View source for Astronomy:HD 156668

{{Short description|Star in the constellation Hercules}}
{{Starbox begin
 | name     = HD 156668
}}
{{Starbox observe 
 | epoch    = [[Astronomy:J2000.0|J2000.0]]
 | constell = [[Astronomy:Hercules (constellation)|Hercules]]
 | ra       = {{RA|17|17|40.49053}}<ref name=aaa474_2_653/>
 | dec      = {{DEC|+29|13.6|38.0243}}<ref name=aaa474_2_653/>
 | appmag_v = +8.424<ref name=apj726_2_73/>
}}
{{Starbox character
 | class    = K3&nbsp;V<ref name=apj726_2_73/>
 | b-v      = 1.015<ref name=apj726_2_73/>
 | u-b      = 0.27
 | variable =
}}
{{Starbox astrometry
 | radial_v = -9.427
 | prop_mo_ra  = -72.481{{±|0.016}}
 | prop_mo_dec = +216.849{{±|0.019}}
|pm_footnote=<ref name=EDR3>{{cite Gaia EDR3|4575374857376398976}}</ref>
 | parallax = 41.1103
 | p_error  = 0.0169
 | parallax_footnote = <ref name=EDR3/>
 | absmag_v = 6.480<ref name=apj726_2_73/>
}}
{{Starbox detail
 | mass     = {{nowrap|0.772 ± 0.020}}<ref name=apj726_2_73/>
 | radius   = {{nowrap|0.720 ± 0.013}}<ref name=apj726_2_73/>
 | gravity  = {{nowrap|4.60 ± 0.12}}<ref name=apj726_2_73/>
 | metal_fe = {{nowrap|+0.05 ± 0.06}}<ref name=apj726_2_73/>
 | age_gyr  = {{nowrap|8.6 ± 4.8}}<ref name=apj726_2_73/>
 | rotation = 51.5&nbsp;days<ref name=apj726_2_73/>
 | luminosity  = {{nowrap|0.230 ± 0.018}}<ref name=apj726_2_73/>
 | temperature = {{nowrap|4850 ± 88}}<ref name=apj726_2_73/>
 | rotational_velocity = {{nowrap|0.50 ± 1.0}}<ref name=apj726_2_73/>
}}
{{Starbox catalog
 | names = BD+29° 2979, HD&nbsp;156668, HIP&nbsp;84607, SAO&nbsp;84984, Wolf 646, [[Astronomy:2MASS|2MASS]] J17174049+2913378<ref name="Simbad"/>
}}
{{Starbox reference
 | Simbad = HD+156668
}}
{{Starbox end}}

'''HD 156668''' is a [[Astronomy:Star|star]] in the northern [[Astronomy:Constellation|constellation]] of [[Astronomy:Hercules (constellation)|Hercules constellation]]. With an [[Astronomy:Apparent visual magnitude|apparent visual magnitude]] of 8.4<ref name=apj726_2_73/> it is too faint to be viewed with the naked eye, but it can be seen with even a small telescope.<ref name=sherrod_koed2003/> The distance to this object has been determined directly using the [[Astronomy:Parallax|parallax]] technique, yielding a value of about {{Convert|80|ly|pc|abbr=off|lk=on}}.<ref name=aaa474_2_653/>

This star has the [[Astronomy:Stellar classification|stellar classification]] of a [[Astronomy:K-type star|K2 dwarf]], with approximately 77% of the mass of the Sun and about 72% of the Sun's diameter. While they are on the [[Astronomy:Main sequence|main sequence]], lower mass stars like this generate energy much more slowly than the Sun. As a result, this star is radiating only 23% of the Sun's bolometric luminosity. HD 156668 is emitting this energy from its [[Astronomy:Stellar atmosphere|outer atmosphere]] at an [[Astronomy:Effective temperature|effective temperature]] of around 4850&nbsp;K,<ref name=apj726_2_73/> giving it the cool orange glow of a [[Astronomy:K-type star|K-type star]].<ref name=csiro/> It is slightly more enriched in [[Chemistry:Iron|iron]] compared to the Sun and is rotating at a leisurely rate of once every 51.5&nbsp;days. Although much older than the Sun, this star is only middle-aged at about 8.6 billion years.<ref name=apj726_2_73/>

The HD 156668 exhibits a stellar activity cycle with a period about 10 years.<ref name=Rosenthal2021/>

==Observations==
From 1911 to 1915, Photographic Stellar Spectrum surveys<ref>The Henry Draper Catalogue, Annie J. Cannon and Edward C. Pickering, ''Annals of Harvard College Observatory'';<br />hours 0 to 3, '''91''' (1918), {{bibcode|1918AnHar..91....1C}};<br />hours 4 to 6, '''92''' (1918), {{bibcode|1918AnHar..92....1C}};<br />hours 7 to 8, '''93''' (1919), {{bibcode|1919AnHar..93....1C}};<br />hours 9 to 11, '''94''' (1919), {{bibcode|1919AnHar..94....1C}};<br />hours 12 to 14, '''95''' (1920), {{bibcode|1920AnHar..95....1C}};<br />hours 15 to 16, '''96''' (1921), {{bibcode|1921AnHar..96....1C}};<br />hours 17 to 18, '''97''' (1922), {{bibcode|1922AnHar..97....1C}};<br />hours 19 to 20, '''98''' (1923), {{bibcode|1923AnHar..98....1C}};<br />hours 21 to 23, '''99''' (1924), {{bibcode|1924AnHar..99....1C}}.</ref> have been carried out by Annie Jump Cannon and Edward Charles Pickering. Anna Mary Palmer who was Henry Draper's wife financed the study, it was also her husband who was responsible for the star's designation in the Henry Draper Catalogue with subsequent extension (HD) although it was first designated as BD+29 2979<ref>{{cite web | title=ARICNS ARI Data Base for Nearby Stars | url=http://www.ari.uni-heidelberg.de/datenbanken/aricns/cnspages/4c05778.htm | work=ASTRONOMISCHES RECHEN-INSTITUT HEIDELBERG | access-date=2010-02-09 | archive-date=2012-03-02 | archive-url=https://web.archive.org/web/20120302204622/http://www.ari.uni-heidelberg.de/datenbanken/aricns/cnspages/4c05778.htm | url-status=dead }}</ref> in the original catalogue that was published in 1863 by Friedrich Wilhelm Argelander due to its position and brightness of 324,198 stars between +90° and −2° declination in the span of an 11-year observation. A number of [[Astronomy:Proper motion|proper motion surveys]] was published in 1957 where 9,867 stars in the Southern hemisphere where high proper motion was detected.<ref>{{cite book | title=A catalogue of 9867 stars in the southern hemisphere with proper motions exceeding 0."2 annually. | bibcode=1957cnes.book.....L | work= Harvard.edu| year=1957 | last1=Luyten | first1=Willem Jacob }}</ref> The name BD was taken from a catalogue of the [[Astronomy:Durchmusterung|Bonner Durchmusterung (Bonn Survey)]]<ref>{{cite web | title= NASA reference publication -- 1297 | work= Bonner Durchmusterung}}</ref> where observations are abbreviated as BD. Latter study expansions were carried out by the Cape Photographic Durchmusterung observatory in Córdoba, Argentina and further extensions by Henry Lee Giclas who designated the star as G 181-34 in the [[Astronomy:Star catalogue|Giclas catalogues]]. From 1990 to 1993, HD 156668 was observed at least twice and was featured at the 6th annual catalog of the Tokyo Photoelectric Meridian Circle (PMC) where it is one of 6649 stars observed.<ref>{{cite web | title= HD 156668 | url= http://server1.sky-map.org/starview?object_type=1&object_id=471769&object_name=HD+156668&locale=HE| work=Sky Map.org | access-date=2010-02-09}}</ref>

==Planetary system==
{{Main|Astronomy:HD 156668 b}}
Andrew Howard announced the discovery of a [[Astronomy:Super-Earth|Super-Earth]] in orbit around HD 156668. This planet is designated HD 156668 b. The announcement was made at the 215th American Astronomical Society meeting on January 4 to 7, 2010 in Washington D.C. The planet orbits its star in only 4.6 days with a distance approximately 0.05 AU away from the parent star. The researchers used the [[Astronomy:Doppler spectroscopy|wobble method]] where the resulting spectrum showed color shifts which was used to approximate the mass of the astronomical object. The observation revealed data that the exoplanet was at least 4.15 Earth masses.<ref>{{cite web | title= Second smallest exoplanet found to date discovered at Keck | url= http://keckobservatory.org/index.php/news/second_smallest_exoplanet_to_date_discovered_at_keck/ | work= W.M. Keck Observatory | access-date= 2010-02-09 | archive-date= 2016-03-10 | archive-url= https://web.archive.org/web/20160310143145/http://keckobservatory.org/index.php/news/second_smallest_exoplanet_to_date_discovered_at_keck | url-status= dead }}</ref> Later observations revealed that the [[Astronomy:Minimum mass|minimum mass]] is now 3.1 Earth masses.<ref name="rebekah10">{{cite journal|arxiv=1005.4050  |title=Radial velocity planets de-aliased. A new, short period for Super-Earth 55 Cnc e |journal=The Astrophysical Journal |volume=722 |issue=1 |pages=937–953 | author=Rebekah| date=2010|display-authors=0|doi=10.1088/0004-637X/722/1/937 |bibcode=2010ApJ...722..937D |s2cid=118592734 }}</ref>

Another planet, HD 156668 c, was discovered in 2021, and presence of additional long-period object is suspected.<ref name=Rosenthal2021>{{citation|arxiv=2105.11583|year=2021|title=The California Legacy Survey. I. A Catalog of 178 Planets from Precision Radial Velocity Monitoring of 719 Nearby Stars over Three Decades|last1=Rosenthal|first1=Lee J.|last2=Fulton|first2=Benjamin J.|last3=Hirsch|first3=Lea A.|last4=Isaacson|first4=Howard T.|last5=Howard|first5=Andrew W.|last6=Dedrick|first6=Cayla M.|last7=Sherstyuk|first7=Ilya A.|last8=Blunt|first8=Sarah C.|last9=Petigura|first9=Erik A.|last10=Knutson|first10=Heather A.|last11=Behmard|first11=Aida|last12=Chontos|first12=Ashley|last13=Crepp|first13=Justin R.|last14=Crossfield|first14=Ian J. M.|last15=Dalba|first15=Paul A.|last16=Fischer|first16=Debra A.|last17=Henry|first17=Gregory W.|last18=Kane|first18=Stephen R.|last19=Kosiarek|first19=Molly|last20=Marcy|first20=Geoffrey W.|last21=Rubenzahl|first21=Ryan A.|last22=Weiss|first22=Lauren M.|last23=Wright|first23=Jason T.|journal=The Astrophysical Journal Supplement Series |volume=255 |issue=1 |page=8 |doi=10.3847/1538-4365/abe23c |bibcode=2021ApJS..255....8R |s2cid=235186973 |doi-access=free }}</ref>
{{OrbitboxPlanet begin
| table_ref = <ref name="rebekah10" /><ref name=Rosenthal2021/>
}}
{{OrbitboxPlanet
| exoplanet = [[Astronomy:HD 156668 b|b]]
| mass_earth = ≥3.1 ± 0.4
| period = 1.26984 ± 0.00007
| semimajor = 0.0211 ± 0.0002
| eccentricity = 0.000
}}
{{OrbitboxPlanet
| exoplanet = c
| mass = ≥0.0991{{±|0.0079|0.0077}}
| period = 811.3{{±|5.2|5.3}}
| semimajor = 1.570{{±|0.017}}
| eccentricity = 0.089{{±|0.04|0.061}}
}}
{{Orbitbox end}}

==References==
{{reflist|35em|refs=

<ref name=aaa474_2_653>{{citation | first1=F. | last1=van Leeuwen |date=November 2007 | title=Validation of the new Hipparcos reduction | journal=Astronomy and Astrophysics | volume=474 | issue=2 | pages=653–664 | bibcode=2007A&A...474..653V | doi=10.1051/0004-6361:20078357 |arxiv = 0708.1752 | s2cid=18759600 }}</ref>

<ref name="Simbad">{{cite simbad | title=HD 156668 | access-date=2010-02-09 }}</ref>

<ref name=apj726_2_73>{{citation | display-authors=1 | last1=Howard | first1=Andrew W. | last2=Johnson | first2=John Asher | last3=Marcy | first3=Geoffrey W. | last4=Fischer | first4=Debra A. | last5=Wright | first5=Jason T. | last6=Henry | first6=Gregory W. | last7=Isaacson | first7=Howard | last8=Valenti | first8=Jeff A. | last9=Anderson | first9=Jay | last10=Piskunov | first10=Nikolai E. | title=The NASA-UC Eta-Earth Program. II. A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses | journal=The Astrophysical Journal | volume=726 | issue= 2 | page=73 |date=January 2011 | doi=10.1088/0004-637X/726/2/73 | bibcode=2011ApJ...726...73H |arxiv = 1003.3444 | s2cid=15559379 }}</ref>

<ref name=sherrod_koed2003>{{citation | first1=P. Clay | last1=Sherrod | first2=Thomas L. | last2=Koed | title=A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations | series=Astronomy Series | publisher=Courier Dover Publications | year=2003 | isbn=0-486-42820-6 | page=9 | url=https://books.google.com/books?id=4zjv84hHNPcC&pg=PA9 }}</ref>

<ref name=csiro>{{citation | title=The Colour of Stars | date=December 21, 2004 | work=Australia Telescope, Outreach and Education | publisher=Commonwealth Scientific and Industrial Research Organisation | url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html | access-date=2012-01-16 | archive-url=https://web.archive.org/web/20120318151427/http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html | archive-date=March 18, 2012 | url-status=dead }}</ref>

}}

==External links==
* {{citation | title=Star : HD 156668 | url=http://exoplanet.eu/star.php?st=HD%20156668 | archive-url=https://archive.today/20130213071749/http://exoplanet.eu/star.php?st=HD+156668 | url-status=dead | archive-date=February 13, 2013 | work=[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] | access-date=2010-02-09 }}
* {{citation | title=BD+29 2979 / HD 156668 | url=http://www.solstation.com/stars3/hd156668.htm | work=Sol Station | access-date=2010-02-09 }}
* {{citation | title= K stars within 100 light-years | url= http://www.solstation.com/stars3/100-ks.htm | work=Sol Station | access-date=2010-02-09}}

{{Stars of Hercules}}

{{DEFAULTSORT:HD 156668}}
[[Category:Henry Draper Catalogue objects|156668]]
[[Category:Hercules (constellation)]]
[[Category:K-type main-sequence stars]]
[[Category:Hipparcos objects|084607]]
[[Category:Durchmusterung objects|BD+29 2979]]
[[Category:2MASS objects|J17174049+2913378]]
[[Category:Wolf objects|0646]]

{{Sourceattribution|HD 156668}}