Supernova Remnants
Supernova Remnants
SupernovaDefinition
A supernova is the catastrophic self-destruction of a star. Left behind from this extremely energetic event is an expanding structure of gas known as a supernova remnant. In some cases, ths supernova explosion is triggered by the collapse of the core of a massive star at the end of its lifetime (core-collapse supernova). In this event, the outer layers of the star are hurled outward to form the diffuse supernova remnant, while the core of the star shrinks down into an extremely dense "compact object" – a neutron star (which sometimes becomes a pulsar) or a black hole. In other cases, a white dwarf in a binary system may accumulate enough mass from its companion star to trigger a similar collapse of the white dwarf (white-dwarf supernova). In such explosions, the star is blown completely apart, with the entire mass of the star flung forth to become the supernova remnant.
In either event, the stellar material is flung forth at speeds of thousands of kilometers per second. As this "ejecta" encounters material in the surroundings – often material cast off by the progenitor star in the later stages of its life – shock waves are produced. A "forward" shock wave travels out ahead of (most of) the ejecta, sweeping up interstellar matter and heating it to temperatures of over a million degrees Kelvin. At the same time, a second shock rebounds from the point of contact. This "reverse" shock front is travelling more slowly than the ejecta, so the ejecta pass through the reverse shock and are also shock-heated to higher temperatures. Intense radiation is generated by the cooling of material which has been recently shock-heated. The compression of magnetic fields by the shock(s), and the acceleration of particles to relativistic speeds, serves to generate additional "synchrotron" radiation.
The supernova of 1006, or SN 1006.
This false-color Chandra image of a supernova remnant shows X-rays produced by high-energy particles (blue) and multimillion degree gas (red/green). In 1006 AD, what was thought to be a "new star" suddenly appeared in the sky and over the course of a few days became brighter than the planet Venus. The supernova of 1006, or SN 1006, may have been the brightest supernova on record.
(Source: NASA/CXC/Rutgers/J.Hughes et al.)
Stages of Expansion
The expansion of a supernova remnant has frequently been described as a succession of four stages:
- The "free expansion" phase, in which the shock is not appreciably slowed by interaction with the surroundings, and therefore maintains a nearly constant velocity and temperature.
- The "energy conserving" (adiabatic, or blast-wave, or Sedov) phase, in which the shock gradually decelerates as it interacts with the surrounding medium. The slower shock does not heat material to as high temperatures, so the remnant as a whole cools down. The kinetic energy and thermal energy of the remnant can both be regarded as conserved quantities; that is, the fraction of the total energy in each remains the same. Instabilities develop along the interface between the ejecta and the swept-up (and shock-heated) interstellar medium, leading to increasing mixing of the two. The basic physics of the shell can be reasonably described through the self-similar solutions of Sedov (1959).
- The "momentum conserving" (or snowplow, or radiative, or Oort) phase, in which the shell has cooled to the point where the radiation of energy becomes efficient, and energy can no longer be regarded as conserved. The radiation of energy leads to rapid cooling behind the shock, and so matter piles up into a thin, dense shell behind the shock front. The momentum of the shell is still conserved, so it continues to expand and sweep up material (like a snowplow sweeps up snow) from the surroundings.
- The "merging" (or dispersal) phase, in which the expansion velocity of the shell falls below the thermal and turbulent velocities in the surrounding interstellar medium (i.e., below the local sound speed). The shell is gradually broken up, becoming physically indistinguishable from the local medium.
More recently, the "momentum conserving" phase has often been divided into two sub-phases: the "pressure-driven snowplow", in which the thin shell first forms, and its expansion is partially propelled by pressure from the remnant's hot interior; and the "momentum-driven snowplow", in which that shell continues to expand and sweep up more material, powered only by its own momentum.
Tycho's Supernova
Tycho's Supernova. (Source: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.)
In 1572, the Danish astronomer Tycho Brahe observed and studied the explosion of a star that became known as Tycho's supernova (Image Above). More than four centuries later, Chandra's image of the supernova remnant shows an expanding bubble of multimillion degree debris (green and red) inside a more rapidly moving shell of extremely high energy electrons (filamentary blue).
The supersonic expansion (about six million miles per hour) of the stellar debris has created two X-ray emitting shock waves - one moving outward into the interstellar gas, and another moving back into the debris. These shock waves produce sudden, large changes in pressure and temperature, like an extreme version of sonic booms produced by the supersonic motion of airplanes.
According to the standard theory, the outward-moving shock wave should be about 2 light years ahead of the stellar debris. What Chandra found instead is that the stellar debris has kept pace with the outer shock and is only about half a light year behind.
The most likely explanation for this behavior is that a large fraction of the energy of the outward-moving shock wave is going into the acceleration of atomic nuclei to speeds approaching the speed of light. The Chandra observations provide the strongest evidence yet that nuclei are indeed accelerated and that the energy contained in the high-speed nuclei in Tycho's remnant is about 100 times that observed in high-speed electrons.
This finding is important for understanding the origin of cosmic rays, the high-energy nuclei which pervade the Galaxy and constantly bombard the Earth. Since their discovery in the early years of the 20th century, many sources of cosmic rays have been proposed, including flares on the sun and similar events on other stars, pulsars, black hole accretion disks, and the prime suspect - supernova shock waves. Chandra's observations of Tycho's supernova remnant strengthen the case for this explanation. [1]
References
- "Tycho's Supernova Remnant: Tycho's Remnant Provides Shocking Evidence for Cosmic Rays" - NASA/Chandra/Harvard-Smithsonian Center for Astrophysics, Revised February 20, 2009.
External Links
- Animations & Video: Supernovas & Supernova Remnants - Chandra X-ray Observatory, NASA/.
- International Supernovae Network
- Latest Supernovae - International Supernovae Network
- Pulsar, Supernova Remnant Project - Astrophysics Group, Australia Telescope National Facility (CSIRO).
- Supernova Remnant Group - X-ray Astrophysics Lab, Astrophysics Science Division, Goaddard Space Flight Center, NASA.
- Supernova Remnant Links - Includes: General Links, Cataloges, Research Groups, Individual Researchers, Preprints, Books, Meetings, Proceedings, Other SN Links and Search Engines. Marcos J. Montes, U.S. Naval Research Laboratory.
- Supernova Taxonomy - Marcos J. Montes, U.S. Naval Research Laboratory.
- Supernovae, Supernova Remnant Research - U.S. Naval Research Laboratory.
- Supernovae - Imagine the Universe, NASA.
- Supernovas & Supernova Research - Chandra X-ray Observatory, NASA/.
- Texas Supernova Research Group - Astronomy Department, University of Texas.
Preview Image
"Supernova Remnant G1.9+0.3" - The expanding remains of a supernova explosion in the Milky Way are shown in this composite image of the supernova remnant G1.9+0.3. NASA's Chandra X-ray Observatory image obtained in early 2007 is shown in orange and the radio image from NRAO's Very Large Array (VLA) from 1985 is in blue. The difference in size between the two images gives clear evidence for expansion, allowing the time since the original supernova explosion (about 140 years) to be estimated. This makes the original explosion the most recent supernova in the Galaxy, as measured in Earth's time-frame (referring to when events are observable at Earth). Equivalently, this is the youngest known supernova remnant in the Galaxy (140 years old), easily beating the previous record of about 330 years for Cassiopeia A. The rapid expansion and young age for G1.9+0.3 was recently confirmed by a new VLA image obtained in early 2008. View full-size image. (Source: NASA/Chandra/VLA. X-ray (NASA/CXC/NCSU/S.Reynolds et al.); Radio (NSF/NRAO/VLA/Cambridge/D.Green et al.)
Citation
Williams, Rosa, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Supernova Remnants." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 25, 2007].
<http://www.cosmosportal.org/articles/view/138771/>
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