What is a supernova?

A supernova is defined as a massive explosion that occurs at the end of a star's life cycle. This phenomenon is particularly associated with stars that have exhausted their nuclear fuel and cannot support their own mass. When such a star becomes unstable, it may undergo a catastrophic collapse, leading to an explosive expulsion of its outer layers into space. This explosion not only marks the end of the star but also contributes to the creation of heavy elements, which can later form new stars and planets, enriching the surrounding interstellar medium.

The other options refer to different astronomical phenomena. The mention of a distant galaxy undergoing rapid change pertains more to galaxies experiencing active star formation or interactions rather than the explosive death of a star. The collision of two stars describes a different event, often leading to phenomena like a nova or a type of supernova, but is not itself a definition of a supernova. Lastly, an area with a high concentration of dark matter relates to astrophysical structures such as galaxy clusters but is unrelated to the explosive remnants of dying stars. Thus, the description of a supernova as a massive explosion marking the death of a star captures its essence accurately.