IK Pegasi

Starbox begin
name=IK Pegasi Starbox image


caption=Location of IK Pegasi. Starbox observe
epoch=J2000
ra=RA|21|26|26.6624cite web
title=SIMBAD Query Result: HD 204188 -- Spectroscopic binary
publisher=Centre de Données astronomiques de Strasbourg
date=June 10, 2007 | work=SIMBAD
url=http://simbad3.u-strasbg.fr/sim-id.pl?protocol=html&Ident=HD+204188
accessdate=2006-05-24 — "Note:" some results were queried via the "Display all measurements" function on the web page.]
dec=DEC| +19|22|32.304
appmag_v=6.078
constell=Pegasus Starbox character
class=A8m:cite journal
last = Kurtz | first = D. W.
title=Metallicism and pulsation - The marginal metallic line stars
journal=Astrophysical Journal
year=1978 | volume=221 | pages=869–880
url=http://adsabs.harvard.edu/abs/1978ApJ...221..869K
accessdate=2007-05-14
doi=10.1086/156090 ] /DA
b-v=0.24/–
u-b=0.03/–
variable=Delta Scuti Starbox astrometry
radial_v=-11.4
prop_mo_ra=80.23
prop_mo_dec=17.28
parallax=21.72
p_error=0.78
parallax_footnote=
absmag_v=2.762Ref_label|A|a|none Starbox detail
mass=1.65cite journal
author=D. Wonnacott, B. J. Kellett, B. Smalley, C. Lloyd
title=Pulsational Activity on Ik-Pegasi
journal=Monthly Notices of the Royal Astronomical Society
year=1994 | volume=267 | issue=4 | pages=1045–1052
url=http://adsabs.harvard.edu/abs/1994MNRAS.267.1045W
accessdate=2007-04-14 ] /1.15cite journal
author=Landsman, W.; Simon, T.; Bergeron, P.
title=The hot white-dwarf companions of HR 1608, HR 8210, and HD 15638
journal=Publications of the Astronomical Society of the Pacific
year=1999 | volume=105 | issue=690 | pages=841–847
url=http://adsabs.harvard.edu/abs/1993PASP..105..841L
accessdate=2007-02-04
doi=10.1086/133242 ]
radius=1.6/0.006cite journal
author=Barstow, M. A.; Holberg, J. B.; Koester, D.
title=Extreme Ultraviolet Spectrophotometry of HD16538 and HR:8210 Ik-Pegasi
journal=Monthly Notices of the Royal Astronomical Society
year=1994
volume=270
issue=3
pages=516
url=http://adsabs.harvard.edu/abs/1994MNRAS.270..516B
accessdate=2007-05-15 ]
luminosity=8.0/0.12Ref_label|B|b|none
temperature=7,700cite journal
author=B. Smalley, K. C. Smith, D. Wonnacott, C. S. Allen
title=The chemical composition of IK Pegasi
journal=Monthly Notices of the Royal Astronomical Society
year=1996 | volume=278 | issue=3 | pages=688–696
url=http://adsabs.harvard.edu/abs/1996MNRAS.278..688S ] /35,500
metal=117/– % Sun
rotation=< 32.5/&ndash; km/s
gravity=4.25/8.95
age=5&ndash;60 × 10⁷ Starbox catalog
names=AB: V* IK Peg, HR 8210, BD +18°4794, HD 204188, SAO 107138, HIP 105860.
B: WD 2124+191, EUVE J2126+193. [cite journal
last = Vallerga | first = John
title=The Stellar Extreme-Ultraviolet Radiation Field
journal=Astrophysical Journal | year=1998 | volume=497
pages=77–115
url=http://adsabs.harvard.edu/abs/1998ApJ...497..921V
accessdate=2007-06-10
doi=10.1086/305496 ]

IK Pegasi (or HR 8210) is a binary star system in the constellation Pegasus. A distance of about 150 light years from the Solar System, it is just luminous enough to be seen with the unaided eye.

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 0.21 astronomical units (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 masses, it may explode as a Type Ia supernova.cite journal
author=Wonnacott, D.; Kellett, B. J.; Stickland, D. J.
title=IK Peg - A nearby, short-period, Sirius-like system
journal=Monthly Notices of the Royal Astronomical Society
year=1993
volume=262
issue=2
pages=277–284
url=http://adsabs.harvard.edu/abs/1993MNRAS.262..277W
accessdate=2007-05-15 ]

Observation

This star system was catalogued in the 1862 "Bonn Durchmusterung" ("Bonn astrometric Survey") as BD +18°4794B. It later appeared in Pickering's 1908 "Harvard Revised Photometry Catalogue" as HR 8210. [cite journal
last = Pickering | first = Edward Charles
title=Revised Harvard photometry : a catalogue of the positions, photometric magnitudes and spectra of 9110 stars, mainly of the magnitude 6.50, and brighter observed with the 2 and 4 inch meridian photometers
journal=Annals of the Astronomical Observatory of Harvard College
year=1908 | volume=50 | pages=182
url=http://adsabs.harvard.edu/abs/1908AnHar..50....1P
accessdate=2007-05-14 ] The designation "IK Pegasi" follows the expanded form of the variable star nomenclature introduced by Friedrich W. Argelander.

Examination of the spectrographic 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. [cite web
author=Staff
url=http://csep10.phys.utk.edu/astr162/lect/binaries/spectroscopic.html
title=Spectroscopic Binaries
publisher =University of Tennessee
accessdate=2007-06-09 ]

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. [cite journal
last = Harper | first = W. E.
title=The orbits of A Persei and HR 8210
journal=Publications of the Dominion Astrophysical Observatory
year=1927 | volume=4 | pages=161–169
url=http://adsabs.harvard.edu/abs/1927PDAO....4..161H
accessdate=2007-05-14 ]

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). [cite journal
author=M. A. C. Perryman, L. Lindegren, J. Kovalevsky, E. Hoeg, U. Bastian, P. L. Bernacca, M. Crézé, F. Donati, M. Grenon, F. van Leeuwen, H. van der Marel, F. Mignard, C. A. Murray, R. S. Le Poole, H. Schrijver, C. Turon, F. Arenou, M. Froeschlé, C. S. Petersen
title=The HIPPARCOS Catalogue
journal=Astronomy and Astrophysics | year=1997 | volume=323
pages=L49–L52
url=http://adsabs.harvard.edu/cgi-bin/bib_query?1997A&A...323L..49P
accessdate=2007-05-14 ] 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.Ref_label|C|c|none 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. [cite book
last=Wilson | first=Ralph Elmer | year=1953
title=General catalogue of stellar radial velocities
publisher=Carnegie Institution of Washington
url=http://adsabs.harvard.edu/abs/1953QB901.W495.....
accessdate=2007-05-14 ] The combination of these two motions gives a space velocity of 20.4 km/s relative to the Sun.Ref_label|D|d|none

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. [cite conference
author=Burleigh, M. R.; Barstow, M. A.; Bond, H. E.; Holberg, J. B.
editor=Provencal, J. L.; Shipman, H. L.; MacDonald, J.; Goodchild, S.
title = Resolving Sirius-like Binaries with the Hubble Space Telescope
booktitle = Proceedings 12th European Workshop on White Dwarfs
pages = 222 | publisher = Astronomy Society of the Pacific
date = July 28–August 1, 1975 | location = San Francisco
url = http://adsabs.harvard.edu/abs/2001ASPC..226..222B
accessdate = 2007-02-27 | id = ISBN 1-58381-058-7 ] Recent measurements with the Extreme Ultraviolet Explorer space telescope gave a more accurate orbital period of 21.72168 ± 0.00009 days.cite journal
author=Vennes, S.; Christian, D. J.; Thorstensen, J. R.
title=Hot White Dwarfs in the Extreme-Ultraviolet Explorer Survey. IV. DA White Dwarfs with Bright Companions
journal=The Astrophysical Journal
year=1998
volume=502
issue=2
pages=763–787
url=http://www.journals.uchicago.edu/doi/full/10.1086/305926
accessdate=2008-09-16
doi=10.1086/305926 ] 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.

IK Pegasi A

The Hertzsprung-Russell diagram (HR diagram) is a plot of luminosity versus a color index for a set of stars. IK Pegasi A is currently a main sequence star&mdash;a term 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.cite journal
author=A. Gautschy, H. Saio
title=Stellar Pulsations Across The HR Diagram: Part 1
journal=Annual Review of Astronomy and Astrophysics
year=1995 | volume=33 | pages=75–114
url=http://adsabs.harvard.edu/abs/1995ARA&A..33...75G
accessdate=2007-05-14
doi=10.1146/annurev.aa.33.090195.000451 ]

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.

right|320px|thumb|The relative dimensions of IK Pegasi A (left), B (lower center) and the Sun (right). [">For an explanation of the star colors, see: cite web
date=December 21 2004
url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html
title=The Colour of Stars
publisher=Australia Telescope Outreach and Education
accessdate=2007-09-26 ] Stars within the portion of the instability strip that crosses the main sequence are called Delta Scuti variables. 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 masses. [cite web
last = Templeton | first = Matthew | year = 2004
url = http://www.aavso.org/vstar/vsots/summer04.shtml
title = Variable Star of the Season: Delta Scuti and the Delta Scuti variables
publisher = AAVSO | accessdate = 2007-01-23 ] 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 elements 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. [cite web
last=Smith | first=Gene | date=April 16, 1999
url=http://casswww.ucsd.edu/public/tutorial/Stars.html
title=Stellar Spectra
publisher=University of California, San Diego Center for Astrophysics & Space Sciences
accessdate=2007-05-19 ] 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. [cite journal
author=J. G. Mayer, J. Hakkila
title=Photometric Effects of Binarity on AM Star Broadband Colors
journal=Bulletin of the American Astronomical Society
year=1994 | volume=26 | pages=868
url=http://adsabs.harvard.edu/abs/1994AAS...184.0607M
accessdate=2007-05-14 ]

Spectral class-A stars are hotter and more massive than the Sun. But, in consequence, their life span on the main sequence is correspondingly brief. For a star with a mass similar to IK Pegasi A (1.65 solar), the expected lifetime on the main sequence is 2&ndash;3 × 10⁹ years, which is about half the current age of the Sun. [cite web
author=Anonymous | year = 2005
url = http://hyperphysics.phy-astr.gsu.edu/hbase/astro/startime.html
title = Stellar Lifetimes
publisher = Georgia State University
accessdate = 2007-02-26 ]

In terms of mass, the relatively young Altair is the nearest star to the Sun that is a stellar analogue of component A&mdash;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 larger, with a semimajor axis of 20 A.U.

IK Pegasi B

The companion star is a dense white dwarf 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 remaining energy, growing cooler and dimmer over the course of many billions of years. [cite web
author=Staff | title =White Dwarfs & Planetary Nebulas
date=August 29, 2006
url =http://chandra.harvard.edu/xray_sources/white_dwarfs.html
publisher =Harvard-Smithsonian Center for Astrophysics
accessdate = 2007-06-09 ]

Evolution

Nearly all small and intermediate-mass stars (below about nine solar masses) will end up as a white dwarf once they have exhausted their supply of thermonuclear fuel. [cite journal
author=Heger, A.; Fryer, C. L.; Woosley, S. E.; Langer, N.; Hartmann, D. H.
title=&sect;3, How Massive Single Stars End Their Life
journal=Astrophysical Journal
year=2003 | volume=591 | issue=1 | pages=288–300
url=http://adsabs.harvard.edu/abs/2003ApJ...591..288H
accessdate=2007-08-14
doi=10.1086/375341 ] 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. [cite web
last = Seligman
first = Courtney
year=2007
url =http://cseligman.com/text/stars/mldiagram.htm
title =The Mass-Luminosity Diagram and the Lifetime of Main-Sequence Stars
accessdate = 2007-05-14] 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 masses.

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.cite web
author=Staff
date =August 29, 2006
url =http://chandra.harvard.edu/edu/formal/stellar_ev/story/index4.html
title =Stellar Evolution - Cycles of Formation and Destruction
publisher =Harvard-Smithsonian Center for Astrophysics
accessdate = 2006-08-10] [cite web
last = Richmond | first = Michael | date =October 5, 2006
url =http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html
title =Late stages of evolution for low-mass stars
publisher =Rochester Institute of Technology
accessdate = 2007-06-07 ] [cite web
last = Darling | first = David
url=http://www.daviddarling.info/encyclopedia/C/carbon_burning.html
title=Carbon burning
publisher=The Internet Encyclopedia of Sciencs
accessdate=2007-08-15 ]

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 5 × 10⁸ km (3 A.U.) in the case of the pulsating AGB star Mira. [cite web
author=Savage, D. Jones, T.; Villard, Ray; Watzke, M.
date = August 6, 1997
url = http://hubblesite.org/newscenter/archive/releases/1997/26/text/
title = Hubble Separates Stars in the Mira Binary System
publisher = HubbleSite News Center | accessdate = 2007-03-01 ] 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.

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. [cite journal
author=Oberhummer, H.; Csótó, A.; Schlattl, H.
title=Stellar Production Rates of Carbon and Its Abundance in the Universe
journal=Science | year=2000 | volume=289
issue=5476 | pages=88–90
url=http://www.sciencemag.org/cgi/content/full/289/5476/88
accessdate=2007-06-07
doi=10.1126/science.289.5476.88
pmid=10884230 ] 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.cite journal
last = Iben | first = Icko, Jr.
title=Single and binary star evolution
journal=Astrophysical Journal Supplement Series
year=1991 | volume=76 | pages=55–114
url=http://adsabs.harvard.edu/abs/1991ApJS...76...55I
accessdate=2007-03-03
doi=10.1086/191565 ]

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. [cite journal
author=Gil-Pons, P.; García-Berro, E.
title=On the formation of oxygen-neon white dwarfs in close binary systems
journal=Astronomy and Astrophysics | year=2001
volume=375 | pages=87–99
url=http://adsabs.harvard.edu/abs/2001astro.ph..6224G
accessdate=2007-05-15
doi=10.1051/0004-6361:20010828 ] [cite journal
author=Woosley, S. E.; Heger, A.
title=The Evolution and Explosion of Massive Stars
journal=Reviews of Modern Physics
year=2002 | volume=74 | issue=4 | pages=1015–1071
url=http://www.ucolick.org/~alex/Preprints/RMP.pdf
format=PDF | accessdate=2007-05-30
doi=10.1103/RevModPhys.74.1015 ] 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. 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&mdash;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.Ref_label|E|e|none Although its radius has not been observed directly, it can be estimated from known theoretical relationships between the mass and radius of white dwarfs, [cite web
url = http://www.sciencebits.com/StellarEquipartition
title =Estimating Stellar Parameters from Energy Equipartition
publisher =ScienceBits
accessdate = 2007-05-15 ] 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.Ref_label|F|f|none

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.Ref_label|G|g|none

The effective surface temperature of IK Pegasi B is estimated to be about 35,500 ± 1,500 K, making it a strong source of ultraviolet radiation.Ref_label|H|h|none Under normal conditions this white dwarf would continue to cool for more than a billion years, while its radius would remain essentially unchanged. [cite web
last =Imamura | first =James N.
date =February 24, 1995
url =http://zebu.uoregon.edu/~imamura/208/feb24/cool.html
title =Cooling of White Dwarfs
publisher =University of Oregon
accessdate = 2007-05-19 ]

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 supernova 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 and will pose no threat. 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.cite journal
author=Gehrels, Neil; Laird, Claude M.; Jackman, Charles H.; Cannizzo, John K.; Mattson, Barbara J.; Chen, Wan
title=Ozone Depletion from Nearby Supernovae
journal=The Astrophysical Journal | year=2003
volume=585 | issue=2 | pages=1169–1176
url=http://adsabs.harvard.edu/abs/2003ApJ...585.1169G
accessdate=2007-06-07
doi=10.1086/346127 ]

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. [cite web
author=K. A. Postnov, L. R. Yungelson | year = 2006
url = http://relativity.livingreviews.org/open?pubNo=lrr-2006-6&page=articlesu8.html
title = The Evolution of Compact Binary Star Systems
publisher = Living Reviews in Relativity
accessdate = 2007-05-16 ]

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 &mdash;a cataclysmic variable star&mdash;and the luminosity of the white dwarf rapidly would increase by several magnitudes for a period of several days or months. [cite web
author=Malatesta, K.; Davis, K. | month =May | year =2001
url =http://www.aavso.org/vstar/vsots/0501.shtml
title =Variable Star Of The Month: A Historical Look at Novae
publisher =AAVSO | accessdate = 2007-05-20] 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.cite web
last = Malatesta | first = Kerri | month = May | year = 2000
url =http://www.aavso.org/vstar/vsots/0500.shtml
title =Variable Star Of The Month&mdash;May, 2000: RS Ophiuchi
publisher =AAVSO | accessdate = 2007-05-15 ] [cite news
first=Susan | last=Hendrix
title=Scientists see Storm Before the Storm in Future Supernova
publisher=NASA | date=July 20, 2007
url=http://www.nasa.gov/vision/universe/starsgalaxies/rxte_supernova.html
accessdate=2007-05-25 ]

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. [cite journal
author=Langer, N.; Deutschmann, A.; Wellstein, S.; Höflich, P.
title=The evolution of main sequence star + white dwarf binary systems towards Type Ia supernovae
journal=Astronomy and Astrophysics | year=2000 | volume=362
pages=1046–1064
url=http://adsabs.harvard.edu/abs/2000astro.ph..8444L
accessdate=2007-05-20 ]

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 (0.5 &times; 10⁶ to 1 &times; 10⁶ K [cite conference
author=Langer, N.; Yoon, S.-C.; Wellstein, S.; Scheithauer, S.
editors=Gänsicke, B. T.; Beuermann, K.; Rein, K.
title =On the evolution of interacting binaries which contain a white dwarf
booktitle =The Physics of Cataclysmic Variables and Related Objects, ASP Conference Proceedings
pages =252
publisher = Astronomical Society of the Pacific
year=2002
location =San Francisco, California
url =http://adsabs.harvard.edu/abs/2002ASPC..261..252L
accessdate = 2007-05-25] ). [cite conference
first=Rosanne | last=Di Stefano | editor=J. Greiner
title=Luminous Supersoft X-Ray Sources as Progenitors of Type Ia Supernovae
booktitle=Proceedings of the International Workshop on Supersoft X-Ray Sources
publisher=Springer-Verlag | date=February 28–March 1, 1996
location=Garching, Germany
url=http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai%3AarXiv.org%3Aastro-ph%2F9701199
format=PDF | accessdate=2007-05-19 | id =ISBN 3540613900]

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. 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. [cite web
author=Fryer, C. L.; New, K. C. B.
date =January 24, 2006
url =http://www.livingreviews.org/Articles/Volume6/2003-2new
title =2.1 Collapse scenario
work=Gravitational Waves from Gravitational Collapse
publisher =Max-Planck-Gesellschaft | accessdate = 2007-06-07 ] If the core is instead made of carbon-oxygen, however, the collapse will cause a substantial fraction of the star to undergo nuclear fusion within a short time. This will be sufficient to unbind the star in a cataclysmic, Type Ia supernova explosion. [cite web
author=Staff | date =August 29, 2006
url =http://chandra.harvard.edu/edu/formal/stellar_ev/story/index8.html
title =Stellar Evolution - Cycles of Formation and Destruction
publisher =Harvard-Smithsonian Center for Astrophysics
accessdate = 2006-08-10 ]

Such a supernova event is not likely to pose a threat to life on the Earth, however. 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. This is outside the radius where a Type Ia supernova is thought to be hazardous.

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&ndash;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. [cite journal
last = Hansen | first = Brad M. S.
title=Type Ia Supernovae and High-Velocity White Dwarfs
journal=The Astrophysical Journal
year=2003 | volume=582 | issue=2 | pages=915–918
url=http://adsabs.harvard.edu/abs/2002astro.ph..6152H
accessdate=2007-02-04
doi=10.1086/344782 ] [cite journal
author=Marietta, E.; Burrows, A.; Fryxell, B.
title=Type Ia Supernova Explosions in Binary Systems: The Impact on the Secondary Star and Its Consequences
journal=The Astrophysical Journal Supplement Series
year=2000 | volume=128 | pages=615–650
url=http://adsabs.harvard.edu/abs/1998RPPh...61...77K
accessdate=2007-02-04
doi=10.1086/313392 ] The supernova explosion will create a remnant of expanding material that will eventually merge with the surrounding interstellar medium. [cite web
author=Staff | date =September 7, 2006
url =http://agile.gsfc.nasa.gov/docs/objects/snrs/snrstext.html
title =Introduction to Supernova Remnants
publisher =NASA/Goddard | accessdate = 2007-05-20 ]

Notes

1. Note_label|A|a|none The absolute magnitude "M_v"is given by::where "V" is the visual magnitude and "π" is the parallax. [cite book
   first=Roger John | last=Tayler | year=1994
   title=The Stars: Their Structure and Evolution
   publisher=Cambridge University Press
   pages=16 | isbn=0521458854 ]
2. Note_label|B|b|none Based upon::where "L" is luminosity, "R" is radius and "T_eff" is the effective temperature. [cite web
   last = Krimm
   first = Hans
   date =August 19, 1997
   url =http://ceres.hsc.edu/homepages/classes/astronomy/spring99/Mathematics/sec20.html
   title =Luminosity, Radius and Temperature
   publisher = Hampden-Sydney College
   accessdate = 2007-05-16 ]
3. Note_label|C|c|none The net proper motion is given by:: mas/y.where $mu\_alpha$ and $mu\_delta$ are the components of proper motion in the RA and Dec., respectively. The resulting transverse velocity is:: km.where "d"(pc) is the distance in parsecs.cite web
   last = Majewski | first = Steven R. | year=2006
   url = http://www.astro.virginia.edu/class/majewski/astr551/lectures/VELOCITIES/velocities.html
   title =Stellar Motions | publisher =University of Virginia
   accessdate = 2007-05-14]
4. Note_label|D|d|none By the Pythagorean theorem, the net velocity is given by::V_r}^2 + {V_t}^2} = sqrt{11.4^2 + 16.9^2} = 20.4, end{smallmatrix} km/s.where $V\_r$ is the radial velocity and $V\_t$ is the transverse velocity, respectively.
5. Note_label|E|e|none The white-dwarf population is narrowly distributed around the mean mass of 0.58 solar masses, and only 2% [cite journal
   author=J. B. Holberg, M. A. Barstow, F. C. Bruhweiler, A. M. Cruise, A. J. Penny
   title=Sirius B: A New, More Accurate View
   journal=The Astrophysical Journal
   year=1998 | volume=497 | issue=2 | pages=935–942
   url=http://www.journals.uchicago.edu/doi/full/10.1086/305489
   accessdate=2007-05-15
   doi=10.1086/305489 ] of all white dwarfs have at least one solar mass.
6. Note_label|F|f|none km.
7. Note_label|G|g|none The surface gravity of the Earth is 9.780 m/s², or 978.0 cm/s² in cgs units. Thus::The log of the gravitational force ratios is 8.95 - 2.99 = 5.96. So::
8. Note_label|H|h|none From Wien's displacement law, the peak emission of a black body at this temperature would be at a wavelength of:: nmwhich lies in the far ultraviolet part of the electromagnetic spectrum.

References

External links

* cite web
last=Davies | first=Ben | year=2006
url=http://ben.davies.net/supernovae2.htm
title=Supernova events | accessdate = 2007-06-01
* cite web
url = http://www.alcyone.de/SIT/bsc/HR8210.html
title = IK Pegasi | publisher = Alcyone
accessdate = 2007-01-18
* cite web
last = Richmond | first = Michael | date =April 8, 2005
url =http://www.tass-survey.org/richmond/answers/snrisks.txt
title =Will a Nearby Supernova Endanger Life on Earth?
publisher =The Amateur Sky Survey
accessdate = 2007-06-07
*cite web
last=Tzekova|first=Svetlana Yordanova|date=2004
url=http://www.eso.org/public/outreach/eduoff/cas/cas2004/casreports-2004/rep-310/#2.%20The%20main%20star%20-%20IK%20Peg%20A
title=IK Pegasi (HR 8210)
publisher=ESO (European Organisation for Astronomical Research in the Southern Hemisphere)
accessdate=2007-09-30