BL Lacertae or ''BL Lac'' is a highly variable, extragalactic active galactic nucleus (AGN or

active galaxy An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars. Such e ...

). It was first discovered by

Cuno Hoffmeister Cuno Hoffmeister (2 February 1892 – 2 January 1968) was a German astronomer, observer and discoverer of variable stars, comets and minor planets, and founder of Sonneberg Observatory. Born in Sonneberg in 1892 to Carl and Marie Hoffmeister, Cu ...

in 1929, but was originally thought to be an irregular

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

in the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

galaxy and so was given a

variable star designation In astronomy, a variable-star designation is a unique identifier given to variable stars. It extends the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the constellation in whic ...

. In 1968, the "star" was identified by John Schmitt at the

David Dunlap Observatory The David Dunlap Observatory (DDO) is an astronomical observatory site in Richmond Hill, Ontario, Canada. Established in 1935, it was owned and operated by the University of Toronto until 2008. It was then acquired by the city of Richmond Hill ...

as a bright, variable radio source. A faint trace of a host

galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...

was also found. In 1974, Oke and

Gunn Gunn may refer to: Places * Gunn City, Missouri, a village * Gunn, Northern Territory, outer suburb of Darwin * Gunn, Alberta, Canada, a hamlet * Gunn Valley, a mountain valley in British Columbia, Canada * Gun Lake (British Columbia), a Cana ...

measured the

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...

of BL Lacertae as z = 0.07, corresponding to a recession velocity of 21,000 km/s with respect to the Milky Way. The redshift figure implies that the object lies at a distance of 900 million light years. BLLacLightCurve

Due to its early discovery, BL Lacertae became the prototype and namesake of the class of

active galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars. Such e ...

known as "

BL Lacertae objects A BL Lacertae object or BL Lac object is a type of active galactic nucleus (AGN) or a galaxy with such an AGN, named after its prototype, BL Lacertae. In contrast to other types of active galactic nuclei, BL Lacs are characterized by rapid and l ...

" 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 A quasar ( ) is an extremely Luminosity, luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by accretion onto a supermassive black hole with a mass rangi ...

. These characteristics are understood to result from

relativistic beaming In physics, relativistic beaming (also known as Doppler beaming, Doppler boosting, or the headlight effect) is the process by which relativistic effects modify the apparent luminosity of emitting matter that is moving at speeds close to th ...

of emission from a jet of plasma ejected from the vicinity of a

supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (). Black holes are a class of astronomical ...

. BL Lac objects are also categorized as a type of

blazar A blazar is an active galactic nucleus (AGN) with a relativistic jet (a jet composed of ionized matter traveling at nearly the speed of light) directed very nearly towards an observer. Relativistic beaming of electromagnetic radiation from the ...

. BL Lacertae changes in

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

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.

References

External links

* * {{DEFAULTSORT:BL Lacertae BL Lacertae objects Lacerta Blazars Lacertae, BL