even the most powerful visible-light telescopes as anything more than faint starlike points of light. But if they were small and far away in space, their power output would have to be immense and difficult to explain. Equally, if they were very small and much closer to this galaxy, it would be easy to explain their apparent power output, but less easy to explain their redshifts and lack of detectable movement against the background of the universe. Schmidt noted that redshift is also associated with the expansion of the universe, as codified in Hubble's law. If the measured redshift was due to expansion, then this would support an interpretation of very distant objects with extraordinarily high luminosity and power output, far beyond any object seen to date. This extreme luminosity would also explain the large radio signal. Schmidt concluded that 3C 273 could either be an individual star around 10 km wide within (or near to) this galaxy, or a distant active galactic nucleus. He stated that a distant and extremely powerful object seemed more likely to be correct. Schmidt's explanation for the high redshift was not widely accepted at the time. A major concern was the enormous amount of energy these objects would have to be radiating, if they were distant. 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 Zeldovich, and even then it was rejected by many astronomers, as at this time the existence of black holes at all was widely seen as theoretical. Various explanations were proposed during the 1960s and 1970s, each with their own problems. It was suggested that quasars were nearby objects, and that their redshift was not due to the expansion of space but rather to light escaping a deep gravitational well. This would require a massive object, which would also explain the high luminosities. However, a star of sufficient mass to produce the measured redshift would be unstable and in excess of the Hayashi limit. Quasars also show forbidden spectral emission lines, previously only seen in hot gaseous nebulae of low density, which would be too diffuse to both generate the observed power and fit within a deep gravitational well. There were also serious concerns regarding the idea of