Epsilon Pegasi

{{Short description|Star in the northern constellation of Pegasus}}

{{Starbox begin

| name=ε Pegasi

}}

{{Starbox image

| image=

{{Location mark

|image=Pegasus constellation map.svg

|float=center

|alt=

|label=

|position=right

|width=280

|mark=Red circle.svg

|mark_width=12

|mark_link=ε Pegasi

|x=762|y=622

}}

|caption=Location of ε Pegasi (circled)

}}

{{Starbox observe

| epoch = J2000

| ra = {{RA|21|44|11.15614}}

| dec = {{DEC|+09|52|30.0311}}

| appmag_v = 2.37 - 2.45

| constell = Pegasus

}}

{{Starbox character

| class = K2 Ib-II{{cite journal

| title=The Perkins catalog of revised MK types for the cooler stars

| journal=Astrophysical Journal Supplement Series

| volume=71 | pages=245 | year=1989

| last1=Keenan | first1=Philip C. | last2=McNeil | first2=Raymond C.

| bibcode=1989ApJS...71..245K | doi=10.1086/191373 }}

| b-v = +1.527

| u-b = +1.722

| variable = LC

|type=Red supergiant}}

{{Starbox astrometry

| radial_v = {{val|3.39|0.06}}

| prop_mo_ra = +26.92

| prop_mo_dec = +0.4

| parallax = 4.73

| p_error = 0.17

| parallax_footnote =

| absmag_v = −4.142

}}

{{Starbox detail

| mass = 11–12

| radius = {{Val|183|6|7}}

| luminosity_visual = 3,895

| luminosity = {{val|8,508|596|fmt=commas}}

| temperature = {{Val|4100|37|fmt=commas}}

| metal_fe = −0.04

| gravity = 1.01

| rotational_velocity = 8

| age_myr = {{val|20.0|4.5}}

}}

{{Starbox catalog

| names=Enif, 8 Pegasi, BD+09°4891, FK5 815, HD 206778, HIP 107315, HR 8308, SAO 127029

}}

{{Starbox reference

| Simbad=HD+206778

}}

Epsilon Pegasi (Latinised from ε Pegasi, abbreviated Epsilon Peg, ε Peg), formally named Enif {{IPAc-en|'|iː|n|ɪ|f}}, is the brightest star in the northern constellation of Pegasus.

With an average apparent visual magnitude of 2.4, this is a second-magnitude star that is readily visible to the naked eye. The distance to this star can be estimated using parallax measurements from the Hipparcos astrometry satellite, yielding a value of around {{Convert|690|ly|pc|abbr=off|lk=on}}.

Nomenclature

ε Pegasi (Latinised to Epsilon Pegasi) is the star's Bayer designation.

It bore the traditional name Enif derived from the Arabic word for 'nose', due to its position as the muzzle of Pegasus. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN){{cite web | url=https://www.iau.org/science/scientific_bodies/working_groups/280/ | title=IAU Working Group on Star Names (WGSN)|access-date=22 May 2016}} to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016{{cite web | url=http://www.pas.rochester.edu/~emamajek/WGSN/WGSN_bulletin1.pdf | title=Bulletin of the IAU Working Group on Star Names, No. 1 |access-date=28 July 2016}} included a table of the first two batches of names approved by the WGSN; which included Enif for this star.{{cite web | url=http://www.pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt | title=IAU Catalog of Star Names |access-date=28 July 2016}}

Other traditional names for the star include Fom al Feras, Latinised to Os Equi.{{cite journal |last=Knobel|first=Edward B. |date=1895|title= Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of|journal=Monthly Notices of the Royal Astronomical Society|volume=55|issue=8 |pages=429–38|bibcode=1895MNRAS..55..429K|doi=10.1093/mnras/55.8.429|doi-access=free}} In Chinese, {{lang|zh|危宿}} ({{lang|zh-Latn|Wēi Sù}}), meaning Rooftop (asterism), refers to an asterism consisting of Epsilon Pegasi, Alpha Aquarii and Theta Pegasi.{{in lang|zh}} 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, {{ISBN|978-986-7332-25-7}}. Consequently, the Chinese name for Epsilon Pegasi itself is {{lang|zh|危宿三}} ({{lang|zh-Latn|Wēi Sù sān}}, {{langx|en|the Third Star of Rooftop}}.){{in lang|zh}} [http://www.lcsd.gov.hk/CE/Museum/Space/Research/StarName/c_research_chinengstars_e_g.htm 香港太空館 - 研究資源 - 亮星中英對照表] {{webarchive|url=https://web.archive.org/web/20110130063007/http://www.lcsd.gov.hk/CE/Museum/Space/Research/StarName/c_research_chinengstars_e_g.htm |date=2011-01-30 }}, Hong Kong Space Museum. Accessed on line November 23, 2010.

Physical characteristics

Epsilon Pegasi is a red supergiant star, as indicated by the stellar classification of K2 Ib. It is estimated to be between 11 and 12 times the Sun's mass and has an enormous size of 183 times the radius of the Sun. From this expanded envelope, it is radiating roughly 8,500 times the luminosity of the Sun at an effective temperature of {{val|4100|fmt=commas|ul=K}}. This temperature is cooler than the Sun, giving it the orange-hued glow of a K-type star.

Epsilon Pegasi is a slow irregular variable star that usually has a brightness between magnitudes 2.37 and 2.45. However, it was once observed very briefly at magnitude 0.7, giving rise to the theory that it (and possibly other supergiants) erupt in massive flares that dwarf those of the Sun. It has also been observed as faint as magnitude 3.5.

The spectrum shows an overabundance of the elements strontium and barium, which may be the result of the s-process of nucleosynthesis in the outer atmosphere of the star. It has a relatively high peculiar velocity of {{val|21.6|u=km/s}}.

Evolution

Epsilon Pegasi has exhausted its core hydrogen and expanded away from the main sequence. It is almost certainly on the horizontal branch fusing helium in its core.{{Cite journal|last1=Stock|first1=S.|last2=Reffert|first2=S.|last3=Quirrenbach|first3=A.|date=2018|title=VizieR Online Data Catalog: Stellar parameters of 372 giant stars (Stock+, 2018)|url=http://adsabs.harvard.edu/abs/2018yCat..36160033S|journal=VizieR On-line Data Catalog|volume=361|doi=10.26093/cds/vizier.36160033 |bibcode=2018yCat..36160033S}} If it loses more mass in the supergiant phase in its evolution, it may shed its outer layers and leave behind an unusual high mass oxygen–neon white dwarf near the Chandrasekhar limit, otherwise it may be able to produce a supernova,{{Cite journal |last1=Asakura |first1=K. |last2=Gando |first2=A. |last3=Gando |first3=Y. |last4=Hachiya |first4=T. |last5=Hayashida |first5=S. |last6=Ikeda |first6=H. |last7=Inoue |first7=K. |last8=Ishidoshiro |first8=K. |last9=Ishikawa |first9=T. |last10=Ishio |first10=S. |last11=Koga |first11=M. |last12=Matsuda |first12=S. |last13=Mitsui |first13=T. |last14=Motoki |first14=D. |last15=Nakamura |first15=K. |date=2016-02-10 |title=KamLAND Sensitivity to Neutrinos from Pre-Supernova Stars |journal=The Astrophysical Journal |volume=818 |issue=1 |pages=91 |doi=10.3847/0004-637X/818/1/91 |doi-access=free |arxiv=1506.01175 |bibcode=2016ApJ...818...91A |issn=0004-637X}} albeit an electron capture supernova.{{Cite journal |last=Nomoto |first=K. |date=1984-02-01 |title=Evolution of 8-10 solar mass stars toward electron capture supernovae. I - Formation of electron-degenerate O + NE + MG cores. |journal=The Astrophysical Journal |volume=277 |pages=791–805 |doi=10.1086/161749 |issn=0004-637X|doi-access=free |bibcode=1984ApJ...277..791N }}

Based on its position on the color-magnitude diagram, Enif may have evolved from a whitish-yellow color to its current red color in the last 2,000 years, though there is currently no historical record supporting this.

Pulfrich effect

Epsilon Pegasi is a fine example to observe the Pulfrich effect. This optical phenomenon is described on page 1372 of Burnham's Celestial Handbook. According to John Herschel: The apparent pendulum-like oscillation of a small star in the same vertical as the large one, when the telescope is swung from side to side.

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}}

Category:K-type supergiants

Category:Pegasus (constellation)

Category:Horizontal-branch stars

Category:Slow irregular variables