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Stellar Evolution - Cycles of Formation and Destruction
Massive Stars (con't.)
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G292.0+1.8
Credit: NASA/CXC/Rutgers/J.Hughes et al. |
Pulsars are spinning neutron stars that have jets of particles moving almost at the speed of light streaming out from the magnetic poles. These jets produce very powerful beams of high energy particles that emit x-rays. For a similar reason that "true north" and "magnetic north" are different on Earth, the magnetic and rotational axes of a pulsar are also misaligned. Therefore, the beam of particles and x-rays from the jets sweep around as the pulsar rotates, just as the spotlight in a lighthouse does. Like a ship in the ocean that sees only regular flashes of
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Animation: X-Ray Pulsar
Credit: NASA/CXC |
light, we see pulsars turn on and off as the beam sweeps over the Earth. The oxygen-rich supernova G292.0+1.8 contains a pulsar. Neutron stars have very intense magnetic fields, about 1,000,000,000,000 times stronger than Earth's own field. The combination of this strong magnetic field and the rapid rotation of the neutron star produces extremely powerful electric fields, with electric potential in excess of 1,000,000,000,000 volts. Electrons are accelerated to high velocities by these strong electric fields. These high-energy electrons produce radiation in two general ways: as a coherent plasma the electrons work together to produce radio emissions, and individually the electrons interact with photons or the magnetic filed to produce high-energy emission such as optical, X-ray and gamma-ray. The pulses of radiation match the rate of the rotation of the neutron star.
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Magnetar Illustration
Credit: Robert Mallozzi (UAH, MSFC) |
Magnetars are neutron stars that have super strong magnetic fields, about 100 trillion times as strong as the Earth's magnetic field. These fields are so intense that the solid neutron star crust buckles and shifts under its influence. The resulting star
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N49
Credit: Hubble Heritage Team (STScI / AURA), Y. Chu (UIUC) et al., NASA |
quakes could repeatedly generate brief flashes of hard X-rays and soft gamma-rays -giving rise to the rare but mysterious "soft gamma repeaters" - because magnetars seem to be rotating too slowly to produce the observed energy output. The Hubble image of N49, a Type II supernova remnant in the Lage Magellanic Cloud, contains a magnetar.
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Cassiopeia A
Credit: NASA/CXC/GSFC/U.Hwang et al. |
One million seconds of x-ray image data were used to construct this view of supernova remnant Cassiopeia A, the expanding debris cloud from a stellar explosion. Cas A's outer green ring, ~10 light-years in diameter, marks the location of the expanding shock from the original supernova explosion. In the upper left portion of the remnant, a structure extends beyond it, evidence that the initial explosion may have also produced energetic jets. Still glowing in x-rays,
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Cassiopeia A
Credit: NASA/CXC/GSFC/U.Hwang et al. |
the tiny point source near the center of Cas A is a neutron
star, the collapsed remains of the stellar core. In the blue-colored Cas A image specially processed to highlight silicon ions, a counter-jet can be seen on the lower right The X-ray spectra show that the jet and counter-jet are rich in silicon atoms and relatively poor in iron atoms. This indicates that the jets formed soon after the initial explosion of the star; otherwise, the jets should have contained large quantities of iron from the star's central regions. The bright blue fingers located near the shock
wave on the lower left are composed almost purely of iron gas. This iron was produced in the central, hottest regions of the star and somehow ejected in a direction almost perpendicular to the jets. The bright source at the center of the image is presumed to be a neutron star created
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Animation: Cassiopeia A supernova
Credti: NASA/CXC/D.Berry & A.Hobart) |
during the supernova. Unlike the rapidly rotating neutron stars in other supernova remnants that are surrounded by dynamic magnetized clouds of electrons called pulsar wind nebulas, this neutron star is quiet, faint, and so far shows no evidence for pulsed radiation. One explanation could be that the explosion that created Cas A produced high-speed jets similar to but less energetic than the hypernova jets thought to produce gamma-ray bursts. During the explosion, the neutron star may have developed an extremely strong magnetic field that helped to accelerate the jets. This super-strong magnetic field later stifled any pulsar wind activity, so the neutron star today resembles other strong-field neutron stars in lacking a pulsar wind nebula; Cas A may contain a magnetar.
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