a strange quark surface and will be pushed away. For a neutron star, the requirement of the hydrodynamical equilibrium will make the core materials to self-adjust themselves to form a stable structure, including a very heavy crust above the superfluid neutron layer, and the backfalling supernova ejecta has little contribution to such a crust. For a strange star, things are quite different. Almost the whole iron core will be completely converted to SQM, since SQM has the property to swallow all the normal matter that directly contacts with it if Witten[@w84]’s hypothesis is right. Also the detonation flame may expel the outer part of a protoneutron star to form a strange star with a bare quark surface[@detonation] at the very beginning. Thus a crust above the quark surface of a strange star then solely depends on the fallback of the supernova ejecta.

There are a lot of uncertainties on the possible fallback of the supernova ejecta. An important issue is whether a reverse shock can be formed. It was argued that one needs to include a reverse shock to fit the light curve of SN 1987A, and such a reverse shock might be due to a sufficiently massive hydrogen envelope 