A stellar core fragment is a massive remnant of a collapsed star, thrown out into space during the process of the star's collapse. The density of a fragment can exceed 10 million metric tons per cubic centimeter, causing it to have immense gravity.

In 2364, the SS Tsiolkovsky was observing the collapse of a red giant into a white dwarf when the gravitational influence of the star caused an outbreak of polywater intoxication that claimed the crew. The USS Enterprise-D, responding to the Tsiolkovsky's situation, was threatened by a stellar core fragment thrown off by the collapsing star. The Enterprise managed to buy enough time to repair its engines and escape by pushing off from the Tsiolkovsky using a repulsor beam. The Tsiolkovsky struck the fragment and was destroyed. (TNG: "The Naked Now")

The Genome colony of Moab IV was menaced by a stellar core fragment in 2368, when one passed through the its star system and its gravity produced earthquakes greater than what the colony's biosphere was designed to withstand. The colony survived with assistance from the crew of the USS Enterprise-D, who reinforced the colony dome and used a multiphasic tractor beam to slightly divert the fragment's path. (TNG: "The Masterpiece Society").

In actual astrophysics, the collapse of a star is preceded by a helium-burning phase, where very high mass stars with more than nine solar masses expand to form red supergiants. Once this fuel is exhausted at the core, the core contracts and can continue to fuse elements heavier until the final stage is reached when the star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, if they are fused they do not release energy—the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission. In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.

An evolved, average-size star will now shed its outer layers as a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 solar masses, it shrinks to a relatively tiny object (about the size of Earth) that is not massive enough for further compression to take place, known as a white dwarf. The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. White dwarfs will eventually fade into black dwarfs over a very long stretch of time.

In larger stars, fusion continues until the iron core has grown so large (more than 1.4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before.

Most of the matter in the star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula) and what remains will be a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a stellar remnant greater than roughly 4 solar masses), a black hole. In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole the matter is in a state that is not currently understood.

The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.