The huge step is a newly precise estimate of the Hubble constant, the rate at which the universe has been expanding since the Big Bang. The more distant that object, the more reliable the result.
Astronomers can easily determine the first number.
The stumbling block has been the second. The distant objects used for this study are pulsating stars called Cepheid variable stars, which blink on and off in periods ranging from one to 70 days. M100 taken with WFPC-2. In 12 sessions over a two-month period, the camera had photographed a field in M100 containing 100,000 identifiable stars. It was Hopkins graduate student Laura Ferrarese, working closely with professor of physics and astronomy Holland Ford, who had the painstaking job of picking out Cepheid stars from among the thousands of stars in each image.