IK Pegasi
Encyclopedia
IK Pegasi is a binary star
system
in the constellation
Pegasus
. It is just luminous enough to be seen with the unaided eye, at a distance of about 150 light years from the Solar System
.
The primary (IK Pegasi A) is a main sequence
, A-class star that displays minor pulsations in luminosity
. It is categorized as a Delta Scuti variable
star and it has a periodic cycle of luminosity variation that repeats itself about 22.9 times per day. 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
. They orbit each other every 21.7 days with an average separation of about 31 million kilometres, or 19 million miles, or 0.21 astronomical unit
s (AU). This is smaller than the orbit of Mercury
around the Sun
.
IK Pegasi B is the nearest known supernova
progenitor candidate. 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. When the white dwarf approaches the Chandrasekhar limit
of 1.44 solar mass
es, it may explode as a Type Ia supernova
.
("Bonn astrometric Survey") as BD +18°4794B. It later appeared in Pickering's
1908 Harvard Revised Photometry Catalogue
as HR 8210. The designation "IK Pegasi" follows the expanded form of the variable star nomenclature
introduced by Friedrich W. Argelander
.
Examination of the spectrograph
ic features of this star showed the characteristic absorption line shift of a binary star system. This shift is created when their orbit carries the member stars toward and then away from the observer, producing a doppler shift
in the wavelength of the line features. 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.
In 1927, the Canadian astronomer William E. Harper
used this technique to measure the period of this single-line spectroscopic binary and determined it to be 21.724 days. He also initially estimated the orbital eccentricity
as 0.027. (Later estimates gave an eccentricity of essentially zero, which is the value for a circular orbit.) The velocity amplitude was measured as 41.5 km/s, which is the maximum velocity of the primary component along the line of sight to the Solar System.
The distance to the IK Pegasi system can be measured directly by observing the tiny parallax
shifts of this system (against the more distant stellar background) as the Earth
orbits around the Sun. This shift was measured to high precision by the Hipparcos
spacecraft, yielding a distance estimate of 150 light years (with an accuracy of ±5 light years). The same spacecraft also measured the proper motion
of this system. This is 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 can be used to compute the transverse velocity of IK Pegasi as 16.9 km/s. The third component, the heliocentric radial velocity
, can be measured by the average red-shift (or blue-shift) of the stellar spectrum. The General Catalogue of Stellar Radial Velocities lists a radial velocity of -11.4 km/s for this system. The combination of these two motions gives a space velocity of 20.4 km/s relative to the Sun.
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. Recent measurements with the Extreme Ultraviolet Explorer
space telescope gave a more accurate orbital period of . The inclination
of this system's orbital plane
is believed to be nearly edge-on (90°) as seen from the Earth. If so it may be possible to observe an eclipse
.
versus a color index
for a set of stars. 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. However, IK Pegasi A lies in a narrow, nearly vertical band of the HR diagram that is known as the instability strip
. Stars in this band oscillate in a coherent manner, resulting in periodic pulsations in the star's luminosity.
The pulsations result from a process called the κ-mechanism. A part of the star's outer atmosphere
becomes optically thick due to partial ionization
of certain elements. When these atoms lose an electron
, the likelihood that they will absorb energy increases. This results in an increase in temperature that causes the atmosphere to expand. The inflated atmosphere becomes less ionized and loses energy, causing it to cool and shrink back down again. The result of this cycle is a periodic pulsation of the atmosphere and a matching variation of the luminosity.
Stars within the portion of the instability strip that crosses the main sequence are called Delta Scuti variable
s. These are named after the prototypical star for such variables: Delta Scuti
. Delta Scuti variables typically range from spectral class A2 to F8, and a stellar luminosity class of III (subgiants) to V (main sequence
stars). They are short-period variables that have a regular pulsation rate between 0.025 and 0.25 days. Delta Scuti stars have an abundance of elements similar to the Sun's (see Population I stars) and between 1.5 and 2.5 solar mass
es. The pulsation rate of IK Pegasi A has been measured at 22.9 cycles per day, or once every 0.044 days.
Astronomers define the metallicity
of a star as the abundance of chemical element
s that have a higher atomic number
than helium. This is measured by a spectroscopic analysis of the atmosphere, followed by a comparison with the results expected from computed stellar models. In the case of IK Pegasus A, the estimated metal abundance is [M/H] = +0.07 ± 0.20. This notation gives the logarithm
of the ratio of metal elements (M) to hydrogen (H), minus the logarithm of the Sun's metal ratio. (Thus if the star matches the metal abundance of the Sun, this value will be zero.) A logarithmic value of 0.07 is equivalent to an actual metallicity ratio of 1.17, so the star is about 17% richer in metallic elements than the Sun. However the margin of error for this result is relatively large.
The spectrum of A-class stars such as IK Pegasi A show strong Balmer lines of hydrogen along with absorption lines of ionized metals, including the K line of ionized calcium
(Ca II) at a wavelength of 393.3 nm
. The spectrum of IK Pegasi A is classified as marginal Am (or "Am:"), which means it displays the characteristics of a spectral class A but is marginally metallic-lined. That is, this star's atmosphere displays slightly (but anomalously) higher than normal absorption line strengths for metallic isotopes. Stars of spectral type Am are often members of close binaries with a companion of about the same mass, as is the case for IK Pegasi.
Spectral class-A stars are hotter and more massive than the Sun. But, in consequence, their life span on the main sequence is correspondingly shorter. For a star with a mass similar to IK Pegasi A (1.65 solar), the expected lifetime on the main sequence is 2–3, which is about half the current age of the Sun.
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 times the solar mass
. The binary system as a whole has some similarities to the nearby system of Sirius
, which has a class-A primary and a white dwarf companion. However, Sirius A is more massive than IK Pegasi A and the orbit of its companion is much larger, with a semimajor axis of 20 A.U.
star. 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.
es) will end up as white dwarfs once they have exhausted their supply of thermonuclear fuel. Such stars spend most of their energy-producing life span as a main sequence
star. The amount of time that a star spends on the main sequence depends primarily on its mass, with the lifespan decreasing with increasing mass. Thus, for IK Pegasi B to have become a white dwarf before component A, it must once have been more massive than component A. In fact, the progenitor of IK Pegasi B is thought to have had a mass between 5 and 8 solar mass
es.
As the hydrogen fuel at the core of the progenitor of IK Pegasi B was consumed, it evolved into a red giant
. The inner core contracted until hydrogen burning commenced in a shell surrounding the helium core. To compensate for the temperature increase, the outer envelope expanded to many times the radius it possessed as a main sequence star. When the core reached a temperature and density where helium could start to undergo fusion this star contracted and became what is termed a horizontal branch
star. That is, it belonged to a group of stars that fall upon a roughly horizontal line on the H-R diagram. The fusion of helium formed an inert core of carbon and oxygen. 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. (This is a track leading to the upper-right corner of the H-R diagram.) If the star had sufficient mass, in time carbon fusion
could begin in the core, producing oxygen
, neon
and magnesium
.
The outer envelope of a red giant or AGB star can expand to several hundred times the radius of the Sun, occupying a radius of about (3 A.U.) in the case of the pulsating AGB star Mira
. This is well beyond the current average separation between the two stars in IK Pegasi, so during this time period the two stars shared a common envelope. As a result, the outer atmosphere of IK Pegasi A may have received an isotope enhancement.
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Some time after an inert oxygen-carbon (or oxygen-magnesium-neon) core formed, thermonuclear fusion began to occur along two shells concentric with the core region; hydrogen was burned along the outermost shell, while helium fusion took place around the inert core. However, this double-shell phase is unstable, so it produced thermal pulses that caused large-scale mass ejections from the star's outer envelope. This ejected material formed an immense cloud of material called a planetary nebula
. 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.
of DA. Due to higher atomic mass
, 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.
At an estimated 1.15 solar masses, IK Pegasi B is considered to be a high-mass white dwarf. Although its radius has not been observed directly, it 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. (A different source gives a value of 0.72%, so there remains some uncertainty in this result.) 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
.
The massive, compact nature of a white dwarf produces a strong surface gravity. Astronomers denote this value by the decimal logarithm
of the gravitational force in cgs units, or log g. For IK Pegasi B, log g is 8.95. By comparison, log g for the Earth is 2.99. Thus the surface gravity on IK Pegasi is over 900,000 times the gravitational force on the Earth.
The effective surface temperature of IK Pegasi B is estimated to be about , 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 essentially unchanged.
or a cataclysmic variable
. At a distance of 150 light years, this makes it the nearest known candidate supernova progenitor to the Earth
. However in the time it will take for the system to evolve to a state where a supernova could occur, it will have moved a considerable distance from Earth but may yet pose a threat.
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. The envelope of a red giant can grow to significant dimensions, extending up to a hundred times its previous radius (or larger). 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 gas, composed primarily of hydrogen and helium, will then accrete onto the surface of the companion. This mass transfer between the stars will also cause their mutual orbit to shrink.
On the surface of the white dwarf, the accreted gas will become compressed and heated. At some point the accumulated gas can reach the conditions necessary for hydrogen fusion to occur, producing a runaway
reaction that will drive a portion of the gas from the surface. This would result in a (recurrent) nova
explosion—a cataclysmic variable star—and the luminosity of the white dwarf rapidly would increase by several magnitudes
for a period of several days or months. An example of such a star system is RS Ophiuchi; a binary system consisting of a red giant and a white dwarf companion. 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.
It is possible that IK Pegasi B will follow a similar pattern. In order to accumulate mass, however, only a portion of the accreted gas can be ejected, so that with each cycle the white dwarf would steadily increase in mass. Thus, even should it behave as a recurring nova, IK Pegasus B could continue to accumulate a growing envelope.
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). In this scenario, the mass transfer rate to the close white dwarf binary is such that a steady fusion burn can be maintained on the surface as the arriving hydrogen is consumed in thermonuclear fusion to produce helium. This category of super-soft sources consist of high-mass white dwarfs with very high surface temperatures ( to ).
Should the white dwarf's mass approach the Chandrasekhar limit
of 1.44 solar mass
es it will no longer be supported by electron degeneracy pressure
and it will undergo a collapse. For a core primarily composed of oxygen, neon and magnesium, the collapsing white dwarf is likely to form a neutron star
. In this case, only a fraction of star's mass will be ejected as a result. 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. This will be sufficient to unbind the star in a cataclysmic, Type Ia supernova explosion.
Such a supernova event may pose some threat to life on the Earth. It is thought that the primary star, IK Pegasi A, 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, for example, this star will be separated from the Sun by more than 500 light years. A Type Ia supernova within a thousand parsecs (3300 light-years) is thought to be able to affect the Earth, but it must be closer than ∼10 parsecs (about thirty light-years) to cause a major harm to the terrestrial biosphere.
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. The resulting relative velocity could be as high as 100–200 km/s, which would place it among the high-velocity members of the galaxy
. The companion will also have lost some mass during the explosion, and its presence may create a gap in the expanding debris. From that point forward it will evolve into a single white dwarf star. The supernova explosion will create a remnant
of expanding material that will eventually merge with the surrounding interstellar medium
.
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
system
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...
in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Pegasus
Pegasus (constellation)
Pegasus is a constellation in the northern sky, named after the winged horse Pegasus in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:...
. It is just luminous enough to be seen with the unaided eye, at a distance of about 150 light years from the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
.
The primary (IK Pegasi A) is a main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
, A-class star that displays minor pulsations in luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
. It is categorized as a Delta Scuti variable
Delta Scuti variable
A Delta Scuti variable is a variable star which exhibits variations in its luminosity due to both radial and non-radial pulsations of the star's surface. Typical brightness fluctuations are from 0.003 to 0.9 magnitudes in V over a period of a few hours, although the amplitude and period of the...
star and it has a periodic cycle of luminosity variation that repeats itself about 22.9 times per day. Its companion (IK Pegasi B) is a massive white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
—a star that has evolved past the main sequence and is no longer generating energy through nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
. They orbit each other every 21.7 days with an average separation of about 31 million kilometres, or 19 million miles, or 0.21 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s (AU). This is smaller than the orbit of Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
around the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
IK Pegasi B is the nearest known supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
progenitor candidate. When the primary begins to evolve into a red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
, it is expected to grow to a radius where the white dwarf can accrete
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
matter from the expanded gaseous envelope. When the white dwarf approaches the Chandrasekhar limit
Chandrasekhar limit
When a star starts running out of fuel, it usually cools off and collapses into one of three compact forms, depending on its total mass:* a White Dwarf, a big lump of Carbon and Oxygen atoms, almost like one huge molecule...
of 1.44 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es, it may explode as a Type Ia supernova
Type Ia supernova
A Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...
.
Observation
This star system was catalogued in the 1862 Bonner DurchmusterungDurchmusterung
In astronomy, Durchmusterung or Bonner Durchmusterung , is the usual name for three comprehensive astrometric star catalogues of the whole sky, processed by the Bonn Observatory from 1859 to 1903....
("Bonn astrometric Survey") as BD +18°4794B. It later appeared in Pickering's
Edward Charles Pickering
Edward Charles Pickering was an American astronomer and physicist, brother of William Henry Pickering.Along with Carl Vogel, Pickering discovered the first spectroscopic binary stars. He wrote Elements of Physical Manipulations .Pickering attended Boston Latin School, and received his B.S. from...
1908 Harvard Revised Photometry Catalogue
Bright Star Catalogue
The Bright Star Catalogue, also known as the Yale Catalogue of Bright Stars or Yale Bright Star Catalogue, is a star catalogue that lists all stars of stellar magnitude 6.5 or brighter, which is roughly every star visible to the naked eye from Earth. It is currently available online in its 5th...
as HR 8210. The designation "IK Pegasi" follows the expanded form of the variable star nomenclature
Variable star designation
Variable stars are named using a variation on the Bayer designation format of an identifying label combined with the Latin genitive of the name of the constellation in which the star lies...
introduced by Friedrich W. Argelander
Friedrich Wilhelm Argelander
Friedrich Wilhelm August Argelander was a German astronomer. He is known for his determinations of stellar brightnesses, positions, and distances.- Life and work :...
.
Examination of the spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
ic features of this star showed the characteristic absorption line shift of a binary star system. This shift is created when their orbit carries the member stars toward and then away from the observer, producing a doppler shift
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
in the wavelength of the line features. 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.
In 1927, the Canadian astronomer William E. Harper
William Edmund Harper
William Edmund Harper, March 20, 1878–June 14, 1940, was a Canadian astronomer.William Harper was born in Dobbington, Ontario. He attended high school in Owen Sound, then taught for three years following his graduation...
used this technique to measure the period of this single-line spectroscopic binary and determined it to be 21.724 days. He also initially estimated the orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
as 0.027. (Later estimates gave an eccentricity of essentially zero, which is the value for a circular orbit.) The velocity amplitude was measured as 41.5 km/s, which is the maximum velocity of the primary component along the line of sight to the Solar System.
The distance to the IK Pegasi system can be measured directly by observing the tiny parallax
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. The term is derived from the Greek παράλλαξις , meaning "alteration"...
shifts of this system (against the more distant stellar background) as the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
orbits around the Sun. This shift was measured to high precision by the Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
spacecraft, yielding a distance estimate of 150 light years (with an accuracy of ±5 light years). The same spacecraft also measured the proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
of this system. This is 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 can be used to compute the transverse velocity of IK Pegasi as 16.9 km/s. The third component, the heliocentric radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
, can be measured by the average red-shift (or blue-shift) of the stellar spectrum. The General Catalogue of Stellar Radial Velocities lists a radial velocity of -11.4 km/s for this system. The combination of these two motions gives a space velocity of 20.4 km/s relative to the Sun.
An attempt was made to photograph the individual components of this binary using the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, but the stars proved too close to resolve. Recent measurements with the Extreme Ultraviolet Explorer
Extreme Ultraviolet Explorer
The Extreme Ultraviolet Explorer was a space telescope for ultraviolet astronomy, launched on June 7, 1992. With instruments for UV radiation between wavelengths of 7 and 76 nm, the EUVE was the first satellite mission especially for the short-wave ultraviolet range...
space telescope gave a more accurate orbital period of . The inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
of this system's orbital plane
Orbital plane (astronomy)
All of the planets, comets, and asteroids in the solar system are in orbit around the Sun. All of those orbits line up with each other making a semi-flat disk called the orbital plane. The orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded...
is believed to be nearly edge-on (90°) as seen from the Earth. If so it may be possible to observe an eclipse
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
.
IK Pegasi A
The Hertzsprung-Russell diagram (HR diagram) is a plot of luminosityLuminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
versus a color index
Color index
In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature...
for a set of stars. IK Pegasi A is currently a main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
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. However, IK Pegasi A lies in a narrow, nearly vertical band of the HR diagram that is known as the instability strip
Instability strip
The Instability strip is a nearly vertical region in the Hertzsprung–Russell diagram which is occupied by pulsating variable stars .The instability strip intersects the main sequence in the region of A...
. Stars in this band oscillate in a coherent manner, resulting in periodic pulsations in the star's luminosity.
The pulsations result from a process called the κ-mechanism. A part of the star's outer atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
becomes optically thick due to partial ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
of certain elements. When these atoms lose an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
, the likelihood that they will absorb energy increases. This results in an increase in temperature that causes the atmosphere to expand. The inflated atmosphere becomes less ionized and loses energy, causing it to cool and shrink back down again. The result of this cycle is a periodic pulsation of the atmosphere and a matching variation of the luminosity.
Delta Scuti variable
A Delta Scuti variable is a variable star which exhibits variations in its luminosity due to both radial and non-radial pulsations of the star's surface. Typical brightness fluctuations are from 0.003 to 0.9 magnitudes in V over a period of a few hours, although the amplitude and period of the...
s. These are named after the prototypical star for such variables: Delta Scuti
Delta Scuti
Delta Scuti is a white, F-type giant star in the constellation Scutum. It is approximately 187 light years from Earth. Delta Scuti is the prototype of the Delta Scuti type variable stars. It is a high-amplitude δ Scuti type pulsator with light variations of about 0.15 minutes...
. Delta Scuti variables typically range from spectral class A2 to F8, and a stellar luminosity class of III (subgiants) to V (main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
stars). They are short-period variables that have a regular pulsation rate between 0.025 and 0.25 days. Delta Scuti stars have an abundance of elements similar to the Sun's (see Population I stars) and between 1.5 and 2.5 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es. The pulsation rate of IK Pegasi A has been measured at 22.9 cycles per day, or once every 0.044 days.
Astronomers define the metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
of a star as the abundance of chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s that have a higher atomic number
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
than helium. This is measured by a spectroscopic analysis of the atmosphere, followed by a comparison with the results expected from computed stellar models. In the case of IK Pegasus A, the estimated metal abundance is [M/H] = +0.07 ± 0.20. This notation gives the logarithm
Logarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...
of the ratio of metal elements (M) to hydrogen (H), minus the logarithm of the Sun's metal ratio. (Thus if the star matches the metal abundance of the Sun, this value will be zero.) A logarithmic value of 0.07 is equivalent to an actual metallicity ratio of 1.17, so the star is about 17% richer in metallic elements than the Sun. However the margin of error for this result is relatively large.
The spectrum of A-class stars such as IK Pegasi A show strong Balmer lines of hydrogen along with absorption lines of ionized metals, including the K line of ionized calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
(Ca II) at a wavelength of 393.3 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
. The spectrum of IK Pegasi A is classified as marginal Am (or "Am:"), which means it displays the characteristics of a spectral class A but is marginally metallic-lined. That is, this star's atmosphere displays slightly (but anomalously) higher than normal absorption line strengths for metallic isotopes. Stars of spectral type Am are often members of close binaries with a companion of about the same mass, as is the case for IK Pegasi.
Spectral class-A stars are hotter and more massive than the Sun. But, in consequence, their life span on the main sequence is correspondingly shorter. For a star with a mass similar to IK Pegasi A (1.65 solar), the expected lifetime on the main sequence is 2–3, which is about half the current age of the Sun.
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 times the solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
. The binary system as a whole has some similarities to the nearby system of Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
, which has a class-A primary and a white dwarf companion. However, Sirius A is more massive than IK Pegasi A and the orbit of its companion is much larger, with a semimajor axis of 20 A.U.
IK Pegasi B
The companion star is a dense white dwarfWhite dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
star. This category of stellar object has reached the end of its evolutionary life span and is no longer generating energy through nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
. 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.
Evolution
Nearly all small and intermediate-mass stars (below about nine solar massSolar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es) will end up as white dwarfs once they have exhausted their supply of thermonuclear fuel. Such stars spend most of their energy-producing life span as a main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
star. The amount of time that a star spends on the main sequence depends primarily on its mass, with the lifespan decreasing with increasing mass. Thus, for IK Pegasi B to have become a white dwarf before component A, it must once have been more massive than component A. In fact, the progenitor of IK Pegasi B is thought to have had a mass between 5 and 8 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es.
As the hydrogen fuel at the core of the progenitor of IK Pegasi B was consumed, it evolved into a red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
. The inner core contracted until hydrogen burning commenced in a shell surrounding the helium core. To compensate for the temperature increase, the outer envelope expanded to many times the radius it possessed as a main sequence star. When the core reached a temperature and density where helium could start to undergo fusion this star contracted and became what is termed a horizontal branch
Horizontal branch
The horizontal branch is a stage of stellar evolution which immediately follows the red giant branch in stars whose masses are similar to the Sun's...
star. That is, it belonged to a group of stars that fall upon a roughly horizontal line on the H-R diagram. The fusion of helium formed an inert core of carbon and oxygen. 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
Asymptotic Giant Branch
The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....
, or AGB. (This is a track leading to the upper-right corner of the H-R diagram.) If the star had sufficient mass, in time carbon fusion
Carbon burning process
The carbon-burning process or carbon fusion is a set of nuclear fusion reactions that take place in massive stars that have used up the lighter elements in their cores...
could begin in the core, producing oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
.
The outer envelope of a red giant or AGB star can expand to several hundred times the radius of the Sun, occupying a radius of about (3 A.U.) in the case of the pulsating AGB star Mira
Mira
Mira also known as Omicron Ceti , is a red giant star estimated 200-400 light years away in the constellation Cetus. Mira is a binary star, consisting of the red giant Mira A along with Mira B. Mira A is also an oscillating variable star and was the first non-supernova variable star discovered,...
. This is well beyond the current average separation between the two stars in IK Pegasi, so during this time period the two stars shared a common envelope. As a result, the outer atmosphere of IK Pegasi A may have received an isotope enhancement.
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
. 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.
Composition and structure
The interior of IK Pegasi B may be composed wholly of carbon and oxygen; alternatively, if its progenitor underwent carbon burning, it may have a core of oxygen and neon, surrounded by a mantle enriched with carbon and oxygen. In either case, the exterior of IK Pegasi B is covered by an atmosphere of almost pure hydrogen, which gives this star its stellar classificationStellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
of DA. Due to higher atomic mass
Atomic mass
The atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....
, any helium in the envelope will have sunk beneath the hydrogen layer. The entire mass of the star is supported by electron degeneracy pressure
Electron degeneracy pressure
Electron degeneracy pressure is a particular manifestation of the more general phenomenon of quantum degeneracy pressure. The Pauli Exclusion Principle disallows two half integer spin particles from occupying the same quantum state at a given time. The resulting emergent repulsive force is...
—a quantum mechanical
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
effect that limits the amount of matter that can be squeezed into a given volume.
At an estimated 1.15 solar masses, IK Pegasi B is considered to be a high-mass white dwarf. Although its radius has not been observed directly, it 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. (A different source gives a value of 0.72%, so there remains some uncertainty in this result.) 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
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
.
The massive, compact nature of a white dwarf produces a strong surface gravity. Astronomers denote this value by the decimal logarithm
Logarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...
of the gravitational force in cgs units, or log g. For IK Pegasi B, log g is 8.95. By comparison, log g for the Earth is 2.99. Thus the surface gravity on IK Pegasi is over 900,000 times the gravitational force on the Earth.
The effective surface temperature of IK Pegasi B is estimated to be about , making it a strong source of ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation. Under normal conditions this white dwarf would continue to cool for more than a billion years, while its radius would remain essentially unchanged.
Future evolution
In a 1993 paper, David Wonnacott, Barry J. Kellett and David J. Stickland identified this system as a candidate to evolve into a Type Ia supernovaType Ia supernova
A Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...
or a cataclysmic variable
Cataclysmic variable star
Cataclysmic variable stars are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state...
. At a distance of 150 light years, this makes it the nearest known candidate supernova progenitor to the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. However in the time it will take for the system to evolve to a state where a supernova could occur, it will have moved a considerable distance from Earth but may yet pose a threat.
Roche lobe
The Roche lobe is the region of space around a star in a binary system within which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material can escape the gravitational pull of the star. If the star is in a binary system then the material...
of its companion, a gaseous accretion disk will form around the white dwarf. This gas, composed primarily of hydrogen and helium, will then accrete onto the surface of the companion. This mass transfer between the stars will also cause their mutual orbit to shrink.
On the surface of the white dwarf, the accreted gas will become compressed and heated. At some point the accumulated gas can reach the conditions necessary for hydrogen fusion to occur, producing a runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...
reaction that will drive a portion of the gas from the surface. This would result in a (recurrent) nova
Nova
A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner...
explosion—a cataclysmic variable star—and the luminosity of the white dwarf rapidly would increase by several magnitudes
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
for a period of several days or months. An example of such a star system is RS Ophiuchi; a binary system consisting of a red giant and a white dwarf companion. 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.
It is possible that IK Pegasi B will follow a similar pattern. In order to accumulate mass, however, only a portion of the accreted gas can be ejected, so that with each cycle the white dwarf would steadily increase in mass. Thus, even should it behave as a recurring nova, IK Pegasus B could continue to accumulate a growing envelope.
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). In this scenario, the mass transfer rate to the close white dwarf binary is such that a steady fusion burn can be maintained on the surface as the arriving hydrogen is consumed in thermonuclear fusion to produce helium. This category of super-soft sources consist of high-mass white dwarfs with very high surface temperatures ( to ).
Should the white dwarf's mass approach the Chandrasekhar limit
Chandrasekhar limit
When a star starts running out of fuel, it usually cools off and collapses into one of three compact forms, depending on its total mass:* a White Dwarf, a big lump of Carbon and Oxygen atoms, almost like one huge molecule...
of 1.44 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es it will no longer be supported by electron degeneracy pressure
Electron degeneracy pressure
Electron degeneracy pressure is a particular manifestation of the more general phenomenon of quantum degeneracy pressure. The Pauli Exclusion Principle disallows two half integer spin particles from occupying the same quantum state at a given time. The resulting emergent repulsive force is...
and it will undergo a collapse. For a core primarily composed of oxygen, neon and magnesium, the collapsing white dwarf is likely to form a neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
. In this case, only a fraction of star's mass will be ejected as a result. 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. This will be sufficient to unbind the star in a cataclysmic, Type Ia supernova explosion.
Such a supernova event may pose some threat to life on the Earth. It is thought that the primary star, IK Pegasi A, 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, for example, this star will be separated from the Sun by more than 500 light years. A Type Ia supernova within a thousand parsecs (3300 light-years) is thought to be able to affect the Earth, but it must be closer than ∼10 parsecs (about thirty light-years) to cause a major harm to the terrestrial biosphere.
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. The resulting relative velocity could be as high as 100–200 km/s, which would place it among the high-velocity members of the galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
. The companion will also have lost some mass during the explosion, and its presence may create a gap in the expanding debris. From that point forward it will evolve into a single white dwarf star. The supernova explosion will create a remnant
Supernova remnant
A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...
of expanding material that will eventually merge with the surrounding interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
.