Examples of Well-known Supernovae
When stars explode as supernovae, they throw out a lot of their material, very often at relativistic speeds (i.e. speeds close to the speed of light, so perhaps as much as thousands of kilometres per second). Over time, this material dissipates and on occasion, it helps to form the raw material for the next round of star formation. In fact, we think our own Sun (and its Solar System) is composed of material that has been "recycled" like this at least twice. For a few thousand years, the material can form a rapidly expanding shell (known as a supernova remnant), which is lit up by energetic photons (usually UV and X-rays) coming from the scene of the original supernova.
The remnants can be spherical as in the case of Cas A (see Figure 1) or more filamentary in structure such as the Crab Nebula (see Figure 2) in the constellation of Taurus. The Crab Nebula is a fantastic example for us to study since we know that it was formed as a result of a supernova in 1054 which was seen by Chinese astronomers even in the daytime. Knowing its age gives scientists a great insight into the evolution of the remnant, as well as the behaviour of the compact object left behind, in this case a pulsar, which spins around 30 times per second.
Kepler's Supernova was discovered in 1604 in our Galaxy, before the invention of the telescope. It was visible to the naked eye and is the most recently discovered supernova in the Milky Way.
SN 1987A was a type II supernova in a nearby dwarf galaxy called the Large Magellanic Cloud. This made it the closest observed supernova in the last few hundred years. Discovered in February 1987, it brightened for over two months and left behind a spectacular hourglass shaped remnant. Astronomers studied it in great detail and at several wavelengths allowing a greater understanding of core-collapse supernovae, however it wasn't until 2019 that the neutron star left behind was discovered.