View source for IK Pegasi

{{Starbox begin
| name=IK Pegasi }}
{{Starbox image
| image=[[Image:Location of IK Pegasi.png|236px]]
| caption=Location of IK Pegasi. }}
{{Starbox observe
| epoch=J2000
| ra={{RA|21|26|26.6624}}<ref name="simbad">{{cite web
 | title=SIMBAD Query Result: HD 204188 -- Spectroscopic binary
 | publisher=Centre de Données astronomiques de Strasbourg | work=SIMBAD
 | url=http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HD+204188
 | accessdate=2009-01-02 }} — ''Note:'' some results were queried via the "Display all measurements" function on the web page.</ref>
| dec={{DEC| +19|22|32.304}}<ref name="simbad" />
| appmag_v=6.078<ref name="simbad" />
| constell=[[Pegasus (constellation)|Pegasus]] }}
{{Starbox character
| class=A8m:<ref name="apj221" />/DA<ref name="mnras270" />
| b-v=0.24<ref name="simbad" />/–
| u-b=0.03<ref name="simbad" />/–
| variable=[[Delta Scuti variable|Delta Scuti]]<ref name="apj221" /> }}
{{Starbox astrometry
| radial_v=-11.4<ref name="simbad" />
| prop_mo_ra=80.23<ref name="simbad" />
| prop_mo_dec=17.28<ref name="simbad" />
| parallax=21.72
| p_error=0.78
| parallax_footnote=<ref name="simbad" />
| absmag_v=2.762{{Ref_label|A|a|none}} }}
{{Starbox detail
| mass=1.65<ref name="mnras267" />/1.15<ref name="pasp105" />
| radius=1.6<ref name="mnras267" />/0.006<ref name="mnras270" />
| luminosity=8.0/0.12{{Ref_label|B|b|none}}
| temperature=7,700<ref name="nras278">{{cite 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 }}</ref>/35,500<ref name="pasp105" />
| metal=117<ref name="nras278" /><ref name="mnras267" />/– % Sun
| rotation=<&nbsp;32.5<ref name="nras278" />/–&nbsp;km/s
| gravity=4.25<ref name="mnras267" />/8.95<ref name="mnras270" />
| age=5–60&nbsp;×&nbsp;10<sup>7</sup><ref name="mnras267" /> }}
{{Starbox catalog
| names='''AB:''' V*&nbsp;IK&nbsp;Peg, [[Harvard Revised catalogue|HR&nbsp;8210]], [[Bonner Durchmusterung|BD&nbsp;+18°4794]], [[Henry Draper catalogue|HD&nbsp;204188]], [[Smithsonian Astrophysical Observatory|SAO&nbsp;107138]], [[Hipparcos catalogue|HIP&nbsp;105860]].<ref name="simbad" />
<br />'''B:''' WD&nbsp;2124+191, EUVE&nbsp;J2126+193.<ref name="apj502">{{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
  }}</ref><ref>{{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 }}</ref> }}
{{Starbox end}}

'''IK Pegasi''' (or '''HR 8210''') is a [[binary star]] [[star system|system]] in the [[constellation]] [[Pegasus (constellation)|Pegasus]]. At a distance of about 150&nbsp;[[light year]]s 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]], [[spectral class|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.<ref name="mnras267">{{cite 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 }}</ref> 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&nbsp;million kilometres, or 19&nbsp;million miles, or 0.21&nbsp;[[astronomical unit]]s (AU). This is smaller than the orbit of [[Mercury (planet)|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 [[Accretion (astrophysics)|accrete]] matter from the expanded gaseous envelope. When the white dwarf approaches the [[Chandrasekhar limit]] of 1.44&nbsp;[[solar mass]]es, it may explode as a [[Type Ia supernova]].<ref name="mnras262">{{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 }}</ref>

==Observation==
This star system was catalogued in the 1862 ''[[Durchmusterung|Bonner Durchmusterung]]'' ("Bonn astrometric Survey") as BD&nbsp;+18°4794B. It later appeared in [[Edward Charles Pickering|Pickering's]] 1908 ''[[Bright Star Catalogue|Harvard Revised Photometry Catalogue]]'' as HR 8210.<ref>{{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 {{convert|4|in|mm|sing=on}} 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 }}</ref> The designation "IK Pegasi" follows the expanded form of the [[variable star designation|variable star nomenclature]] introduced by [[Friedrich Wilhelm Argelander|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 effect|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.<ref>{{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 }}</ref>

In 1927, the Canadian astronomer [[William Edmund Harper|William E. Harper]] used this technique to measure the period of this single-line spectroscopic binary and determined it to be 21.724&nbsp;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.)<ref name="mnras262" /> The velocity amplitude was measured as 41.5&nbsp;km/s, which is the maximum velocity of the primary component along the line of sight to the Solar System.<ref>{{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 }}</ref>

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&nbsp;[[light year]]s (with an accuracy of ±5&nbsp;light years).<ref>{{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 }}</ref> 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&nbsp;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&nbsp;km/s for this system.<ref>{{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 }}</ref> The combination of these two motions gives a [[space velocity]] of 20.4&nbsp;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.<ref>{{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 }}</ref> Recent measurements with the [[Extreme Ultraviolet Explorer]] [[space telescope]] gave a more accurate orbital period of 21.72168&nbsp;±&nbsp;0.00009&nbsp;days.<ref name="apj502">{{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
  }}</ref> The [[inclination]] of this system's [[Orbital plane (astronomy)|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]].<ref name="pasp105" />

==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—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.<ref name="araa33">{{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 }}</ref>

The pulsations result from a process called the ''κ''-mechanism. A part of the star's outer [[atmosphere]] becomes [[Optical density|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.<ref name="araa33" />

[[File:Size IK Peg.svg|right|320px|thumb|The relative dimensions of IK Pegasi A (left), B (lower center) and the Sun (right).<ref>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 }}</ref>]]
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 ([[subgiant]]s) to V ([[main sequence]] stars). They are short-period variables that have a regular pulsation rate between 0.025 and 0.25&nbsp;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&nbsp;[[solar mass]]es.<ref>{{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 }}</ref> The pulsation rate of IK Pegasi A has been measured at 22.9 cycles per day, or once every 0.044 days.<ref name="mnras267" />

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]&nbsp;=&nbsp;+0.07&nbsp;±&nbsp;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.<ref name="mnras267" /> 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 line]]s of hydrogen along with absorption lines of ionized metals, including the K&nbsp;line of ionized [[calcium]] (Ca&nbsp;II) at a wavelength of 393.3&nbsp;[[nanometre|nm]].<ref>{{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 }}</ref> 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.<ref name="apj221">{{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 }}</ref> 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.<ref>{{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 }}</ref>

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&nbsp;×&nbsp;10<sup>9</sup>&nbsp;years, which is about half the current age of the Sun.<ref>{{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 }}</ref>

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&nbsp;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 excess energy, mainly stored heat, growing cooler and dimmer over the course of many billions of years.<ref>{{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 }}</ref>

===Evolution===
Nearly all small and intermediate-mass stars (below about nine [[solar mass]]es) will end up as white dwarfs once they have exhausted their supply of [[thermonuclear]] fuel.<ref>{{cite journal
 | author=Heger, A.; Fryer, C. L.; Woosley, S. E.; Langer, N.; Hartmann, D. H.
 | title=§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 }}</ref> 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.<ref>{{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
}}</ref> 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&nbsp;[[solar mass]]es.<ref name="mnras262" />

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 burning process|carbon fusion]] could begin in the  core, producing [[oxygen]], [[neon]] and [[magnesium]].<ref name="evolution">{{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
}}</ref><ref>{{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 }}</ref><ref>{{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 }}</ref>

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&nbsp;×&nbsp;10<sup>8</sup>&nbsp;km (3 A.U.) in the case of the pulsating AGB star [[Mira]].<ref>{{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 }}</ref> 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.<ref name="pasp105">{{cite 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 }}</ref>

[[Image:NGC7293 (2004).jpg|right|thumb|The [[Helix Nebula]] is being created by a star evolving into a white dwarf. ''[[NASA]] & [[ESA]] image.'']]
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.<ref>{{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 }}</ref> 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.<ref name="apjs76">{{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 }}</ref>

===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.<ref>{{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 }}</ref><ref>{{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 }}</ref>  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.<ref name="mnras270">{{cite 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 }}</ref> The entire mass of the star is supported by [[electron degeneracy pressure]]—a [[quantum mechanics|quantum mechanical]] effect that limits the amount of matter that can be squeezed into a given volume.

[[File:ChandrasekharLimitGraph.svg|left|280px|thumb|This graph shows the theoretical radius of a white dwarf, given its mass. The green curve is for a [[Special relativity|relativistic]] electron gas model.]]
At an estimated 1.15&nbsp;[[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,<ref>{{cite web
 | url = http://www.sciencebits.com/StellarEquipartition
 | title =Estimating Stellar Parameters from Energy Equipartition
 | publisher =ScienceBits
 | accessdate = 2007-05-15 }}</ref> giving a value of about 0.60% of the [[solar radius|Sun's radius]].<ref name="mnras270" /> (A different source gives a value of 0.72%, so there remains some uncertainty in this result.)<ref name="mnras267" /> 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 [[Centimeter gram second system of units|cgs units]], or log ''g''. For IK Pegasi B, log ''g'' is 8.95.<ref name="mnras270" />  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&nbsp;±&nbsp;1,500&nbsp;K,<ref name="pasp105" /> making it a strong source of [[ultraviolet]] radiation.<ref name="mnras270" />{{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.<ref>{{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 }}</ref>

==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 star|cataclysmic variable]].<ref name="mnras262" /> At a distance of 150&nbsp;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&nbsp;light years of the Earth to effectively destroy the Earth's [[ozone layer]], which would severely impact the planet's [[biosphere]].<ref name="apj585">{{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 }}</ref>

[[Image:Mira 1997 UV.jpg|right|thumb|This [[Hubble Space Telescope]] image shows the pulsating AGB ([[Asymptotic giant branch]])  star Mira. ''NASA image.'']]
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.<ref>{{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 }}</ref>

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 [[thermal runaway|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 [[Apparent magnitude|magnitudes]] for a period of several days or months.<ref>{{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}}</ref> 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.<ref name="vsom0501">{{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—May, 2000: RS Ophiuchi
 | publisher =AAVSO | accessdate = 2007-05-15 }}</ref><ref>{{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 }}</ref>

It is possible that IK Pegasi B will follow a similar pattern.<ref name="vsom0501" /> 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.<ref>{{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 }}</ref>

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&nbsp;×&nbsp;10<sup>6</sup> to 1&nbsp;×&nbsp;10<sup>6</sup>&nbsp;K<ref>{{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
}}</ref>).<ref>{{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
}}</ref>

Should the white dwarf's mass approach the [[Chandrasekhar limit]] of 1.44&nbsp;[[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.<ref>{{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 }}</ref> 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.<ref>{{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 }}</ref>

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&nbsp;km/s. This is equivalent to moving a distance of one light year every 14,700&nbsp;years. After 5&nbsp;million years, for example, this star will be separated from the Sun by more than 500&nbsp;light years. This is outside the radius where a Type Ia supernova is thought to be hazardous.<ref name="apj585" />

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 [[Milky Way|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.<ref>{{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 }}</ref><ref>{{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 }}</ref> The supernova explosion will create a [[Supernova remnant|remnant]] of expanding material that will eventually merge with the surrounding [[interstellar medium]].<ref>{{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 }}</ref>

==Notes==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}} The absolute magnitude ''M<sub>v</sub>''
is given by:
:<math>\begin{smallmatrix} M_v = V + 5(\log_{10} \pi + 1) = 2.762 \end{smallmatrix}</math>
where ''V'' is the visual magnitude and ''π'' is the parallax.<ref>{{cite book
 | first=Roger John | last=Tayler | year=1994
 | title=The Stars: Their Structure and Evolution
 | publisher=Cambridge University Press
 | pages=16 | isbn=0521458854 }}</ref></li>

<li>{{Note_label|B|b|none}} Based upon:
:<math>\begin{smallmatrix} \frac{L}{L_{sun}} = \left ( \frac{R}{R_{sun}} \right )^2 \left ( \frac{T_{eff}}{T_{sun}} \right )^4 \end{smallmatrix}</math>
where ''L'' is luminosity, ''R'' is radius and ''T<sub>eff</sub>'' is the effective temperature.<ref>{{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 }}</ref></li>

<li>{{Note_label|C|c|none}} The net proper motion is given by:
:<math>\begin{smallmatrix} \mu = \sqrt{ {\mu_\delta}^2 + {\mu_\alpha}^2 \cdot \cos^2 \delta } = 77.63\, \end{smallmatrix}</math>&nbsp;mas/y.
where <math>\mu_\alpha</math> and <math>\mu_\delta</math> are the components of proper motion in the RA and Dec., respectively. The resulting transverse velocity is:
:<math>\begin{smallmatrix} V_t = \mu \cdot 4.74 d\,(\operatorname{pc}) = 16.9\, \end{smallmatrix}</math>&nbsp;km.
where ''d''(pc) is the distance in parsecs.<ref name="majewski">{{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
}}</ref></li>

<li>{{Note_label|D|d|none}} By the [[Pythagorean theorem]], the net velocity is given by:
:<math>\begin{smallmatrix} V = \sqrt{{V_r}^2 + {V_t}^2} = \sqrt{11.4^2 + 16.9^2} = 20.4\, \end{smallmatrix}</math>&nbsp;km/s.
where <math>V_r</math> is the radial velocity and <math>V_t</math> is the transverse velocity, respectively.</li>

<li>{{Note_label|E|e|none}} The white-dwarf population is narrowly distributed around the mean mass of 0.58 solar masses, and only 2%<ref>{{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 }}</ref> of all white dwarfs have at least one solar mass.</li>

<li>{{Note_label|F|f|none}} <math>\begin{smallmatrix} R_{\star} = 0.006 \cdot (6.96 \times 10^8)\,\mbox{m}\;\approx 4,200\, \end{smallmatrix}</math>&nbsp;km.</li>

<li>{{Note_label|G|g|none}} The surface gravity of the Earth is 9.780 m/s<sup>2</sup>, or 978.0 cm/s<sup>2</sup> in cgs units. Thus:
:<math>\begin{smallmatrix} \log\ \operatorname{g}=\log\ 978.0=2.99 \end{smallmatrix}</math>
The log of the gravitational force ratios is 8.95 - 2.99 = 5.96. So:
:<math>\begin{smallmatrix} 10^{5.96} \approx 912,000 \end{smallmatrix}</math></li>

<li>{{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:
:<math>\begin{smallmatrix} \lambda_b = (2.898 \times 10^6 \operatorname{nm\ K})/(35,500\ \operatorname{K}) \approx 82\, \end{smallmatrix}</math>&nbsp;nm
which lies in the far ultraviolet part of the [[electromagnetic spectrum]].</li>

</ol>
</div>

==References==
{{reflist|2}}

==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}}

{{featured article}}

[[Category:Pegasus constellation]]
[[Category:Spectroscopic binaries]]
[[Category:A-type main sequence stars]]
[[Category:White dwarfs]]
[[Category:Supernovae]]

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