description of class 8.6 BL Lacs WUPPE PROGRAM CLASS 8.6

BL LACERTAE OBJECTS

UW Astronomer: A. Weitenbeck

BL Lac objects are galaxies with an active nucleus which is so bright that most of the light which we observe comes from the nucleus rather than the rest of the galaxy. In some BL Lacs the nucleus is so bright that the fuzzy blob (= how a faint galaxy looks on a photograph) of the galaxy is not visible; BL Lacertae itself is such a case: on a picture, it looks starlike, and so has a variable star name; in other cases the fuzz of the galaxy may be visible around the outside. The usual model adopted for this kind of active galaxy is that the active nucleus is shooting out a jet almost directly toward us (instead of well off the line of sight, as can be observed in other kinds of radio galaxies). Jets vary in brightness and in polarization, as material is shot out along them.