We attempt to solve the question of star
formation triggers and star formation laws by
studying samples of simple objects and defining
carefully the possible external effects. Among the
star formation (SF) triggers there are some that
can operate only in large disk galaxies. These are
shear instabilities and density waves, and we can
eliminate them if we restrict the sample to
diskless objects of low mass. Such galaxies, which
do show star formation, are late-type dwarf
galaxies (DGs).
Other SF triggers are related to the
neighborhood a galaxy finds itself in. Such
triggers are galaxy-galaxy collisions and
galaxy-intracluster matter interactions
(stripping, shocks, etc.). These also can be
eliminated by properly choosing the sample to
study; one selects galaxies from neigh-borhoods of
widely different densities (of galaxies) and
compares their SF parameters.
We selected our first samples in a region
of relatively high galaxy density, where a
complete morphological classification of objects
was available, and suitable depth in brightness
sampling could readily be achieved. This is the
Virgo cluster, where Binggeli, Sandage and Tammann
(1985, BST) provide a sample of more than 2000
DGs. Among those, some 25% are of late type and
are classified by BST as blue compact dwarfs
(BCDs), as Magellanic irregulars (Im) of five
possible sub-classes, or as combinations of those
two classifications.