BL Lacertae
Published on Dec 4, 2006 at 4:28 pm.
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Filed under active galaxies, galaxies.
This time of year, high overhead about sunset for observers in the Northern Hemisphere is the constellation Lacerta. The constellation Lacerta is supposed to be a lizard. It is a rather underwhelming constellation, with no really bright stars. However, Lacerta is home to a number of interesting objects. One of these is BL Lacertae.
The name BL Lacertae is a variable star designation. But, BL Lacertae isn’t a star! Cuno Hoffmeister discovered what appeared to be a variable star in 1926. The object was studied for a number of years to determine what type of variable it was, but it refused to behave like a normal variable star. It varies in magnitude from about 14 to about 17, with changes of magnitude happening in only weeks. But, the variation was seemingly random, not the regular sorts of variation that most variable stars have. By 1941, the mystery “star” was assigned a variable star designation: BL Lacertae. But, by 1969, a radio source was found to lie at the same coordinates as BL Lacertae, and soon thereafter, deeper images showed that there was a faint galaxy surrounding the mystery object. Eventually, astronomers realized that they were looking at the active core of a distant galaxy.
BL Lacertae appears as a point source in small to medium telescopes, and so it is no wonder that it was mistaken for a star for so many years. 
About the time that it was found to be associated with a galaxy, several other similar objects were being found. Collectively, in the 1970’s, they were called blazars to distinquish them from the then still pretty mysterious quasars. The light from BL Lacertae is dominated by non-thermal emissions. For a long time, absorption lines were not even detected, further distinquishing blazars from quasars.
We now believe that the source of the energetic emissions from blazars, or BL Lacertae Objects (as they are often called), is a supermassive black hole at the center of the galaxy. As material falls into the black hole, massive magnetic fields are generated in the accretion disk of the black hole. Not all of the material in the accretion disk actually makes it into the black hole. In fact, black holes are pretty messy eaters, with much of the material falling in simply missing the black hole and being shot into space along jets directed perpendicular to the accretion disk. The magnetic field in the accretion disk further collimates the jets. The paticles shooting out along the jets are moving at very near the speed of light, and so as they slow down, they beam radiation in the direction of travel. 
This relativistic beaming makes the jets look much brighter when you are looking straight into them. That is what we think we are seeing when we see blazars — the jets being directed nearly right at us. VLBA studies of BL Lacertae haves shown evidence of jets spewing from the galaxy. Blazars appear to be very similar, in fact, to Seyfert galaxies, another kind of active galaxy, except that blazars tend to be elliptical galaxies and Seyferts tend to be spiral galaxies. Blazars probably settle down to be radio lobe elliptical galaxies like M87.
So, if you are a serious amateur astronomer, and you really want a challenge, go try to find BL Lacertae.Â
-Astroprof
(Images available from the NASA/IPAC Extragalactic Database-NED, Light curve and observing chart courtesy AAVSO)
