  We report on the experimental observation of the dipolar collapse of a
quantum gas which sets in when we reduce the contact interaction below some
critical value using a Feshbach resonance. Due to the anisotropy of the
dipole-dipole interaction, the stability of a dipolar Bose-Einstein condensate
depends not only on the strength of the contact interaction, but also on the
trapping geometry. We investigate the stability diagram and find good agreement
with a universal stability threshold arising from a simple theoretical model.
Using a pancake-shaped trap with the dipoles oriented along the short axis of
the trap, we are able to tune the scattering length to zero, stabilizing a
purely dipolar quantum gas.
