Quasars are active galactic nuclei. Markarian 231, for example, consists of a pair of ultra-massive black holes that have masses of 4 and 150 million solar masses, orbiting each other at an enormous speed at a distance of 600 million light-years from our Solar System and emitting electromagnetic radiation in the whole spectrum.
The brightest objects in the universe are too far away to study them in high resolution. Therefore, astronomers were excited about the discovery of microquasars. What proper quasars present at the scale of whole galaxies, microquasars mimic at the scale of a solar system.
SS 433, one such microquasar, is at a distance of only 18,000 light-years from Earth. It consists of a black hole that has a mass of several solar masses, was produced in a supernova, and is siphoning off matter from a spectral class A super giant. This irregular pair circles each other every 13 days, generating very high-energy jets.
In the case of SS 433, these jets point into space so that they and also their source can be observed perfectly from Earth. Due to this arrangement, astronomers could determine that the brightest sources are not the two celestial bodies of SS 433, instead the points where the jets intersect the matter ejected by the supernova explosion.