Also, 3C 273 was bright enough to detect on archival photographs dating back to the 1900s; it was found to be variable on yearly timescales, implying that a substantial fraction of the light was emitted from a region less than 1 light-year in size, tiny compared to a galaxy. Although it raised many questions, Schmidt's discovery quickly revolutionized quasar observation.
The redshifts of quasars are of cosmological origin. The term originated as a contraction of quasi-stellar [star-like] radio source – because quasars were first identified during the 1950s as sources of radio-wave emission of unknown physical origin – and when identified in photographic images at visible wavelengths, they resembled faint, star-like points of light.
But when radio astronomy commenced in the 1950s, astronomers detected, among the galaxies, a small number of anomalous objects with properties that defied explanation. The objects emitted large amounts of radiation of many frequencies, but no source could be located optically, or in some cases only a faint and point-like object somewhat like a distant star.
They were described as "quasi-stellar [meaning: star-like] radio sources", or "quasi-stellar objects" (QSOs), a name which reflected their unknown nature, and this became shortened to "quasar". === Early observations (1960s and earlier) === The first quasars (3C 48 and 3C 273) were discovered in the late 1950s, as radio sources in all-sky radio surveys.
By 1960, hundreds of these objects had been recorded and published in the Third Cambridge Catalogue while astronomers scanned the skies for their optical counterparts.
In the 1960s no commonly accepted mechanism could account for this.
The currently accepted explanation, that it is due to matter in an accretion disc falling into a supermassive black hole, was only suggested in 1964 by Edwin Salpeter and Yakov Zel'dovich, and even then it was rejected by many astronomers, because in the 1960s, the existence of black holes was still widely seen as theoretical and too exotic, and because it was not yet confirmed that many galaxies (including our own) have supermassive black holes at their center.
A common alternative explanation was that the redshifts were caused by extreme mass (gravitational redshifting explained by general relativity) and not by extreme velocity (explained by special relativity). Various explanations were proposed during the 1960s and 1970s, each with their own problems.
The anomalous spectrum defied interpretation. British-Australian astronomer John Bolton made many early observations of quasars, including a breakthrough in 1962.
In 1963, a definite identification of the radio source 3C 48 with an optical object was published by Allan Sandage and Thomas A.
Shortly afterwards, two more quasar spectra in 1964 and five more in 1965 were also confirmed as ordinary light that had been redshifted to an extreme degree.
The currently accepted explanation, that it is due to matter in an accretion disc falling into a supermassive black hole, was only suggested in 1964 by Edwin Salpeter and Yakov Zel'dovich, and even then it was rejected by many astronomers, because in the 1960s, the existence of black holes was still widely seen as theoretical and too exotic, and because it was not yet confirmed that many galaxies (including our own) have supermassive black holes at their center.
Most quasars, with the exception of 3C 273, whose average apparent magnitude is 12.9, cannot be seen with small telescopes. Quasars are believed—and in many cases confirmed—to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, as suggested in 1964 by Edwin Salpeter and Yakov Zel'dovich.
Shortly afterwards, two more quasar spectra in 1964 and five more in 1965 were also confirmed as ordinary light that had been redshifted to an extreme degree.
This discovery by Maarten Schmidt in 1967 was early strong evidence against steady-state cosmology and in favor of the Big Bang cosmology.
A common alternative explanation was that the redshifts were caused by extreme mass (gravitational redshifting explained by general relativity) and not by extreme velocity (explained by special relativity). Various explanations were proposed during the 1960s and 1970s, each with their own problems.
By 1987 it was "well accepted" that this was the correct explanation for quasars, and the cosmological distance and energy output of quasars was accepted by almost all researchers. ===Modern observations (1970s onward)=== Later it was found that not all quasars have strong radio emission; in fact only about 10% are "radio-loud".
Such quasars are called blazars. The hyperluminous quasar APM 08279+5255 was, when discovered in 1998, given an absolute magnitude of −32.2.
In 2017, the quasar, ULAS J1342+0928, was detected at redshift z = 7.54.
In 2020, the quasar, Pōniuāʻena, was detected from a time only 700 million years after the Big Bang, and with an estimated mass of 1.5 billion times the mass of our Sun.
In early 2021, the quasar, J0313-1806, with a 1.6 billion solar-mass black hole was reported at z = 7.64, 670 million years after the Big Bang.
All observed quasar spectra have redshifts between 0.056 and 7.64 (as of 2021).
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