{{Short description|Star in the constellation Boötes}} {{Starbox begin}} {{Starbox image | image = [[Image:BLBooLightCurve.png|250px]] | caption = A [[Photometric_system#Photometric_letters|visual band]] [[light curve]] for BL Boötes, adapted from Schmidt (2002)<ref name="Schmidt"/> }} {{Starbox observe | epoch=J2000 | constell = [[Boötes]] | ra = {{RA|14|05|40.463}}<ref name=dr3/> | dec = {{DEC|+28|29|12.28}}<ref name=dr3/> | appmag_v = 14.45 - 15.10<ref name=AAVSOBL/> }} {{Starbox character | type = | class = kA2/3hA9/F0V<ref name=AAVSOBL/> | r-i = | v-r = | b-v = 0.12 - 0.25<ref name=zinn1976/> | u-b = 0.16 - 0.05<ref name=zinn1976/> | variable = [[Anomalous Cepheid]]<ref name=AAVSOBL/> }} {{Starbox astrometry | radial_v = +115<ref name=zinn1982/> {{nowrap|(+90 to +160)<ref name=mccarthy1997/>}} | prop_mo_ra = −5.303<ref name=dr3/> | prop_mo_dec = −0.770<ref name=dr3/> | parallax = 0.0114 | p_error = 0.0220 | parallax_footnote = <ref name=dr3>{{cite Gaia DR3|1452625254531322752}}</ref> | dist_pc = 16,000<ref name=mccarthy1997/> | absmag_v = −1.27<ref name=mccarthy1997/> }} {{Starbox detail | mass = 1.56<ref name=mccarthy1997/> | temperature = 7,010<ref name=mccarthy1997/> | radius = 11.0<ref name=zinn1976>{{cite journal |doi=10.1086/111916 |title=Variable 19 in NGC 5466 : An anomalous cepheid in a globular cluster |year=1976 |last1=Zinn |first1=R. |last2=Dahn |first2=C. C. |journal=The Astronomical Journal |volume=81 |page=527 |bibcode=1976AJ.....81..527Z }}</ref> | gravity = 2.22<ref name=mccarthy1997/> | metal_fe = −1.92<ref name=mccarthy1997/> | luminosity = 278<ref name=mccarthy1997/> | rotational_velocity = <18<ref name=zinn1982/> | component2 = minimum | temperature2 = 6,405<ref name=mccarthy1997/> | gravity2 = 2.55<ref name=mccarthy1997/> }} {{Starbox reference | Simbad=BL+Boo | ARICNS = }} {{Starbox end}}

'''BL Boötis''' (abbreviated to '''BL Boo''') is a [[pulsating star]] in the constellation [[Boötes]]. It is the prototype of a class of anomalous [[Cepheids]] which is intermediate in the [[H-R diagram]] between the type I [[classical Cepheid]]s and the [[type II Cepheid]]s.

It varies from magnitude 14.45 to 15.10 over 0.82 days.<ref name=AAVSOBL>{{cite web|url=https://www.aavso.org/vsx/index.php?view=detail.top&oid=4392|title=BL Boötis |author =Otero, Sebastian Alberto |date=23 November 2011 |work=AAVSO Website|publisher=[[American Association of Variable Star Observers]]|access-date=19 January 2019}}</ref> It is located 4 arcminutes from the centre of (and assumed to be a member star of) the [[globular cluster]] [[NGC 5466]]. Its variability was first noted in 1961 by Russian astronomer Nikolaĭ Efimovich Kurochkin, who gave it the [[variable star designation]] BL Boötis. However, he thought it was an [[eclipsing binary]]. It was subsequently thought to be an [[RR Lyrae variable]] by T.I. Gryzunova in 1971.<ref name="zinn76">{{cite journal|last=Zinn|first=Robert|author2=Dahn, Conard C. |date=1976|title=Variable 19 in NGC 5466: an anomalous cepheid in a globular cluster|journal=Astronomical Journal|volume=81|pages=527&ndash;33, 565|bibcode=1976AJ.....81..527Z|doi=10.1086/111916}}</ref>

Robert Zinn confirmed it was a member of the globular cluster and found it was too blue to be an [[RR Lyrae variable]]. He gave it the name V19 within the cluster.<ref name="zinn76"/> He calculated its [[solar mass|mass]] to be around 1.56 times and its [[luminosity]] to be around 278 times that of the Sun; its [[absolute magnitude]] is -1.27.<ref name="zinn82">{{cite journal|last=Zinn|first=Robert|author2=King, Christopher R. |date=1982|title=The mass of the anomalous cepheid in the globular cluster NGC 5466|journal=Astrophysical Journal|volume=262|pages=700–08|bibcode=1982ApJ...262..700Z|doi=10.1086/160462|doi-access=free}}</ref> The [[stellar spectrum|spectrum]] has been compared to an A2 or A3 [[main sequence]] star on the basis of its [[Calcium K line|K lines]] and A9 or F0 on the basis of its hydrogen lines. The discrepancy is due to a strong deficiency in metals, around 100 times lower than that of the [[Sun]]. Despite the spectral classification, it is not thought to be a main-sequence star, at least not a normal one. It is larger and more luminous than its spectrum would indicate, comparable to a [[horizontal branch]] star but more massive than comparable stars on the horizontal branch.<ref name=zinn1976/> The pulsations are thought to be in the [[overtone|first overtone]].<ref name=zinn1982>{{cite journal |doi=10.1086/160462 |title=The mass of the anomalous cepheid in the globular cluster NGC 5466 |year=1982 |last1=Zinn |first1=R. |last2=King |first2=C. R. |journal=The Astrophysical Journal |volume=262 |page=700 |bibcode=1982ApJ...262..700Z |doi-access=free }}</ref>

BL Boötis has been designated the prototype of a rare class of variable star known as an [[anomalous Cepheid]] or BL Boötis variable.<ref name=good/> These stars are somewhat similar to [[Cepheid variable]]s, but they do not have the same relationship between their period and luminosity. Their periods are similar to the ab subtypes of RR Lyrae variables; however, they are far brighter than these stars. Anomalous Cepheids are [[metallicity|metal poor]] and have masses not much larger than the Sun's, on average, 1.5 [[solar mass]]es.<ref name=good>{{cite book | last = Good | first = Gerry A. | year = 2003 | title = Observing Variable Stars | url = https://archive.org/details/observingvariabl0000good/page/61 | url-access = registration | publisher = Springer | isbn = 978-1-85233-498-7 | pages = [https://archive.org/details/observingvariabl0000good/page/61 61, 69&ndash;70] }}</ref> The origin of these stars is uncertain, but thought to possibly be from the merger of two stars.<ref>{{cite book|last=Balona|first= L. A. |title=Challenges in Stellar Pulsation|publisher=Bentham Science Publishers|date=2010|page=135|isbn=978-1-60805-185-4|url=https://books.google.com/books?id=tvmWBWm2MIwC&pg=PA135e}}</ref> Detailed examination of the spectrum of BL Boötis with the Keck-1 telescope at the [[W. M. Keck Observatory]] showed that its [[effective temperature|effective (surface) temperature]] is around 6450 K at minimum light. It also showed that the chemical composition was consistent with ageing metal-poor (Population II) stars and hence cast doubt on the origin as a result of a stellar merger. The radial velocity is lower than would be expected if it were from a stellar merger.<ref name=mccarthy1997>{{cite journal | title = The Chemical Composition and Period Change Rate of the Anomalous Cepheid V19 in NGC 54661 |author1=McCarthy, James K. |author2=Nemec, James M. | journal= The Astrophysical Journal |volume = 482 |issue = 1 |pages= 203&ndash;29 |year=1997 | bibcode =1997ApJ...482..203M | doi=10.1086/304118|doi-access=free }}</ref>

==References== <references> <ref name="Schmidt">{{cite journal |last1=Schmidt |first1=Edward G. |title=The Intermediate-Period Cepheid Strip Stars |journal=The Astronomical Journal |date=February 2002 |volume=123 |issue=2 |pages=965–982 |doi=10.1086/338439 |bibcode=2002AJ....123..965S |s2cid=43762088 |url=https://ui.adsabs.harvard.edu/abs/2002AJ....123..965S/abstract |access-date=4 November 2021|doi-access=free }}</ref> </references>

{{Stars of Boötes}}

{{DEFAULTSORT:BL Bootis}} [[Category:Boötes]] [[Category:Objects with variable star designations|Bootis, BL]] [[Category:Cepheid variables]] [[Category:A-type stars]]