BL Lacertae objects are radio galaxies characterized by rapidly varying luminosity, no emission lines in the spectrum, and point-like appearance. As with blazars, they are aligned so that their plasma jets point in our direction. (BL Lacertae objects can be defined as a blazar whose spectrum is dominated by the frequencies of (to us) visible light.)

(The name is actually a remnant from an original misidentification. The first observed BL Lacertae galaxy was mistaken for a variable star in our own galaxy that was named BL Lacertae. Only later was it realized that it was not a variable star but a distant galaxy exhibiting these peculiar properties. Nevertheless, the name stuck to the class of objects exhibiting the same features.)

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