... Most of the spirals discussed in the following are well-known starburst galaxies (M 82, NGC 253, etc.) In addition to their nuclear activity, which makes them so popular, the selected galaxies have several other phenomena in common, such as bars, gas outflow, and possibly nuclear rings. ... I compare the group of nearby starburst galaxies with distant ultra-luminous galaxies (Arp 220, NGC 6240, etc.), in which strong tidal interactions cause the radial gas inflow. ...

metrologia The leap second: its history and possible future R. A. Nelson, D. D. McCarthy , S. Malys, J. Levine, B. Guinot, H. F. Fliegel , R. L. Beard and T. R. Bartholome w Abstract. This paper reviews the theoretica l motivatio n for the leap second in the context of the historica l evolutio n of time measurement . Th e periodic insertio n of a leap second step into th e scale of Coordinate d Universal Time (UTC) necessitates frequent changes in comple x timekeepin g systems an d is currentl y th e