  After a brief non technical introduction of the basic properties of strange
quark matter (SQM) in compact stars, we consider some of the late important
advances in the field, and discuss some recent astrophysical observational data
that could shed new light on the possible presence of SQM in compact stars. We
show that above a threshold value of the gravitational mass a neutron star
(pure hadronic star) is metastable to the decay (conversion) to an hybrid
neutron star or to a strange star. We explore the consequences of the
metastability of "massive" neutron stars and of the existence of stable compact
"quark" stars (hybrid neutron stars or strange stars) on the concept of
limiting mass of compact stars, and we give an extension of this concept with
respect to the "classical" one given in 1939 by Oppenheimer and Volkoff.
