This year 2018 has great historical and current significance for stellar spectral classification. Two hundred years ago in Reggio Emilia, Italy, was born Angelo Secchi, a pioneer of observing and classifying the spectra of stars. At the beginning of the IAU, almost a hundred years ago, one of its original Commissions was entitled the Spectral Classification of Stars, from which was generated Commission 45, Spectral Classification and Multi-band Colour Indices. And seventy-five years ago, was published the system-changing MKK, An Atlas of Stellar Spectra. Through this necessarily brief, historical view we shall recall how spectral classification, supported internationally by the IAU, continually updated its techniques, while remaining anchored to standards. This has ensured that the MK classification process stays very relevant to the initial characterizing of stars in the 21st century era of large spectral surveys.

Hypervelocity stars (HVSs) are characterized by a total velocity in excess of the Galactic escape speed, and with trajectories consistent with coming from the Galactic Centre. We apply a novel data mining routine, an artificial neural network, to discover HVSs in the TGAS subset of the first data release of the Gaia satellite, using only the astrometry of the stars. We find 80 stars with a predicted probability >90% of being HVSs, and we retrieved radial velocities for 47 of those. We discover 14 objects with a total velocity in the Galactic rest frame >400 km s−1, and 5 of these have a probability >50% of being unbound from the Milky Way. Tracing back orbits in different Galactic potentials, we discover 1 HVS candidate, 5 bound HVS candidates, and 5 runaway star candidates with remarkably high velocities, between 400 and 780 km s−1. We wait for future Gaia releases to confirm the goodness of our sample and to increase the number of HVS candidates.