  Recent progress in the seismic interpretation of field beta Cep stars has
resulted in improvements of the physics in the stellar structure and evolution
models of massive stars. Further asteroseismic constraints can be obtained from
studying ensembles of stars in a young open cluster, which all have similar
age, distance and chemical composition. We present an observational
asteroseismology study based on the discovery of numerous multi-periodic and
mono-periodic B-stars in the open cluster NGC 884. We describe a thorough
investigation of the pulsational properties of all B-type stars in the cluster.
Overall, our detailed frequency analysis resulted in 115 detected frequencies
in 65 stars. We found 36 mono-periodic, 16 bi-periodic, 10 tri-periodic, and 2
quadru-periodic stars and one star with 9 independent frequencies. We also
derived the amplitudes and phases of all detected frequencies in the U, B, V
and I filter, if available. We achieved unambiguous identifications of the mode
degree for twelve of the detected frequencies in nine of the pulsators.
Imposing the identified degrees and measured frequencies of the radial, dipole
and quadrupole modes of five pulsators led to a seismic cluster age estimate of
log(age/yr) =7.12-7.28 from a comparison with stellar models. Our study is a
proof-of-concept for and illustrates the current status of ensemble
asteroseismology of a young open cluster.
