Supernova Remnants

A totally different origin is ascribed to nebulae connected with the explosion of very massive stars. Stars with masses of 8 or more solar masses cease their life in a gigantic explosion, leaving behind a gaseous remnant. This supernova remnant expands into the interstellar medium in most cases as a fast moving shell, with velocities of 10000 to 20000 km/s. This produces strong shocks, in which the gas is heated up by the collisions of the fast moving particles with those of the interstellar medium. This in turn gives rise to ionization so that the gas emits characteristic radiation, similar to emission nebulae.

Crab Nebula M 1 (NGC 1952)
Maksutov-Newton, Canon 1100Da, 5.4 hours

Very massive stars (more than five solar masses) cease their life in a spectacular way, viz. in a supernova explosion, leaving behind an extremely compact neutron star (which sometimes can be seen as a pulsar) and a gaseous structure expanding at a high velocity. A prime example of such an object is the Crab Nebula. The white-bluish light from the centre of this nebula is synchrotron radiation, the red elements being remnants of the atmosphere of the original star containing mostly ionised helium and hydrogen (and some heavier elements). The distance to the Crab Nebula is about 1000 ly. In 1054, a Chinese astronomer observed the supernova exactly at that position, and today radio astronomers observe a pulsar right there!

The arrow points at the precise location of the pulsar in the Crab Nebula. It is not particularly bright, since the super-thin pair of beams, much sharper than a laser beam, sweeps across the earth only very swiftly as the pulsar rotates (like a lighthouse, but way faster, namely 30 times per second). Hence we see the pulsar only for a small fraction of the exposure. The pulsar is a neutron star, the extremely compact relic of a gigantic supernova explosion. Its enormously strong magnetic field drags along all charged particles in the surroundings, accelerating them to nearly the speed of light! Imagine: the whole mass of the sun (333333 times that of the earth) is compressed into a sphere with a diameter of the city of Cologne and rotates 30 times a second. And the fastest known pulsar does this 672 times per second! Astrophysics can be exciting and crazy.

Cygnus Loop (NGC 6992)
Evostar 72, Canon 1100Da, 11.0 hours

The so-called Cygnus Loop, located at a distance of about 1500 ly, is the remnant of a supernova explosion that took place in the constellation of Cygnus about 10,000 to 20,000 years ago. The purple colour represents ionised hydrogen, whereas the blue colour shows ionised oxygen! Picture taken with the APO refractor.

Cygnus Loop (NGC 6992)
Maksutov-Newton, Canon 1100Da, 14.1 hours

What we see here is the eastern shockwave of the supernova remnant.