BL Lacertae
This exciting object was discovered in 1929 by the German astronomer Cuno Hoffmeister, who at first thought it was a variable star - therefore the typical
name for variable stars "BL Lac". That the "star" is something else than thought became clear in 1968, when BL Lac was identified as a very strong radio
source.
Today it is certain that "BL Lac" belongs to the group of Active Galactic Nuclei (AGN). These special galaxies are characterized by the fact that they
host a very small active region in the center, which is usually not larger than our solar system. Responsible for this is a supermassive black hole (SMBH),
which accretes enormous amounts of gas. Of course, this results in a very high luminosity, which is why objects of this type can be seen at very large
distances. In the case of "BL Lac" the distance is about 900 million lightyears with a measured redshift of z=0.07 and an escape velocity of 21.000 km/s.
So every 20 seconds "BL Lac" moves away from us by the distance Earth-Moon.
The brightness of "BL Lac" usually varies between 13.0mag and 15.5mag. In 2021 an outburst occurred, which increased the brightness to 11.9mag.
The object could be seen in relatively small telescopes at that time.
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In my 20" telescope, "BL Lac" is usually a very easy object to spot with averted vision at magnifications of V=200x. However, more than a star-shaped
point of light cannot be seen at 200x. Under very dark skies at magnifications of 270x and more an extremely faint halo can be noticed, which can be
seen briefly every now and then. This is the host galaxy, which harbors the AGN.
