{{Short description|Star in the constellation Draco}} {{Starbox begin}} {{Starbox image | image= {{Location mark |image=Draco constellation map.svg|alt=|float=center|width=280 |label=|position=right |mark=Red circle.svg|mark_width=10|mark_link=18 Dra |x=472|y=718 }} |caption=Location of 18 Dra (circled in red) }} {{Starbox observe | epoch = J2000 | constell = [[Draco (constellation)|Draco]] | ra = {{RA|16|40|55.11952}}<ref name=GaiaDR2/> | dec = {{DEC|+64|35|20.5824}}<ref name=GaiaDR2/> | appmag_v = 4.84<ref name=Anderson2012/> }} {{Starbox character | type = [[horizontal branch]]<ref name=reffert2015/> | class = {{nowrap|K0 III CN−0.5 CH−2 Ca1}}<ref name=perkins1989/> | b-v = {{Val|1.212|0.003}}<ref name=Anderson2012/> | u-b = }} {{Starbox astrometry | radial_v = {{Val|−1.37|0.09}}<ref name=deBruijne2012/> | prop_mo_ra = −0.599<ref name=GaiaDR2/> | prop_mo_dec = −17.436<ref name=GaiaDR2/> | parallax = 4.5045 | p_error = 0.1730 | parallax_footnote = <ref name=GaiaDR2/> | absmag_v = −1.92<ref name=Anderson2012/> }} {{Starbox detail | source = <ref name=reffert2015/> | mass = {{Val|3.81|0.38}} | radius = {{Val|46.83|1.74}} | luminosity = {{Val|786.7|56.3}} | gravity = {{Val|1.69|0.06}} | temperature = {{Val|4471|23|fmt=commas}} | metal_fe = {{Val|−0.13|0.10}} | rotational_velocity = | age_myr = {{Val|280|80}} }} {{Starbox catalog | names = {{odlist | B=g Dra | F=18 Dra | BD=+64°1145 | FK5=3326 | HD=151101 | HIP=81660 | HR=6223 | SAO=17188 }}<ref name=SIMBAD/> }} {{Starbox reference | Simbad = g+Dra }} {{Starbox end}}
[[Image:18 Draconis.jpg|thumb|left|18 Draconis in optical light]] '''18 Draconis''' is a likely [[binary star]]<ref name=Eggleton2008/> system in the northern [[circumpolar constellation]] of [[Draco (constellation)|Draco]]. With an [[apparent visual magnitude]] of 4.84,<ref name=Anderson2012/> it is just bright enough to be faintly visible to the naked eye. The distance to this system, as estimated from an annual [[stellar parallax|parallax shift]] of {{Val|4.5|ul=mas}},<ref name=GaiaDR2/> is roughly 720 [[light year]]s (220 [[parsecs]]). It is moving closer to the [[Sun]] with a heliocentric [[radial velocity]] of −1.4 km/s,<ref name=deBruijne2012/> and is a probable member of the [[Sirius]] stream of co-moving stars.<ref name=Famaey2005/>
The visible component has a [[stellar classification]] of {{nowrap|K0 III CN−0.5 CH−2 Ca1}},<ref name=perkins1989/> indicating it is an [[stellar evolution|evolved]] [[K-type star|K-type]] [[giant star]] with some [[Chemically peculiar star|abundance peculiarities]] in its [[stellar atmosphere|atmosphere]]. At the age of around 280 million years, it is most likely (99% chance) on the [[horizontal branch]].<ref name=reffert2015/> It is a [[barium star]], which suggests it may have a degenerate [[white dwarf]] companion from which it accreted materials during an earlier stage of its evolution.<ref name=gomez/><ref name=Eggleton2008/> 18 Dra has an estimated 3.8 times the [[mass of the Sun]] and has expanded to 47 times the [[Sun's radius]]. The star is radiating 787 times the [[Sun's luminosity]] from its enlarged [[photosphere]] at an [[effective temperature]] of 4,471 K.<ref name=reffert2015/>
==References== <references> <ref name=SIMBAD>{{cite simbad | title=g Dra | access-date=2018-07-17 }}</ref>
<ref name=GaiaDR2>{{cite DR2}}</ref>
<ref name=reffert2015>{{citation | display-authors=1 | last1=Reffert | first1=Sabine | last2=Bergmann | first2=Christoph | last3=Quirrenbach | first3=Andreas | last4=Trifonov | first4=Trifon | last5=Künstler | first5=Andreas | title=Precise radial velocities of giant stars. VII. Occurrence rate of giant extrasolar planets as a function of mass and metallicity | journal=Astronomy & Astrophysics | volume=574 | pages=A116 | year=2015 | bibcode=2015A&A...574A.116R | doi=10.1051/0004-6361/201322360 | arxiv=1412.4634 | postscript=. | hdl=10722/215277| s2cid=59334290 }}</ref>
<ref name=deBruijne2012>{{citation | last1=de Bruijne | first1=J. H. J. | last2=Eilers | first2=A.-C. | title=Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project | journal=Astronomy & Astrophysics | volume=546 | id=A61 | pages=14 | date=October 2012 | doi=10.1051/0004-6361/201219219 | bibcode=2012A&A...546A..61D | arxiv=1208.3048 | s2cid=59451347 | postscript=. }}</ref>
<ref name=Eggleton2008>{{citation | title=A catalogue of multiplicity among bright stellar systems | last1=Eggleton | first1=P. P. | last2=Tokovinin | first2=A. A. | journal=Monthly Notices of the Royal Astronomical Society | volume=389 | issue=2 | pages=869 | date=2008 | doi=10.1111/j.1365-2966.2008.13596.x | doi-access=free | postscript=. | bibcode=2008MNRAS.389..869E | arxiv=0806.2878 | s2cid=14878976 }}</ref>
<ref name=gomez>{{citation | display-authors=1 | last1=Gomez | first1=A. E. | last2=Luri | first2=X. | last3=Grenier | first3=S. | last4=Prevot | first4=L. | last5=Mennessier | first5=M. O. | last6=Figueras | first6=F. | last7=Torra | first7=J. | title=Absolute magnitudes and kinematics of barium stars | journal=Astronomy and Astrophysics | volume=319 | pages=881 | year=1997 | bibcode=1997A&A...319..881G | postscript=. }}</ref>
<ref name=Famaey2005>{{citation | display-authors=1 | last1=Famaey | first1=B. | last2=Jorissen | first2=A. | last3=Luri | first3=X. | last4=Mayor | first4=M. | last5=Udry | first5=S. | last6=Dejonghe | first6=H. | last7=Turon | first7=C. | title=Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters | journal=Astronomy and Astrophysics | volume=430 | pages=165–186 | date=2005 | doi=10.1051/0004-6361:20041272 | bibcode=2005A&A...430..165F | postscript=. | arxiv=astro-ph/0409579 | s2cid=17804304 }}</ref>
<ref name=perkins1989>{{citation | postscript=. | 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 | s2cid=123149047 }}</ref>
<ref name=Anderson2012>{{citation | title=XHIP: An extended hipparcos compilation | last1=Anderson | first1=E. | last2=Francis | first2=Ch. | journal=Astronomy Letters | volume=38 | issue=5 | pages=331 | year=2012 | bibcode=2012AstL...38..331A | doi=10.1134/S1063773712050015 | arxiv=1108.4971 | s2cid=119257644 | postscript=. }}</ref> </references>
{{Stars of Draco}}
[[Category:K-type giants]] [[Category:Barium stars]] [[Category:Binary stars]] [[Category:Draco (constellation)]] [[Category:Bayer objects|Draconis, g]] [[Category:Durchmusterung objects]] [[Category:Flamsteed objects|Draconis, 18]] [[Category:Henry Draper Catalogue objects|151101]] [[Category:Hipparcos objects|081660]] [[Category:Bright Star Catalogue objects|6223]] [[Category:Horizontal-branch stars]]