BL Lacertae 
Lacerta, the lizard, is a small constellation that’s lodged between Cygnus and Andromeda. The hazy band of the Milky Way runs through it, so you might expect its best-known objects to reside inside our galaxy. In fact, though, none of Lacerta’s stars is particularly bright, so its best-known object is a strange galaxy nearly a billion light-years beyond its stars.
For decades, astronomers mistook that object for a mere star in the Milky Way. But in 1929, they discovered that its light varied from night to night. They christened the object BL Lacertae.
In the 1960s, though, astronomers realized that BL Lacertae was no star. Instead, they detected the fuzzy glow of a galaxy around it. What’s more, the object emitted lots of radio waves — something that no “normal” star does.
Other distant galaxies resemble BL Lacertae — so much so that BL Lacertae serves as the prototype for an entire class of these objects. All of them shine profusely at all wavelengths, from radio to visible light to gamma rays.
Each galaxy is powered by a large black hole at its center. Material falling into the black hole gets heated to extreme temperatures, so it emits huge amounts of radiation. Some of this material shoots away from the black hole as high-speed jets. In BL Lacertae, the jet happens to be aimed right at us, so even slight changes in its angle can cause the variability that first caught astronomers’ attention nearly a century ago.
Script by Ken Croswell, Copyright 2012