quasar

A quasar (from quasi-stellar object or QSO) is an object with the point-like appearance of a star, but with a redshift indicating a far greater distance than that of a visible individual star. First noticed in the 1960s, quasars are now identified to be active galactic nuclei (AGNs). Observed redshifts range from 0.05 to greater than 7, putting them in the range of 600 million to near 13 billion light-years' distant (and years old). The Balmer series spectral lines are often useful to determine their redshift, typically H-alpha. They appear bright enough that some quasars might equal the Sun's apparent brightness as seen from Earth at a distance of 30 light-years. However, they do not necessarily shine so brightly in all directions. Their strong light can be lensed by nearer objects and can be useful for studying the objects doing the lensing. As a very-distant-but-observable light source, their light is useful in studying material it passes through.

The term quasi-stellar source (or QSS or quasi-stellar radio source, also sometimes using the prefix QSR) refers to a quasar that is also a strong radio source. The terms radio loud quasar (RLQ) and radio quiet quasar (RQQ) are also used. In fact, the term quasar was originally coined for such radio sources (but now is often used with the above broader meaning), and the term quasi-stellar galaxy (QSG) formerly was used for radio-quiet quasars. (Note that in radio astronomy, the word source is typically assumed to specifically mean a radio source).

Another classification of quasars is between red quasars and blue quasars. The former are presumed to be within dusty galaxies, and the latter, those not so dusty, so the bluer light of the AGN is visible to us.