Starship Asterisk*

http://apod.nasa.gov/apod/ap060602.html wrote:
Explanation: IC 443 is typical of the aftermath of a stellar explosion, the ultimate fate of massive stars. Seen in this false-color composite image, the supernova remnant is still glowing across the spectrum, from radio (blue) to optical (red) to x-ray (green) energies -- even though light from the stellar explosion that created the expanding cosmic cloud first reached planet Earth thousands of years ago. The odd thing about IC 443 is the apparent motion of its dense neutron star, the collapsed remnant of the stellar core. The close-up inset shows the swept-back wake created as the neutron star hurtles through the hot gas, but that direction is not aligned with the direction toward the apparent center of the remnant. The misalignment suggests that the explosion site was offset from the center or that fast-moving gas in the nebula has influenced the wake. The wide view of IC 443, also known as the Jellyfish nebula, spans about 65 light-years at the supernova remnant's estimated distance of 5,000 light-years.

http://apod.nasa.gov/apod/ap970502.html wrote:
Explanation: The life-cycles of stars help drive the ecology of our Galaxy, churning, processing, and redistributing matter. Massive stars reach a spectacular evolutionary endpoint - supernovae explosions which blast off their outer layers, violently merging stellar material with the gas and dust of the Milky Way. The supernova remnant IC 443 is typical of the aftermath. Seen in this false color X-ray image are the shocked, expanding shells of gas from a star which exploded thousands of years ago. Known to be interacting with galactic molecular clouds, the expanding supernova remnant was also recently discovered to have regions of intense higher energy X-ray emission (coded blue in this map) near the molecular cloud boundaries. This X-ray emission may indicate that electrons are being accelerated within the remnant, gaining in energy as they surf back and forth across the expanding shock wave. If so, IC 443 could also be one source of our Galaxy's puzzling high energy cosmic-rays.

http://en.wikipedia.org/wiki/IC_443 wrote:
<<IC 443 (also known as the Jellyfish Nebula) is a Galactic supernova remnant (SNR) in the constellation Gemini. On the plan of the sky, it is located near the star Eta Geminorum. Its distance is roughly 5,000 light years (~5×1016 km) from Earth.

IC 443 is thought to be the remains of a supernova occurred 3,000 - 30,000 years ago. The same supernova event likely created the neutron star CXOU J061705.3+222127, the collapsed remnant of the stellar core.

IC 443 is one of the best-studied case of supernova remnant interacting with surrounding molecular clouds.

IC 443 is an extended source, having an angular diameter of 50 arcmin (by comparison, the full moon is 30 arcmin across). At the estimated distance of 5,000 ly (1,500 parsec) from Earth, it corresponds to a physical size of roughly 70 light years (20 parsec).

The SNR optical and radio morphology is shell-like (e.g. a prototypical shell-like SNR is SN 1006), consisting of two connected sub-shells with different centers and radii. A third larger sub-shell, initially attributed to IC 443, is now recognized as a different and older (100,000 years) SNR, called G189.6+3.3.

Notably, IC 443 X-ray morphology is centrally peaked and a very soft X-ray shell is barely visible. Unlike plerion remnants, e.g. the Crab Nebula, the inner X-ray emission is not dominated by the central pulsar wind nebula. It has indeed a thermal origin. IC 443 shows very similar features to the class of mixed morphology SNRs.

Both optical and X-ray emission are heavily absorbed by a giant molecular cloud in the foreground, crossing the whole remnant body from northwest to southeast.

The remnant's age is still uncertain. There is some agreement that the progenitor supernova happened between 3,000 and 30,000 years ago. Recent Chandra and XMM-Newton observations identified a plerion nebula, close to the remnant southern rim. The point source near the apex of the nebula is a neutron star, relic of a SN explosion. The location in a star forming region and the presence of a neutron star favor a Type II supernova, the ultimate fate of a massive star, as the progenitor explosion.

The SNR IC 443 is located in the Galactic anticenter direction (l=189.1°), close to the Galacatic plane (b=+3.0°). Many objects lie in the same region of sky: the HII region S249, several young stars members of the GEM OB1 association, and an older SNR (G189.6+3.3).

The remnant is evolving in a rich and complex environment, which strongly affects its morphology. Multi-wavelenght observations show the presence of sharp density gradients and different clouds geometries in the surroundings of IC 443. Massive stars are known to be short lived (roughly 30 million years), ending their life when they are still embedded within the progenitor cloud. The more massive stars (O-type) probably clear the circum-stellar environment by powerful stellar winds or photoionizing radiation. Early B-type stars, with a typical mass between 8 and 12 solar masses, are not capable of this, and they likely interact with the primordial molecular cloud when they explode. Thus, it is not surprising that the SNR IC 443, which is thought to be the aftermath of a stellar explosion, evolved in such complex environment. For instance, an appreciable fraction of supernova remnants lies close to dense molecular clouds (~50 out of 265 in the Green catalogue), and most of them (~60%) show clear signs of interaction with the adjacent cloud.

X-ray and the optical images are characterized by a dark lane, crossing IC 443 from northwest to southeast. Emission from quiescent molecular gas has been observed toward the same direction[9], and it is likely due to a giant molecular cloud, located between the remnant and the observer. This is the main source of extinction of the low energy SNR emission.

In the southeast the blast wave is interacting with a very dense (~10,000 cm-3) and clumpy molecular cloud, such that the emitting shocked gas has a ring-like shape. The blast wave has been strongly decelerated by the cloud and is moving with an estimated velocity of roughly 30-40 km s-1. OH (1720 MHz) maser emission, which is a robust tracer of interaction between SNRs and dense molecular clouds, has been detected in this region. Interestingly, a source of gamma-ray radiation is spatially coincident with IC 443 and the maser emission region, though is not well understood whether it is physically associated with the remnant or not. In the northeast, where the brightest optical filaments are located, the SNR is interacting with a very different environment. The forward shock has encountered with a wall of neutral hydrogen (HI), and is propagating into a less dense medium (~10-1,000 cm-3) with a much higher velocity (80-100 km s-1)[10] than in the southern ridge. In the western region, the shock wave breakouts into a more homogeneous and rarefied medium.>>