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Sandage, Allan Rex

radio quasars galaxies universe

(1926– ) US astronomer: made first identification of an optical object with a quasar.

Sandage studied at the University of Illinois and the California Institute of Technology before joining the Hale Observatories, initially as an assistant to . Although radio galaxies had been discovered in the 1940s, it was some years before compact radio emitters (‘quasi-stellar radio sources’ or quasars) were found by use of improved radio telescopes, and only in 1960 that the first strong compact radio source (3C 48) was matched with the position of an optical bright star, by Sandage. Others were located in the 1960s, and showed they have massive redshifts, implying high speeds of recession; this is now regarded as a defining feature, rather than intense radio emission, which Sandage showed is absent for 90% of quasars (so the word is better now regarded as defining ‘quasi-stellar objects’). Typically quasars are bluish objects, intensely luminous, double-lobed in shape, emitting synchrotron radiation which is variable over weeks or months.

Their origin and huge energy generation is mysterious: consensus opinion regards them as the most distant visible objects in the universe, so that they are now seen as they were billions of years ago. They are best explained as the cores of infant galaxies, having a massive black hole at their centre. The galaxies may represent a more mature stage. Intensive study of quasars continues; over 1500 are now known.

Early in his career, working as Hubble’s successor and refining his studies on the size and age of the universe, Sandage showed that its expansion was accelerating less than had been thought: and in 1956 he deduced an age for the universe (about 15 billion years) which has not been much changed by later studies.

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