QSOs

Quasi-stellar objects, or QSOs, are a class of extremely luminous active galactic nuclei that appear point-like in optical images. They are powered by accretion of matter onto supermassive black holes at the centers of distant galaxies, producing energy that can outshine the host galaxy across the electromagnetic spectrum.

The term “quasi-stellar” arises from their star-like appearance in early optical surveys. Their extragalactic nature was established in the 1960s when Maarten Schmidt identified the spectrum of the radio source 3C 273 as having unusually broad emission lines shifted by a significant redshift. Although many QSOs are radio emitters, the majority are radio-quiet, and the class is commonly referred to as quasars.

Physical properties of QSOs are governed by accretion onto supermassive black holes with masses ranging from about a million to tens of billions of solar masses. Their luminosities can reach up to roughly 10^40 watts (about 10^47 erg/s). QSOs emit across the spectrum, from radio to X-ray, with the optical/ultraviolet continuum often accompanied by broad emission lines from fast-moving gas in the broad-line region. Variability can occur on timescales from days to years. A subset shows relativistic jets and is then described as radio-loud; objects with jets seen close to our line of sight are related to blazars.

Cosmologically, QSOs serve as important probes of the early universe and the intergalactic medium, thanks to their high redshifts and bright continua. They were discovered and cataloged in large surveys such as the Sloan Digital Sky Survey, and they remain a key area of study for black hole growth, galaxy evolution, and large-scale structure.

QSOs are a central component of active galactic nucleus research and are regarded within unified models that connect various AGN classes, including Seyfert galaxies and blazars.