supernova remnant

A supernova remnant is an after-effect of a supernova that occurs for some, essentially a type of nebula. It consists of the supernova's ejected debris (material ejected from the exploding star, often at relativistic speeds), along with interstellar medium (ISM) affected by the debris. The remnant may be optically visible if a source of light is nearby that is reflected, or if the debris collision with other material causes one or both to heat from the shock, increasing the thermal emission. The aftermath of a supernova (i.e., the beginning of its life as a remnant) is modeled as passing through three phases:

• free expansion phase (nearly identical to expansion into a perfect vacuum).
• Sedov-Taylor phase (basically an adiabatic process).
• snowplow phase (gathers ISM and cools through emitted EMR).

Example supernova remnants:

• Crab Nebula (M1).
• Cassiopeia A.
• Remnant of SN 1987A.

Some classification:

• shell-type remnant - like Cassiopeia A, appearing as a circle in the sky, the appearance of EMR emitted by a shell-shaped volume surrounding the supernova location, the EMR resulting from the shock of debris meeting ISM.
• crab-type remnant - like the Crab Nebula, with substantial EMR from closer to the supernova location, a result of a central pulsar.
• composite remnant - showing characteristics of both.

A different term, stellar remnant, is used for a black hole or neutron star left from a supernova. These remain after any supernova remnant has dissipated.

A nova remnant is the analogous debris sometimes produced by a nova (which is typically a flash triggered by accretion onto a white dwarf). A nova remnant is smaller, slower, and fades faster than a supernova remnant, but may leave a brighter central star to illuminate it.

The abbreviation SNR is also used for signal-to-noise ratio.