La Distance des Etoiles au dix-huitième Siècle: L'Echelle des Magnitudes de John Michell

En dépit de la difficulté présentée par les mesures d'éclat, John Michell construisit au dix-huitième siècle une échelle de magnitude étonnante de précision et de rigueur. Mais il n'indique pas ses propres méthodes. Sans doute transposa-t-il aux étoiles les méthodes de Bouguer dont l'ouvrage faisait alors autorité en matière de photométrie. Son travail, dont il se contente d'exposer très brièvement les résultats sans en développer les principes, restera ignoré des autres astronomes.     

A powerful bursting radio source towards the Galactic Centre

Transient astronomical sources are typically powered by compact objects and usually signify highly explosive or dynamic events. Although high-time-resolution observations are often possible in radio astronomy, they are usually limited to quite narrow fields of view. The dynamic radio sky is therefore poorly sampled, in contrast to the situation in the X-ray and gamma-ray bands in which wide-field instruments routinely detect transient sources. Here we report a transient radio source, GCRT J1745-3009, which was detected during a moderately wide-field monitoring programme of the Galactic Centre region at 0.33 GHz. The characteristics of its bursts are unlike those known for any other class of radio transient. If located in or near the Galactic Centre, its brightness temperature (approximately 10(16) K) and the implied energy density within GCRT J1745-3009 vastly exceed those observed in most other classes of radio astronomical sources, and are consistent with coherent emission processes that are rarely observed. We conclude that it represents a hitherto unknown class of transient radio sources, the first of possibly many new classes that may be discovered by emerging wide-field radio telescopes.