View source for BL Lacertae

{{Short description|Active galaxy in the constellation Lacerta}}
{{For|the music group|BL Lacerta}}
{{Quasar
| name = BL Lacertae
| image = [[Image:BL Lacertae Pan-STARRS DR1.jpg|300px]]
| caption= The active galaxy BL Lacertae (center), from the [[Pan-STARRS]] Data Release 1. The bright object at the left is a foreground star within the Milky Way. Green pixels are artifacts due to data processing.
| epoch = [[J2000]]
| ra = {{RA|22|02|43.3}}
| dec = {{DEC|+42|16|40}}
| constellation name = [[Lacerta]]
| z = 0.07
| type = [[BL Lac object|BL Lac]]
| dist_ly = {{convert|0.9|Gly|Gpc|3|lk=on|abbr=on}}
| appmag_v = var. ~14 to ~17
| names = OY+401, VR 42.22.01
}}
{{Sky|22|02|43.3|+|42|16|40|1000000}}
'''BL Lacertae'''  or ''BL Lac'' is a highly variable, extragalactic active galactic nucleus (AGN or [[active galaxy]]). It was first discovered by [[Cuno Hoffmeister]] in 1929,<ref>{{cite journal | last= Hoffmeister |first= Cuno |year= 1929 |title=354 neue Veränderliche|journal= Astronomische Nachrichten |volume= 236 | issue= 15 |pages= 233–244 | bibcode=1929AN....236..233H |doi=10.1002/asna.19292361502}}</ref> but was originally thought to be an irregular [[variable star]] in the [[Milky Way]] galaxy and so was given a [[variable star designation]]. In 1968, the "star" was identified by John Schmitt at the [[David Dunlap Observatory]] as a bright, variable [[astronomical radio source|radio source]]. A faint trace of a host [[galaxy]] was also found.<ref>{{cite journal|last=Schmitt |first=John L. |date=May 1968 |title=BL Lac identified as a Radio Source |journal=Nature |volume=218 |issue=5142 |pages=663 |doi=10.1038/218663a0 |bibcode=1968Natur.218..663S |s2cid=4213061 |doi-access=free }}</ref> In 1974, Oke and [[James E. Gunn (astronomer)|Gunn]] measured the [[redshift]] of BL Lacertae as z = 0.07, corresponding to a recession velocity of 21,000&nbsp;km/s with respect to the Milky Way.<ref>{{cite journal | last = Oke | first = J. B. |author2=Gunn, J. E. |
year = 1974 | title = The Distance of BL Lacertae | journal = Astrophysical Journal Letters | volume = 189 | pages = 5 | bibcode = 1974ApJ...189L...5O | doi = 10.1086/181450 | doi-access = free }}</ref> The redshift figure implies that the object lies at a distance of 900 million light years.
[[File:BLLacLightCurve.png|thumb|left|A [[Photometric_system#Photometric_letters|visual band]] [[light curve]] for BL Lacertae, plotted from AAVSO data<ref name="aavso"/>]]
Due to its early discovery, BL Lacertae became the prototype and namesake of the class of [[Active galactic nucleus|active galactic nuclei]] known as  "[[BL Lac object|BL Lacertae objects]]" or "BL Lac objects".  This class is distinguished by rapid and high-amplitude brightness variations and by optical spectra devoid (or nearly devoid) of the broad emission lines characteristic of [[quasars]].  These characteristics are understood to result from [[relativistic beaming]] of emission from a [[Astrophysical jet|jet]] of plasma ejected from the vicinity of a [[supermassive black hole]]. BL Lac objects are also categorized as a type of [[blazar]].
BL Lacertae changes in [[apparent magnitude]] over fairly small time periods, typically between values of 14 and 17. In January 2021, it exhibited extreme flaring behavior and was reported to reach magnitude 11.45 in the R filter band.<ref>{{cite web |last1=Bonnoli |first1=Giacomo |title=ATel #14329 |url=http://www.astronomerstelegram.org/?read=14329 |website=The Astronomer's Telegram |access-date=20 January 2021}}</ref>
{{clear left}}
==References==
{{reflist|refs=
<ref name="aavso">{{cite web |title=Download Data |url=https://www.aavso.org/data-download |website=aavso.org |publisher=AAVSO |access-date=1 October 2021}}</ref>
}}
==External links==
* {{cite web|url=https://www.aavso.org/vsots_bllac|title=BL Lacertae|website=aavso.org|date=13 April 2010}}
* {{cite simbad|title=NAME BL Lac|accessdate=3 April 2018}}
{{Stars of Lacerta}}
{{Authority control}}
{{DEFAULTSORT:BL Lacertae}}
[[Category:BL Lacertae objects]]
[[Category:Lacerta]]
[[Category:Blazars]]
[[Category:Objects with variable star designations|Lacertae, BL]]