[7] Its companion (IK Pegasi B) is a massive white dwarf—a star that has evolved past the main sequence and is no longer generating energy through nuclear fusion.

When the primary begins to evolve into a red giant, it is expected to grow to a radius where the white dwarf can accrete matter from the expanded gaseous envelope.

The measurement of this shift allows astronomers to determine the relative orbital velocity of at least one of the stars even though they are unable to resolve the individual components.

[2] An attempt was made to photograph the individual components of this binary using the Hubble Space Telescope, but the stars proved too close to resolve.

IK Pegasi A is currently a main sequence star—a term that is used to describe a nearly linear grouping of core hydrogen-fusing stars based on their position on the HR diagram.

That is, this star's atmosphere displays slightly (but anomalously) higher than normal absorption line strengths for metallic isotopes.

For a star with a mass similar to IK Pegasi A (1.65 M☉), the expected lifetime on the main sequence is 2–3 × 109 years, which is about half the current age of the Sun.

[27] In terms of mass, the relatively young Altair is the nearest star to the Sun that is a stellar analogue of component A—it has an estimated 1.7 M☉.

This category of stellar object has reached the end of its evolutionary life span and is no longer generating energy through nuclear fusion.

Instead, under normal circumstances, a white dwarf will steadily radiate away its excess energy, mainly stored heat, growing cooler and dimmer over the course of many billions of years.

To compensate for the temperature increase, the outer envelope expanded to many times the radius it possessed as a main sequence star.

When helium was exhausted in the core a helium-burning shell formed in addition to the hydrogen-burning one and the star moved to what astronomers term the asymptotic giant branch, or AGB.

However, this double-shell phase is unstable, so it produced thermal pulses that caused large-scale mass ejections from the star's outer envelope.

All but a small fraction of the hydrogen envelope was driven away from the star, leaving behind a white dwarf remnant composed primarily of the inert core.

[37][38] In either case, the exterior of IK Pegasi B is covered by an atmosphere of almost pure hydrogen, which gives this star its stellar classification of DA.

[6] The entire mass of the star is supported by electron degeneracy pressure—a quantum mechanical effect that limits the amount of matter that can be squeezed into a given volume.

)[7] Thus this star packs a mass greater than the Sun into a volume roughly the size of the Earth, giving an indication of this object's extreme density.

[nb 5] The effective surface temperature of IK Pegasi B is estimated to be about 35,500 ± 1,500 K,[9] making it a strong source of ultraviolet radiation.

[6][nb 6] Under normal conditions this white dwarf would continue to cool for more than a billion years, while its radius would remain essentially unchanged.

At some point in the future, IK Pegasi A will consume the hydrogen fuel at its core and start to evolve away from the main sequence to form a red giant.

Once IK Pegasi A expands to the point where its outer envelope overflows the Roche lobe of its companion, a gaseous accretion disk will form around the white dwarf.

This would result in a (recurrent) nova explosion—a cataclysmic variable star—and the luminosity of the white dwarf would rapidly increase by several magnitudes for a period of several days or months.

RS Ophiuchi has flared into a (recurrent) nova on at least six occasions, each time accreting the critical mass of hydrogen needed to produce a runaway explosion.

[45] An alternate model that allows the white dwarf to steadily accumulate mass without erupting as a nova is called the close-binary supersoft x-ray source (CBSS).

[47] Should the white dwarf's mass approach the Chandrasekhar limit of 1.4 M☉ it will no longer be supported by electron degeneracy pressure and it will undergo a collapse.

[48] If the core is instead made of carbon-oxygen, however, increasing pressure and temperature will initiate carbon fusion in the center prior to attainment of the Chandrasekhar limit.

The dramatic result is a runaway nuclear fusion reaction that consumes a substantial fraction of the star within a short time.

[50] Following a supernova explosion, the remnant of the donor star (IK Pegasus A) would continue with the final velocity it possessed when it was a member of a close orbiting binary system.