BL Lacertae
Encyclopedia
BL Lacertae or BL Lac is a highly variable, extragalactic AGN (active galactic nucleus or active galaxy). It was first discovered by Cuno Hoffmeister
in 1929, 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 radio source. A faint trace of a host galaxy
was also found. In 1974, Oke and Gunn
measured the redshift
of BL Lacertae as z = 0.07, corresponding to a recession velocity of 21,000 km/s with respect to the Milky Way.
BL Lacertae is the original member of a type of blazar
known as "BL Lacertae objects
"; also termed simply "BL Lac objects". This class is distinguished by optical spectra devoid of the broad emission lines characteristic of quasars. On occasion, however, BL Lacertae has been observed to display weak emission lines.
BL Lacertae changes in apparent magnitude
over fairly small time periods between values of 14 and 17. Its location in the sky for epoch 2000.0 is at RA
, DEC
.
Cuno Hoffmeister
Cuno Hoffmeister was a German astronomerand founder of Sonneberg Observatory.Born in Sonneberg in 1892, Hoffmeister obtained his first telescope in 1905 and became an avid amateur astronomer. After his father lost most of his money in 1914, Hoffmeister had to leave school in 1916 to start an...
in 1929, but was originally thought to be an irregular variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
in the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
galaxy and so was given a variable star designation
Variable star designation
Variable stars are named using a variation on the Bayer designation format of an identifying label combined with the Latin genitive of the name of the constellation in which the star lies...
. In 1968, the "star" was identified by John Schmitt at the David Dunlap Observatory
David Dunlap Observatory
The David Dunlap Observatory is a large astronomical observatory site once owned by the University of Toronto, located just north of the city in Richmond Hill, Ontario within a estate. Its primary instrument is a 74-inch reflector telescope, at one time the second largest telescope in the world,...
as a bright, variable radio source. A faint trace of a host galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
was also found. In 1974, Oke and Gunn
James E. Gunn
James Edward Gunn is the Eugene Higgins Professor of Astronomy at Princeton University. Gunn's early theoretical work in astronomy has helped establish the current understanding of how galaxies form, and the properties of the space between galaxies...
measured the redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
of BL Lacertae as z = 0.07, corresponding to a recession velocity of 21,000 km/s with respect to the Milky Way.
BL Lacertae is the original member of a type of blazar
Blazar
A blazar is a very compact quasar associated with a presumed supermassive black hole at the center of an active, giant elliptical galaxy...
known as "BL Lacertae objects
BL Lac object
A BL Lacertae object or BL Lac object is a type of active galaxy with an active galactic nucleus and is named after its prototype, BL Lacertae. In contrast to other types of active galactic nuclei, BL Lacs are characterized by rapid and large-amplitude flux variability and significant optical...
"; also termed simply "BL Lac objects". This class is distinguished by optical spectra devoid of the broad emission lines characteristic of quasars. On occasion, however, BL Lacertae has been observed to display weak emission lines.
BL Lacertae changes in apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
over fairly small time periods between values of 14 and 17. Its location in the sky for epoch 2000.0 is at RA
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
, DEC
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
.