IK Pegasi B: The Nearest Supernova Candidate

IK Pegasi is a binary star system in the constellation Pegasus. White dwarf IK Pegasi B, a massive star that is no longer generating energy through nuclear fusion, is the nearest known supernova candidate. When the primary evolves into a red giant, it will grow to a radius where the white dwarf can attract more matter from the expanded envelope. When the white dwarf approaches the limit of 1.44 solar masses, it is going to explode as a Type Ia supernova.


In IK Pegasi binary system, gas is being stripped away
from a giant star to form an accretion disc around
a compact companion (NASA image).

The primary is a main sequence star that displays minor pulsations in luminosity. It is categorized as a Delta Scuti variable star with a period of about an hour. Its companion is a massive white dwarf — a star that has evolved past the main sequence. They orbit each other every 21.7 days with a separation of about astronomical units. This is smaller than the orbit of Mercury around the Sun.

The distance to the IK Pegasi system can be measured directly by observing its parallax shifts against the distant stellar background as the Earth orbits around the Sun. This shift was measured to high precision by the Hipparcos spacecraft, and the distance was estimated as 150 light years. Hipparcos also measured the proper motion — the small angular motion of IK Pegasi across the sky because of its motion through space. The combination of the distance and proper motion of this system was used to compute the transverse velocity of IK Pegasi as 16.9 km/s.

The interior of IK Pegasi B may be composed wholly of carbon and oxygen, or alternatively, it may have a core of oxygen and neon, surrounded by a mantle enriched with carbon and oxygen. The exterior is covered by an atmosphere of almost pure hydrogen. Any helium in the envelope will have sunk beneath the hydrogen layer. 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.


A comparison between the IK Pegasi B (center), its companion
IK Pegasi A (left) and the Sun (right). (Credit: RJHall)

IK Pegasi B is considered to be a high-mass white dwarf, at an estimated 1.15 solar masses. Its radius can be estimated from known theoretical relationships between the mass and radius of white dwarfs, giving a value of about 0.60% of the Sun's radius. Thus this star packs a mass greater than the Sun into a volume roughly the size of the Earth. The massive, compact nature of a white dwarf produces a strong surface gravity — over 900,000 times the gravitational force on the Earth. The surface temperature is about 35,500K, making it a strong source of ultraviolet radiation. Under normal conditions this white dwarf would continue to cool for more than a billion years, while its radius would remain unchanged.

At some point in the future, IK Pegasi A will consume the hydrogen fuel at its core and form a red giant. The envelope of a red giant can extend up to a hundred times its previous radius. 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 mass transfer between the stars will also cause their mutual orbit to shrink. Should the white dwarf's mass approach the Chandrasekhar limit of 1.44 solar masses it will no longer be supported by electron degeneracy pressure and it will undergo a collapse. If the core is made of carbon-oxygen, 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. This will be sufficient to unbind the star in a cataclysmic, Type Ia supernova explosion.

A supernova would need to be within about 26 light years of the Earth to effectively destroy the Earth's ozone layer, which would severely impact the planet's biosphere. IK Pegasi system is not likely to pose a threat to life on the Earth, however. It is thought that the primary star is unlikely to evolve into a red giant in the immediate future. As shown previously, the space velocity of this star relative to the Sun is 20.4 km/s. This is equivalent to moving a distance of one light year every 14,700 years. After 5 million years, this star will be separated from the Sun by more than 500 light years. This is outside the radius where a Type Ia supernova is thought to be hazardous.


This video shows a thermonuclear flame burning its way through a white dwarf star. The flame produces hot ash, which buoyantly rises as the flame burns. The ash breaks out of but remains gravitationally bound to the surface of the star and collides at a point on the opposite side of the star from the breakout location. The blue shows the approximate surface of the star and the orange shows the interface between the star and the hot ash produced by the flame. (Credit: DOE NNSA ASC/Alliance Flash Center at the University of Chicago)