AC Herculis, is an

RV Tauri variable RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima. History and discovery German astronomer Friedrich Wilhelm Argelander monitored the distinctive variations in brightne ...

and

spectroscopic binary A binary star is a system of two star, stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separa ...

star in the constellation of

Hercules Hercules (, ) is the Roman equivalent of the Greek divine hero Heracles, son of Jupiter and the mortal Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures. The Romans adapted the Gr ...

. It varies in brightness between

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...

s 6.85 and 9.0. AC Her is an RVa star, meaning it is an RV Tauri variable whose maximum and minimum magnitudes do not slowly vary over hundreds of days. It also is a very clear example of a common type of RV Tauri light curve where the maximum following a deep minimum is brighter than the maximum following a shallow minimum. In each period of 75.46 days it has two maxima and two minima. AC Her is also a binary star, although the secondary can only be detected by its effect on the radial velocity of the primary. The invisible secondary is more massive than the supergiant primary, so the primary moves at relatively high velocity in its three years and three months orbit. The two stars are also surrounded by a dusty disc filling the region between 34 and 200

astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...

s (AU). Little is known of the secondary star except that its mass is around , deduced from the mass ratio of the binary system and the modelled mass of the primary star. The primary itself is calculated to have a mass of , but a luminosity of . It is slightly cooler than the sun, although this varies by over a thousand K as the star pulsates. The total system mass can be estimated from the dynamics of the disc, and this gives a value of , slightly lower than from other methods.

References

External links

AAVSO article
{{DEFAULTSORT:AC Herculis Hercules (constellation) Herculis, AC RV Tauri variables F-type supergiants 090697 170756 BD+21 3459 IRAS catalogue objects J18301623+2152007 Spectroscopic binaries