Blazars (BL Lacertae Objects)


There is a class of objects that, unlike quasars which have emission lines in their spectra, have a continuous spectra and which vary in the same way that quasars do.  In many cases they have been identified with radio sources.  They are called Lacertids, in honor of the first one discovered, BL Lacertae, located in the constellation Lacerta (the Lizard) [1].

The object BL Lacertae (BL Lac for short) had been in the catalogue of variable stars for some time.  It was originally discovered by Cuno Hoffmeister in 1929 [2].  In 1965 Arp and Burbidge found, that 3C 120, a radio source detected at Cambridge in 1959, is a Seyfert galaxy.  In 1968 Penston noticed that the object was located at the same position as the variable star BW Tau in the constellation Taurus (discovered in 1940 by Hanley and Shapley at Harvard College Observatory).  This is sited as the first discovery of class of object.  Also in 1968, J. Schmitt [3] noticed that there was a variable radio source located at the same position as BL Lac.  The radio source VRO 42.22.01 had been detected at the Vermilion River Observatory by MacLeod et al.  in 1965 [4].  BL Lac was not a periodic variable, but rather it's intensity varied irregularly with no apparent pattern to the brightening and dimming.  When the spectra of this "variable star" was taken, it was discovered that the optical spectra were featureless.  There were no emission lines as from quasars and no absorption lines as found in most stars.  Peter Strittmatter (University of Arizona) and several others had identified four other objects all similar to BL Lac by 1972.

Later in 1974, two Caltech astronomers (James E. Gunn and J. B. Oke) determined that BL Lac was actually located in a normal elliptical galaxy by blocking the light from the central part of the object.  The light from the surrounding area had absorption lines that indicated a red shift of 0.07.  At the time,  this indicated that the object was about 420 megaparsecs away.  This indicated that the core at its brightest had a luminosity of 1046 ergs/sec.

Ed Spiegel jokingly referred to these objects as blazars and it seems to have stuck.

Current Theories

 BL Lacertae objects are now classified as a type of Active Galactic Nuclei (AGN).

Current Definitions[5]:

[1] Shipman, Harry L., Black Holes, Quasars, and the Universe, 2nd edition (1980), ISBN: 0-395-28499-6

[2] Hoffmeister, C., AN 236, 233 (1929)

[3] Schmitt, J.L., Nature 218, 663 (1968)

[4] MacLeod, J.M., et al., AJ 70, 756 (1965)

[5] TheBelmontSociety™ "Quasars & AGN"

Related Links:

Extragalactic Objects Discovered As Variable Stars, Wolfgang Steinicke, January 2000


Course: PH227 Galaxies,  Blazars, Dr. C.R. Kaiser

The Blazar Link Collection of the Perugia Observatory

CHARA, The Center for High Angular Resolution Astronomy 

Gamma-Ray Astronomy at Berry Collage

The Belmont Society™ -  Quazars & AGN

AAVSO - Variable Star of the Month, BL Lacertae

Active Galactic Nuclei and the Amateur.

Beginner's Guide to AGN

Last Modified January 25, 2009
Author: Allen Jensen
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