Quasars (quasi-stellar radio sources) are among the most energetic and luminous members of the class of objects known as AGN. These “island universes” are found at the centers of distant galaxies and are powered by supermassive black holes (SMBHs) that serve as their central engines.

Physical Properties:

The Central Engine and Mechanism: The immense energy output of a quasar is produced by a supermassive black hole converting gravitational potential energy into radiation. This process makes quasars so intrinsically bright that they can be observed across vast intergalactic distances. Although supermassive black holes appear to reside in the centers of most large galaxies, only those with sufficient accretion activity exhibit the extreme characteristics of a quasar.

Luminosity and Violent Variability: Quasars are characterized by their high luminosity and frequently exhibit rapid, violent variability. For example, the quasar 3C 446 has been observed to change its optical luminosity by a factor of 40 in as little as 10 days. Mathematically, such rapid fluctuations ($ \Delta t_{obs} $) provide an upper limit on the size of the energy-emitting region, as information cannot travel faster than the speed of light ($ c $). This indicates that the core emission regions of quasars are remarkably compact.

Redshift and Cosmological Expansion: Most quasars are located at great distances from the Milky Way and exhibit very high redshift.These high redshifts correspond to large apparent recessional speeds—over 96% of the speed of light for SDSS 1030+0524—which are primarily due to the expansion of space (cosmological redshift) rather than the object’s motion through space.

Probing the Universe: Because quasars are visible at such extreme distances, they serve as vital tools for astronomers to probe the early universe. Their light undergoes scintillation (flickering) as it travels through the interstellar and intergalactic medium, and their spectra allow researchers to study conditions in the Universe when it was only a fraction of its current age. Furthermore, time dilation effects mean that a change in luminosity observed over a time $ \Delta t_{obs} $ actually occurred over a shorter period in the quasar’s rest frame: $ \Delta t_{rest} = \frac{\Delta t_{obs}}{z + 1} $.

Observational Characteristics:

Spectroscopy: Quasar spectra often feature strong, broad emission lines, such as the Lyman-alpha hydrogen line, which are broadened by the high-velocity environments near the central black hole.

Nonthermal Radiation: Much of the emission, particularly at radio wavelengths, is nonthermal, involving relativistic particles interacting with magnetic fields.

Standard Candles: Due to their high luminosity, they act as beacons that help define the large-scale structure of the cosmos.