Quasars, also known as quasi-stellar objects, are compact astronomical sources whose radiation output is exceptionally high and whose spectra show very large redshifts.
They initially appeared similar to stars and were first recognised in the 1960s because of their unusually strong radio emission. Later observations showed that quasars emit strongly over a much wider range of wavelengths.
| Property | Observation |
|---|---|
| Redshift | Their optical spectra show very large shifts towards longer wavelengths. |
| Luminosity | The amount of radiation they produce is exceptionally large. |
| Appearance | They were initially observed as compact, star-like sources. |
| Size | The source region is small compared with the enormous amount of energy being emitted. |
| Radiation | They emit strongly across the electromagnetic spectrum rather than only in the radio region. |
The combination of a small emitting region and an enormous luminosity distinguishes a quasar from an ordinary star.
Exam Tip: For a description of a quasar, include several linked observations: a compact or star-like appearance, very high luminosity and a very large optical redshift.