  We investigate collective emission from coherently driven ultracold $ ^{88} $
Sr atoms. We perform two sets of experiments, using a strong and weak
transition that are insensitive and sensitive, respectively, to atomic motion
at one microKelvin. We observe highly directional forward emission with a peak
intensity that is enhanced, for the strong transition, by > $ 10 ^3 $ compared
to that in the transverse direction. This is accompanied by substantial
broadening of spectral lines. For the weak transition, the forward enhancement
is substantially reduced due to motion. Meanwhile, a density-dependent
frequency shift of the weak transition (~10% of the natural linewidth) is
observed. In contrast, this shift is suppressed to <1% of the natural linewidth
for the strong transition. Along the transverse direction, we observe strong
polarization dependences of the fluorescence intensity and line broadening for
both transitions. The measurements are reproduced with a theoretical model
treating the atoms as coherent, interacting, radiating dipoles.
