Quasars (quasi-stellar objects) were discovered in the 1960s as very bright radio sources. When astronomers looked for them with optical telescopes they found faint, star-like points of light, but with very unusual spectra.Their spectra show very large optical red shifts. By Hubble's law this means they are extremely far away, making quasars among the most distant objects we can measure. The most distant are billions of light years away, so we see them as they were when the universe was young.Because they are so far away yet still visible, their power output must be enormous: a single quasar can emit more power than an entire galaxy of hundreds of billions of stars.Quasar brightness can vary over days, so the emitting region must be very small, no bigger than the distance light travels in a few days, about the size of the Solar System.The current model is that a quasar is an active galactic nucleus: a supermassive black hole at the centre of a distant galaxy, surrounded by a disc of gas and dust spiralling in. Matter falling in is heated to extreme temperatures and radiates huge amounts of energy, and jets of charged particles produce the radio emission.
Worked example: a quasar has red shift z = 0.16. Estimate its distance.
v = zc = 0.16 × 3.00 × 108 = 4.8 × 107 m s-1 = 48 000 km s-1
d = v/H = 48 000 / 65 ≈ 740 Mpc
Worked example: a quasar has red shift z = 0.16. Estimate its distance.v = zc = 0.16 × 3.00 × 108 = 4.8 × 107 m s-1 = 48 000 km s-1
d = v/H = 48 000 / 65 ≈ 740 Mpc