Collapsed Nuclei

We discuss the observational consistency, possible properties, and detection of collapsed nuclei ${C}_{A}$. These may be considered as elementary particles with mass number $A>1$ and of much smaller radius than ordinary nuclei ${N}_{A}$. The existence of ${C}_{A}$ of (perhaps much) lower energy than ${N}_{A}$ is observationally consistent if ${N}_{A}$ are very long-lived isomers against collapse because of a "saturation" barrier between ${C}_{A}$ and ${N}_{A}$. Barrier-penetrability estimates show that sufficiently long lifetimes $\ensuremath{\gtrsim}{10}^{31}$ sec are plausible for $A\ensuremath{\gtrsim}16\ensuremath{-}40$. The properties of ${C}_{A}$ are discussed using composite baryon and quark models; small charges and hypercharges and, especially, neutral ${C}_{A}$ are possible. ${C}_{A}$ can be effectively a source or sink of baryons. Some astrophysical implications are briefly discussed, in particular the possible large scale presence of ${C}_{A}$ and the possibility that accelerated collapse in massive objects may be a source of energy comparable to the rest mass.