Pulse of the Universe - Neutron Stars

Imploding Star & Exploding Supernova

The Stellar Object, Pre and Post-Formation

The explosive force is the largest known in the universe and as such, the shockwave it creates travels at speeds in excess of 50 million kilometers per hour. At this velocity it slams into and races through the now expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst that can be as intense as the light of several billion Suns. This is a type II supernova and it's caused by the sudden stop and immediate reversal in the process of implosion. What is left — a highly rotating core of dense material called a Neutron Star. Image Credit: Chandra X-Ray Observatory   [6]

But implosion from what and how?

To begin, the neutron star is a product of the process of stellar evolution — from protostar, through red giant to supernova/nebula — this type of star is nearing the end of it's stellar life cycle. Qutoing from our earlier section entitled Red Giant Stars:

"This star is old, it's age measured in billions of years. The fuel that was once so abundant, hydrogen, is running out. During it's life time the star's own thermonuclear engine has been producing the heavier element helium which is now moving inwards, until only a shell of hydrogen remains around the exterior of the core. As this happens, the star's own gravity begins to shrink this core, causing an increase in tempeture and beginning a period of hydrogen to helium burning with the results that it's outer layers begin to expand greatly..."

Small Mass Stars
What happens next depends on the size or mass of the star as this directly relates to how long it's layers remain and just what elements the star will continue to fuse, a process that causes the material to compress in and on the interior core of the star, causing it to become denser and denser. In stellar evolution a star, like our sun, burns through it's hydrogen supply over billions of years in a process we know as fusion. When this runs out the heavier element helium will ignite as a result of the density around the core having increased, causing the remaining supply of hydrogen to burn even faster and creating even more heat. This heat causes the outer layers of the star to expand away from the hot core, cooling rapaidly as the suface area of the star becomes too great for the energy output of the core to continue heating. At this point the star has become a red giant and red giant stars with an average mass of our sun will only continue to fuse helium to carbon ( the limit of elements it will be hot enough to ignite ) until the helium burning, igniting intermittently, finally stops. That "intermittent" ignition of the helium is what causes the surface layers of the red giant to be blown off and away from the core, creating those cool nebulae images seen at APOD and in text books.

If our star has a solar mass of less than 1.4, called the Chandrasekhar limit it will eventual collaspe upon itself, stopping when it's electrons, wanting to remain away from it's nuclei, overcome the process of collaspe through a force known as electron degeneracy pressure, thereby creating a white dwarf star. On the other hand, a star with mass greater than the Chandrasekhar limit of 1.4 will have far too much pressure from gravity at it's core, causing the electrons to be squeezed right into the protons, creating a core of almost nothing but neutrons. At this point, the star will either have expelled it's outer layers, remaining under the 1.4 mass limit ( this occurs for stars whose orignal mass is around 2-5 ) or else, have literally blown itself apart in an explosion that can be seen across 13 billion light years — the supernova. Click the filmstrip icon at left to view an animation of a supernova explosion (Quicktime format) which dissolves into the Chandra First Light Image of the supernova remnant Cassiopeia A. Credit: Chandra X-Ray Observatory

Larger Mass Stars
For stars with 10, 100 or even larger masses, something else occurs in the above process — they keep right on burning, fusing heavier and heavier elements, right through to the last element available, the element iron 56 or and this is where the process comes to a screeching halt, almost. What has happened up to this point is that all the previous elements that have been ignited have been exhausted, their remains gravtionally compressed towards the core of the star ( remember that a star is held together through the force of it's own gravity ) creating a fantastic amount of pressure upon it, so much so that in some cases enough pressure is built up to ignite even the iron at the core, but this final ignition just speeds up the process of collaspe for, unlike all the other elements, iron 56 requires more engery to fuse than it produces.

Whether the final process ignites the iron 56 or not, the end is still the same for the star. With the thermonuclear fusion now gone the very force that has held the star together, gravity, is also gone. All of the star's material mass, from the core outward, fall or collaspe inward, the elementary atomic particles becoming more tightly packed together than ever before. What it then comes down to is the initial mass of the star as those with masses higher than 20-30 form a black hole and those with less mass form either a neutron star or white dwarf.

"With the core collapsed, each subsequent layer also collapses inward. But the major effect comes from the fact that in some ways the core behaves like a rubber ball. As the core falls in on itself, electrons and nuclei are squeezed together into neutrons, and even the neutrons are squeezed into each other--but they push back! and the core actually bounces outward. The shockwaves from the collapse and the bounce (both are spherical) meet and reinforce, and the star very rapidly tears itself apart in what is called a Type II Supernova. During the explosion, enough energy is available to fuse iron into all the elements up to uranium and beyond. This matter gets distributed, with at least 80% of the star's original mass, in a huge nebula called a supernova remnant.

If the stellar remnant (really just the core of the former star) is small, it will become a white dwarf or a neutron star. But if the stellar remnant is bigger than 3 solar masses, gravity overcomes everything and the star collapses to nothing! Only the gravity remains! This mighty odd-sounding state of affairs is called a black hole."   [7]

Neutron Star CXO J164710.2-455216

"The optical image (above left) of Westerlund 1 shows a dense cluster of young stars, several with masses of about 40 suns. Some astronomers speculated that repeated collisions between such massive stars in the cluster might have led to formation of an intermediate-mass black hole, more massive than 100 suns. A search of the cluster with Chandra (above) found no evidence for this type of black hole. Instead they found a neutron star (CXO J164710.2-455216), a discovery which may severely limit the range of stellar masses that lead to the formation of stellar black holes. In the image at right, the neutron star - a dense whirling ball of neutrons about 12 miles in diameter - revealed itself through periodic X-ray pulsations (every 10.6 seconds). A neutron star is left behind after a massive star completes its evolution and goes supernova. Since extremely massive stars evolve more rapidly than less massive ones, and the progenitor of the neutron star has already exploded as a supernova, its mass must have been greater than 40 solar masses." Click on the filmstrip icon at right to view a short movie of the Westerlund 1 images from Chandra. Images from the Chandra X-Ray Observatory, credit: NASA/CXC/UCLA/M.Muno et al.   [8]

Name: ARA CLUSTER, Open (galactic) Cluster, location is RA: 16 47 04.00 Dec: -45 51 04.9, Galatic Long. & Lat. is 339.5493 -00.4042 with a spectral type of O. It is also known as Cl Westerlund 1, ESO 277-12, C 1644-457 and Cl VDBH 197. Query: C.D.S. - SIMBAD4 rel 1.121 - 2009.05.11CEST22:26:21 Image: Aladin Previewer At right is a frame from a visualization showing "a fly-through of Cas A based on the 3-D representation constructed from Chandra and Spitzer data. It begins with an artists rendition of the neutron star previously detected by Chandra. Next, new features unseen in traditional 2-D data sets are visible, including details of how the parent star exploded. The green region is mostly iron observed in X-rays; the yellow region is mostly argon and silicon seen in X-rays, optical and infared; the red region is cooler debris seen in the infared and the blue region is the outer blast wave, most prominent in X-rays." [9]