A space-time oscillation created by the motion of matter, as predicted by Einstein's general relativity. When an object accelerates, it creates ripples in space-time, just like a boat causes ripples i
n a lake. Gravitational waves are extremely weak even for the most massive objects like supermassive black holes. They had been inferred from observing a binary pulsar in which the components slow down, due to losing energy from emitting gravitational waves. Gravitational waves were directly detected for the first time on September 14, 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO). Since then several other events have been detected by LIGO and Laser Interferometer Space Antenna (LISA).
The darkening, or brightening, of a region on a star due to localized decrease, or increase, in the effective gravity. Gravity darkening is explained by the von Zeipel theorem, whereby on stellar surf
ace the radiative flux is proportional to the effective gravity. This means that in rotating stars regions close to the pole are brighter (and have higher temperature) than regions close to the equator. Gravity darkening occurs also in corotating binary systems, where the tidal force leads to both gravity darkening and gravity brightening. The effects are often seen in binary star light curves. Recent theoretical work has shown that gravity darkening is not well represented by the von Zeipel theorem. This is supported by new interferometric observations of some rapidly rotating stars indicating that the von Zeipel theorem seems to overestimate the temperature difference between the poles and equator.