Controversy about the distance of quasars
The 1963 discovery of quasi-stellar radio sources – quasars – generated a new controversy about distance. First identified as bright radio sources apparently associated with stars, they soon turned out to have high redshifts. If the redshifts were interpreted according to Hubble’s law, these were objects at huge distances, and hence with huge luminosities. Since some quasars were varying their light output on the timescale of months or even days, they had to be extremely compact systems, not much bigger than the solar system, yet generating much more power than our entire galaxy.
These strange properties led some astronomers to question whether the quasars really were at huge distances. Could the redshifts be caused by strong gravitational fields, or were quasars really fast-moving local objects thrown out in some spectacular galactic explosion?
Gradually evidence was accumulated that showed that at least some quasars are located in galaxies at the same redshift as the quasar. At the same time theorists began to model quasars as the product of massive black holes, 100 million times the mass of the Sun, in the nuclei of galaxies. Black holes originate as the remnants massive stars leave behind at the end of their lives and are so named because their gravitational fields are so strong that not even light travels quickly enough to escape their grip. Lower mass black holes are thought to be able to merge into larger ones, such as those found in the centre of galaxies.
Here the massive black holes might have formed initially from the collapse of a dense star cluster and have then grown by sucking in gas from their parent galaxies. Gas falling towards the black hole forms a rotating disc around it, which radiates at optical, ultraviolet and X-ray wavelengths. Today we have strong evidence for the black hole model and have even identified a smaller black hole, of one million solar masses, in the centre of our own galaxy.
Quasars have been detected out to redshift 7 (so far away that we see them as they were 12.9 billion years ago) and are among the most distant objects known in the universe. Today we think they are intimately connected with the formation and evolution of galaxies, with the black holes in their centres helping to regulate star formation.
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