in five galaxies was a gas-poor galaxy. So, where do all of today's gas-poor galaxies come from? Scientists suspect that some kind of transforming process must have taken place, but because galaxy evolution occurs over billions of years, scientists have so far not been able to see the transformation at work.
New observations with Hubble provide one of the best examples to date of this metamorphosis. While looking at the galaxy cluster Abell 2667, astronomers found an odd-looking spiral galaxy (shown here in the upper left hand corner of the image) that ploughs through the cluster after being accelerated to at least 3.5 million km/h by the enormous combined gravity of the cluster's hundreds of galaxies.
"By combining Hubble observations with various ground & space based telescopes, we have been able to shed some light on the evolutionary history of galaxies", said Luca Cortese of Cardiff University, United Kingdom.
As the galaxy speeds through, its gas and stars are being stripped away by the tidal forces exerted by the cluster -- just as the tidal forces exerted by the moon and Sun push and pull the Earth's oceans. Also contributing to this destructive process is the pressure of the cluster's hot gas plasma reaching temperatures as high as 10-100 million degrees.
Both processes -- the tidal forces and the aptly named "ram pressure stripping" resulting from the action of the hot cluster gas -- resemble those affecting comets in our Solar System. For this reason, scientists have nicknamed this peculiar spiral with its tail the "Comet Galaxy".
"This unique galaxy, has an extended stream of bright blue knots and diffuse wisps of young stars driven away by the tidal forces and the 'ram pressure stripping' of the hot dense gas," said Jean-Paul Kneib, a study collaborator from the Laboratoire d'Astrophysique de Marseille.
Even though its mass is slightly larger than that of the Milky Way, the spiral will inevitably lose all its gas and dust as well as its chance of generating new stars later, and become a gas-poor galaxy with an old population of red stars. However, in the midst of all this destruction, the cluster's strong forces have triggered a baby-boom of star formation.
Scientists estimate that the total duration of the transformation process is close to one billion years. What is seen now in the Hubble image is roughly 200 million years into the process.
The strong gravitational pull exerted by the galaxy cluster's collective mass has bent the light of other, more distant galaxies and distorted their shapes - an effect called gravitational lensing. The giant bright banana-shaped arc seen just to the right of the centre in the photo corresponds to the magnified and distorted image of a distant galaxy that lies behind the cluster's core.
At the cluster's centre another rare feature can be seen: the vivid blue light from millions of stars created in a so-called cooling flow. Some of the hot cluster gas is cooling in a filamentary structure as it falls into the cluster's core, setting off the birth of lots of bright blue stars outshining their environment. This may be the clearest picture of this phenomenon yet.
News Release
Hubble sees “Comet Galaxy” being ripped apart by galaxy cluster
(Credit: NASA, ESA, Jean-Paul Kneib (Laboratoire d'Astrophysique de Marseille))