BL Lacertae objects (BL Lac for short) exhibit no emission lines but have a strong continuum stretching from rf through X-ray frequencies. Here are spectra of BL Lac and other active galaxies - see the following description. Here is the full spectrum for the BL Lac object Markarian 421, illustrating the broad range of strong emission:

• Spectrum
• Explanation

They can exhibit dramatic variability. Here are some light curves at different wavelengths illustrating the variability in intensity of BL Lac and some other active galaxies. Here is an explanation of these light curves.

The name remains from an original mis-identification of the prototype of such galaxies as 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 with peculiar properties, but the name stuck to the class of objects exhibiting the same features.

Information supplied by: http://csep10.phys.utk.edu