Introduction to Supernova Remnants (2024)

Introduction to Supernova Remnants (1)

What's on This Page

  • What is a Supernova Explosion?
  • What is a Supernova Remnant?
  • How does a Supernova Remnant Evolve?
  • Modeling SNR
  • How Does X-ray Astronomy Fit In?
  • Further SNR Resources and References

What is a Supernova Explosion (SNe)?

super - 1: very large or powerful (a superatomic bomb) 2: exhibiting the characteristics of its type to an extreme or excessive degree (supersecrecy)

nova - Etymology: New Latin, feminine of Latin novus,new. Plural: novae 1: a star that suddenly increases its light output tremendously and then fades away to its former obscurity in a few or years

- Merrian-Webster Dictionary

Supernovae were named because they were objects that appeared to be 'new'stars, that hadn't been observed before in the well-known heavens. Ofcourse, the name is actually a bit ironic, since supernovae are actuallystars at the end of their lifecycle, stars that are going out with a bang,so to speak.

A SNe is the instantaneous release of ~1051 ergs(1031 Megatons) of energy, the result of either thecatastrophic collapse of a massive star or runnaway nuclear burning on thesurface of a white dwarf. Although only a small fraction of the energy isreleased in the form of visible light, this is enough to make it appear asif a new star has appeared in the sky. The effects of the SNe areprofound and far-reaching, and it is possible to see the remains of SNethat occured many hundreds of years ago.

Do you want a more basic description of supernova remnants?

Classification of SNe

Introduction to Supernova Remnants (2)The classification of SNe is an observational one. SNe are broken down into two groupsbased on the presence or absence of Hydrogen Balmer lines in their spectra at maximumbrightness. Those without Balmer lines are classified as Type I SNe and those withBalmer lines are classified as Type II. Supernovae of Type I are further divided intoType Ia, Ib and Ic. Although the classification is a purely observational one, thereare underlying similarities between the progenitor stars (Type Ia are understood to below mass while Type Ib, Ic and II are high mass) that are responsible for the observedclasses of SNe.

The Life and Death of a Star

A star is engaged in a continual struggle against collapse due to the gravitationalforce of its own mass. In the early stages of its life (the main sequence stage), astar resists gravity with thermal pressure, as the fusion of elements in its corereleases energy which heats up the star's gas. The hot gas expands, exerting anoutward pressure that balances the inward force of gravity. How this battle of gravityvs. outward pressure eventually concludes depends upon the initial mass of the star.

Massive Star

At some point in a massive star's life, the center of the star has beenconverted to iron and nuclear fusion in the core is no longer anexothermal process. Nuclear fusion ceases and, without this source ofthermal pressure, gravity, exerting its inexorable pull on the star, seemsto be winning the battle. The star continues its collapse. Then astrange reaction takes place during which electrons and protons are pushedso close together that they merge to become neutrons, releasing energy inthe form of neutrinos. There is no available space between the neutrons:they are supported by neutron degenerate pressure. This halts thegravitational collapse and the outer, more tenuous stellar material'bounces' upon hitting the degenerate core, much like a wave hitting a seawall bounces back on itself. The conversion of the central core toneutrons releases 1051 ergs of neutron binding potentialenergy, and after the bounce, the outer layers of the star are violentlyejected into the ISM.

If the star starts with between 5 and 12 times the mass of our sun, theneutron degeneracy pressure in the core is thought to be able to withstandthe gravitational pressure of the star remaining after the supernovaexplosion. In this case, a neutron star is left in the center of the SNR.If the neutron star is rotating, it may become a pulsar, emittingradiation in a beam which sweeps the earth as the pulsar rotates. If thestar is massive enough, even neutron degenerate pressure will not be ableto hold up against gravitational collapse, and the remains continue to besqueezed inward by gravity, forrming a singularity, or black hole.

Low Mass Stars

In the usual scheme of things, we expect low mass stars to settle down,slowly fizzle out and become white dwarfs. Their masses are too low tobring about the collapse to a neutron star and the resulting spectacle ofa supernova explosion. They resist gravity's pull with electrondegenerate pressure, where the electrons are squeezed until there is nomore space between them. The gravitational pressure of the star is notenough to cause the conversion to neutrons and the star has reached astable equilibrium. However, if the white dwarf is in a binary systemwith a red giant, it is possible for the dwarf to accrete matter from itscompanion. If enough matter falls on it that it exceeds the Chandresekarlimit for white dwarfs, the star is no longer stable, gravity willovercome the resistance of electron degeneracy pressure and the star willcollapse. The collapse raises the temperature until carbon and oxygen inthe core start to fuse, igniting a deflagration wave of runnaway nuclearburning which propagates through the core in seconds. The nuclear fusionreactions create about a solar mass of radioactive 56Ni. Theenergy released is on the order of 1052 ergs, and the whitedwarf is completely disrupted in the process. The star can outshineentire galaxies while the nuclear fusion proceeds. This is believed to bethe mechanism for Type Ia SNe.

What is a Supernova Remnant (SNR)?

Most (99%) of the energy of a SNe is released in the formof energetic neutrinos; the remaining energy is converted into kineticenergy, accelerating the stellar material to speeds greater than the speedof sound and causing a shock wave to move outwards from the central star. The high velocity stellar material plows outwards into the ISM,compressing and heating ambient gas and sweeping it up much as a snow plowcompacts and sweeps up snow. The ISM becomes enriched with the stellarmaterial blown off in the explosion. The expanding material, and anyadditional material the blast collects as it travels through theinterstellar medium, forms a supernova remnant (SNR).

Although in a simple model of SNRs the explosion energy is depositedevenly in all directions and the ambient material is swept up in auniform, spherical shell, in reality, remnants are much more complicated. Small differences in the initial conditions of the progenitors, includingprevious mass ejections, can have important effects on ejecta propagation. External conditions, such as density enhancements, in the ISM in which theSN explodes can shape and mold the morphology of the remnant. AndRayleigh-Taylor instabilities can cause the surface of the forward shockto become rippled, mixing the ambient and stellar gases. It is notsurprising that there are various types of SNR: simple Shell type remnants with nothing in theircenters, Crab-like remnants or plerions with pulsars in their centersand Composite remnants , a combination ofthe first two.

How does the SNR Evolve?

As the ejecta expand out from the star, it passes through the surroundinginterstellar medium, heating it from 107 to 108 K,sufficient to separate electrons from their atoms and to generate thermalX-rays. The interstellar material is accelerated by the shock wave andwill be propelled away from the supernova site at somewhat less than theshock wave's initial velocity. This makes for a thin expanding shellaround the supernova site encasing a relatively low density interior.

Tycho supernova remnant (SN1572)
While the material swept up by the shock is much less than the mass of thestellar ejecta, the expansion of the stellar ejecta proceeds atessentially a constant velocity equal to the initial shock wave speed,typically of the order of 10,000 km/s. This is known as the "freeexpansion" phase and may last for approximately 200 years, at whichpoint the shock wave has swept up as much interstellar material as theinitial stellar ejecta. The supernova remnant at this time will be about10 light years in radius. Although the remnant is radiating thermal X-rayand synchrotron radiation across a broad range of the electromagneticspectrum (from radio to X-rays), the initial energy of the shock wave willhave diminished very little. Line emission from the radioactive isotopesgenerated in the supernovae contribute significantly to the total apparentbrightness of the remnant in the early years, but do not significantlyeffect the shock wave.
As the remnant sweeps upambient mass equal to the mass of the stellar ejecta, the wave will beginto slow and the remnant enters a phase known as adiabatic expansion, orthe Sedov-Taylor or blast wave phase. The internal energy of the shockcontinues to be very large compared to radiation losses from thermal andsynchrotron radiation, so the total energy remains nearly constant. Therate of expansion is determined solely the initial energy of the shockwave and the density of the interstellar medium.

As the shock wave cools, it will become more efficient at radiatingenergy. Once the temperature drops below 20000 K or so, some electronswill be able to recombine with carbon and oxygen ions, enablingultraviolet line emission which is a much more efficient radiationmechanism than the thermal X-rays and synchrotron radiation. Hence thisnew phase is known as the radiative phase during which X-ray radiationbecomes much less apparent and the remnant cools and disperses into thesurrounding medium over the course of the next 10000 years.

Supernova remnants are extremely important for our understanding of ourGalaxy. They are the source of much of the energy that heats up theinterstellar medium. They are believed to be responsible for theacceleration of galactic cosmic rays. Heavy elements (up to iron) createdby fusion in the stellar core are dumped into the galaxy by the mixing ofthe ejecta and ISM material in the remnant. Enriched, heated gas from SNRmaterial is reprocessed to form new stars. Elements heavier than iron arecreated in the powerful blast of a SN explosion and are then dredged up bythe shock wave and mixed into the ISM. Most of the elements (except forHydrogen) in your body, for example, are stellar material that has beenredistributed in a SNR. Understanding how SNR evolve can thus help us tounderstand many important phenomena.

Modeling SNR

One technique for understanding SNR is to develop a "model" SNR, which issimplified but contains important characteristics common to all SNR, suchas the initial energy of the explosion. Such models, which can be eitheranalytical (as an equation, for example, of the size of the remnant as afunction of time) or numerical (simulations) can predict X-rays or otheremission we would expect to see. These predictions can then be comparedwith observed SNR to see how well the model fits.

Analytical models are few, since the equations governing the motion of theSNR gas are very complex and cannot be solved for except in very limitedspecial cases (spherical geometry and constant energy, for example). In anumerical model, the evolution of the ejecta and ISM gas, fit onto acomputational grid, is followed over time, by using the conservationequations of mass, momentum and energy . Numerical models are constrainedby limits on the time and space it takes to run the simulation and thenstore the data. It is not difficult, for example, to perform simulationsthat have spherical or cylindrical symmetry (1-dimensional and2-dimensional, respectively), but many of the more complicated structuresfound in real SNR are destroyed by the imposition of this kind ofsymmetry. Fully 3-dimensional simulations, on the other hand, arecomputationally very expensive, particularly if they are of sufficientresolution (small enough numerical grid size) so that small features suchas small scale turbulence, do not get averaged out.

Despite the difficulties, many good SNR models have been developed, whichincrease our understanding of how they evolve and how they interact withthe surrounding ISM, including the following:

  • Lyerley et al. (abstract for a Bulletin of the AmericanAstronomical Society article entitled Nonequilibrium-IonizationX-ray Spectra from a Sedov-Taylor Blast Wave Model for a Supernova Remnant )
  • Truelove & McKee (abstract for a Bulletin of the American Astronomical Society article entitled Evolution of NonradiativeSupernova Remnants )
  • Reynolds, S. P. (abstract for a Bulletin of the AmericanAstronomical Society article entitled Theoretical Calculations of SynchrotronX-rays from Supernova Remnants: Images and Spectra)

How Does X-ray Astronomy Fit in?

The Puppis A remnant with the ROSAT neutron star discovery

Optical image of the Crab Nebula

There are several mechanisms by which SNR generate X-rays. The ejecta, heated and comressed by the powerful forward shock,generates thermal and line emission in the X-ray region of the spectrum.Electrons, accelerated by the SNR shock to relativistic speeds emit synchrotron radiation when they change direction in the presence of a magnetic field. And a neutron star formed in a Type II SNe may also bea significant source of X-rays.

The X-rays emitted by SNR give us many clues that help us understand SNR,their progenitor stars, and their interaction with the ISM. We usuallyknow little about the progenitor star, however, X-ray spectra of the shockheated ejecta in the remnant offer direct information about thecomposition of the progenitor star. X-ray spectra can also giveinformation on the composition of the ISM into which the remnant isexpanding, if and to what extent the elemental layers of the progenitorstar are mixed with the gas of the ISM. By studying the X-ray morphologiesof SNR, we can learn about how thermal conduction acts in SNR and in theISM. Looking for X-ray synchrotron radiation is a way of getting directevidence for cosmic ray acceleration in SNR shocks.

The hot material, the radioactive isotopes, free electrons moving in thestrong magnetic field of the neutron star... all of these processes produce X-rays and gamma rays which reveal the nature of the SNR.

Further SNR Resources and References

Additions or Comments: Have we left anything out? Is there something you would like to have addedto this page (a link to your own group's research page, for example...)?

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Last modified: Wednesday, 11-May-2011 14:17:37 EDT

Introduction to Supernova Remnants (2024)
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