The young σ Orionis cluster in the Ori OB 1b association is one of the most important clusters for understanding the stellar and substellar formation and evolution. In this work, we used 197 low-resolution optical spectra of 167 stars. We determined spectral types and measured Li I and Hα equivalent widths (EWs). We used Gaia DR2 astrometry

We report the study of an “Einstein Cross” configuration first identified in a set of HST images by Cerny et al. Deep spectroscopic observations obtained at the Spanish 10.4 m Gran Telescopio Canarias telescope, allowed us to demonstrate the lens nature of the system, that consists of a Lyman-break galaxy (LBG), not a quasi-stellar object as is

The criteria for spectral classification of B-type stars were established almost 80 years ago from low-resolution spectra on photographic plates. Since Hipparcos measured accurate distances for nearby stars, it has been clear that, although sound in general terms, the current set of MK standard stars in the B range has many inconsistencies, with

The mechanisms producing fast variability of the γ-ray emission in active galactic nuclei (AGNs) are under debate. The MAGIC telescopes detected a fast, very-high-energy (VHE, E > 100 GeV) γ-ray flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of (1.5 ± 0.3) × 10-10 photons cm-2 s-1 and halving time of 26 ± 8 min. The MAGIC

The dark ages of the Universe end with the formation of the first generation of stars residing in primeval galaxies. These objects were the first to produce ultraviolet ionizing photons in a period when the cosmic gas changed from a neutral state to an ionized one, known as Epoch of Reionization (EoR). A pivotal aspect to comprehend the EoR is to

Aims: Our scientific goal is to provide a 3D map of the nearest open cluster to the Sun, the Hyades, combining the recent release of Gaia astrometric data, ground-based parallaxes of sub-stellar member candidates and photometric data from surveys which cover large areas of the cluster. Methods: We combined the second Gaia release with ground-based