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
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.