Quasars are the brightest objects in the universe and display a mysterious diversity in their appearance that has puzzled astronomers for over two decades. Scientists believe that the mystery can now be solved.
The mystery can be solved by looking at two simple features of quasars - how quickly matter is being fed into the quasars and the direction from which they are being seen.
Quasars are super-massive black holes, up to billions of times the mass of the sun, that live at the hearts of distant, massive galaxies.
Though previous studies found that the physical properties of quasars follow definite, regular trends, for some puzzling reasons, they can vary greatly in appearance in visible and ultraviolet light.
To help solve the mystery, scientists examined the largest sample of quasar observations yet - data from over 20,000 quasars.
"Our work solves a two-decade-long mystery in quasar research," said Yue Shen, lead author and astronomer at the Carnegie Observatories in Pasadena, California.
The first factor is the so-called Eddington ratio - the luminosity of a quasar compared with its mass.
This ratio predicts how quickly matter is falling into a quasar and was long suspected to play a major role in why quasars often varied in appearance.
The other factor is the direction from which astronomers look at a quasar.
"These findings will help improve future measurements of black hole mass to help scientists better understand the cosmic growth of super-massive black holes and their place in galaxy formation," said Luis Ho from the Kavli Institute for Astronomy and Astrophysics at Peking University.
The study appeared in the journal Nature.
