  The nature of pulsar-like compact stars is essentially a central question of
the fundamental strong interaction (explained in quantum chromo-dynamics) at
low energy scale, the solution of which still remains a challenge though
tremendous efforts have been tried. This kind of compact objects could actually
be strange quark stars if strange quark matter in bulk may constitute the true
ground state of the strong-interaction matter rather than Fe^56 (the so-called
Witten's conjecture). From astrophysical points of view, however, it is
proposed that strange cluster matter could be absolutely stable and thus those
compact stars could be strange cluster stars in fact. This proposal could be
regarded as a general Witten's conjecture: strange matter in bulk could be
absolutely stable, in which quarks are either free (for strange quark matter)
or localized (for strange cluster matter). Strange cluster with
three-light-flavor symmetry is renamed strangeon, being coined by combining
"strange nucleon" for the sake of simplicity. A strangeon star can then be
thought as a 3-flavored gigantic nucleus, and strangeons are its constituent as
an analogy of nucleons which are the constituent of a normal (micro) nucleus.
The observational consequences of strangeon stars show that different
manifestations of pulsar-like compact stars could be understood in the regime
of strangeon stars, and we are expecting more evidence for strangeon star by
advanced facilities (e.g., FAST, SKA, and eXTP).
