  One of the two breakout reactions from the hot CNOcycle is 15O(a,g)19Ne,
which at low temperatures depends strongly on the resonance strength of the
4.033 MeV state in 19Ne. An experimental upper limit has been placed on its
strength, but the lower limit on the resonance strength and thereby the
astrophysical reaction rate is unconstrained experimentally. However, this
breakout reaction is crucial to the thermonuclear runaway which causes type I
X-ray bursts on accreting neutron stars. In this paper we exploit astronomical
observations in an attempt to constrain the relevant nuclear physics and deduce
a lower limit on the reaction rate. Our sensitivity study implies that if the
rate were sufficiently small, accreting material would burn stably without
bursts. The existence of type I X-ray bursts and superbursts consequently
suggests a lower limit on the 15O(a,g)19Ne reaction rate at low temperatures.
