BL Lacertae Object
BL Lacertae objects, abbreviated BL Lac, are one subclass of active galactic nuclei (AGN), the extremely energetic nuclei of active galaxies. Roughly 40 BL Lac objects are known. Perhaps the most obvious property of BL Lac objects is that they look like stars. Astronomers originally thought the prototype, BL Lac, was a star. In fact, BL Lacertae is normally a variable star designation, two letters followed by a constellation name, because astronomers originally thought that BL Lac was a star whose brightness varies, a variable star. BL Lac objects do however have properties that are clearly not stellar.
Unlike most stars, BL Lac objects are very strong sources of radio and infrared emission. This emission, which is called synchrotron emission, arises from electrons traveling near the speed of light in spiral paths in strong magnetic fields. Synchrotron emission generally is polarized, so BL Lac objects have polarized emission. When light or other electromagnetic radiation is polarized, the directions of the oscillations are the same. The amount of polarization and the brightness of BL Lac objects is highly variable. This variability is very rapid and erratic. They can change very significantly in times as short as 24 hours or less. The rapid variability tells us that the energy source is very small. Nothing can travel faster than the speed of light, including whatever signal or mechanism causes the BL Lac object to change its brightness. Therefore, if a BL Lac object changes its brightness significantly in a day its energy source must be less than one light day in radius.
The spectrum of a BL Lac object contains very few if any absorption or emission lines, which are caused by interstellar gas. Their essentially featureless spectra tell us that there is very little interstellar gas around BL Lac objects. There is evidence for a faint fuzziness in some pictures of BL Lac objects. This fuzziness is most likely the host galaxy, of which the BL Lac object is the active nucleus.
Currently, the most popular explanation of BL Lac objects is that they are the central very energetic nuclei of galaxies. The small central energy source in the nucleus is probably a supermassive black hole. However, astronomers are still very uncertain of their nature. We need to continue studying BL Lacertae objects to better understand them.