ER Ursae Majoris is a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

in the northern

circumpolar constellation A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets below the horizon due to its apparent proximity to one of the celestial poles. Circumpolar stars are therefore visible from said location toward the nearest po ...

Ursa Major Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...

, abbreviated ER UMa. It is a prototype system for a subclass of SU Ursae Majoris

dwarf nova A U Geminorum-type variable star, or dwarf nova (pl. novae) is one of several types of cataclysmic variable star, consisting of a close binary star system in which one of the components is a white dwarf that accretes matter from its companion. Dw ...

e. The system ranges in brightness from a peak

apparent visual 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 lig ...

of 12.4 down to 15.2, which is too faint to be visible to the naked eye. The distance to this system, based on

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...

measurements, is approximately 1,163

light year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s. This system was identified as an ultraviolet excess object as part of the Palomar-Green (PG) survey by R. F. Green and associate in 1986. It was given the catalog identifier PG 0943+521, and was confirmed to be

cataclysmic variable In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since ones with an outburst brightness visible to ...

. In 1992, it was determined this is a

that ranges in brightness from magnitude 12.3 down to 15.2. F. A. Ringwald in 1993 found a candidate

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

of 0.1997 days based on

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...

variation, but with some uncertainty. In 1995, T. Kato and C. Kunjaya confirmed this is a SU Ursae Majoris-type dwarf nova, and noted the unusual nature of this system, finding it has a long superoutburst lasting about 20 days and the supercycle (the time between superoutbursts) is very short at around 43 days. Large

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...

superhump In astronomy, a superhump is a periodic brightness variation in a cataclysmic variable star system, with a period within a few percent of the orbital period of the system. History Superhumps were first seen in SU Ursae Majoris (SU UMa) stars, a ...

s were found to occur near the start of a superoutburst, with a brightness increase of around 0.35 magnitude. The properties of the system suggest a high mass transfer rate and the

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

component is hotter than in other typical dwarf novae. During periods of quiescence, the accretion rate is ·yr⁻¹.

References

Further reading