AM Herculis star
Eclipses in AM Her systems provide a graphic illustration of this stream geometry. Light curves reveal that the tiny, and thus rapidly eclipsed, accretion spot at the stream-facing pole emits about half the total radiated energy of the system, most of the rest coming from the extended stream, which enters and leaves eclipse more gradually. The optical variations in an AM Her star, described as "flickering," may range over 4 to 5 magnitudes.
The prototype, which lies in the constellation Hercules and has an orbital period of 3.1 hours, was discovered by Max Wolf in 1923, then listed in the General Catalogue of Variable Stars as an irregular variable with a range from 12th to 14th magnitude. This listing remained unchanged until 1976, when the true nature of AM Her began to emerge. It was found to be the optical counterpart of an X-ray source, 3U 1809+50, discovered by the Uhuru satellite. Some of the optical features of AM Her's light curve are explainable in terms of the red dwarf secondary. First, the red dwarf is distorted into an egg-shape by the attraction of its companion, toward which the long axis of the egg points. When the secondary is seen broadside, it appears slightly brighter than when end on; hence, as the entire system rotates, there are two long, weak brightness maxima and two long shallow minima per period. Second, there are sometimes brightness fluctuations due to heating of the red dwarf's surface by X-rays from the primary. This "hot spot" is periodically lost from view on the far side of the rotating secondary. The short term flickering is due to the turbulent nature of the mass transfer in the system.
Related entry• variable stars
Related category• TYPES OF STAR
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