A shell collapsar is a hypothetical compact astrophysical object, which might constitute an alternative explanation for observations of astronomical black hole candidates. Being of neutron star size (~11km), it strongly distorts light rays like a black hole, but has no central point-like singularity. Matter has collapsed onto the event horizon forming a shell there of ultra-high density. The model of the shell collapsar was first proposed by Trevor W. Marshall in 2009 ^[1] and 2012 ^[2] and more firmly established in 2016.^[3] It predicts neutron star masses beyond the Tolman–Oppenheimer–Volkoff limit of 2.1 M☉.^[4] so providing an alternative to stellar-mass black holes for the highly compact bodies merging to give gravitational wave signals.

A shell collapsar may be void inside^[5] as time dilation delays indefinitely the entry of mass-particles. Marshall's solution however, predicts intense gravitational field energy there, arising from Einsteinian gravitation in the highly non-linear regime. As time dilation is extreme on the collapsar surface (just outside the event horizon), accreting neutron matter can be viewed as freezing onto the outer shell, as described in the ‘frozar’ model^[6][7] Zakir conceives that outward gravitational forces arise in the interior and push inner matter into the ‘frozar’ shell.^[8]

The shell collapsar is a special case of a gravastar. With the gravastar, an exotic form of matter stabilizes the object with the equation of state of dark energy inside. The shell collapsar comes to a similar result with ordinary neutron star matter and simply Einstein's field equations describing intense gravitational energy density, comparable as E/c² to the neutron matter density.