  Two blue-straggler sequences discovered in globular cluster M30 provide a
strong constraint on the formation mechanisms of blue stragglers. We study the
formation of the blue-straggler binaries through binary evolution, and find
that binary evolution can contribute to the blue stragglers in both of the
sequences. Whether a blue straggler is located in the blue sequence or red
sequence depends on the contribution of the mass donor to the total luminosity
of the binary, which is generally observed as a single star in globular
clusters. The blue stragglers in the blue sequence have a cool white-dwarf
companion, while the majority ($\sim 60\%$) of the objects in the red sequence
are the binaries that are still experiencing mass transfer, but there are also
some objects that the donors have just finished the mass transfer (the
stripped-core stars, $\sim 10\%$) or the blue stragglers (the accretors) have
evolved away from the blue sequence ($\sim 30\%$). Meanwhile, W UMa contact
binaries found in both sequences may be explained by various mass ratios, that
is, W UMa contact binaries in the red sequence have two components with
comparable masses (e.g. mass ratio $q\sim$ 0.3-1.0), while those in the blue
sequence have low mass ratio (e.g. $q<$ 0.3). However, the fraction of blue
sequence in M30 cannot be reproduced by binary population synthesis if we
assumed the initial parameters of a binary sample to be the same as that of the
field. This possible indicates that dynamical effects on binary systems is very
important in globular clusters.
