from Earth (separated by a few arcseconds or less), they are commonly referred to as a "double quasar". When the two are also close together in space (i.e. observed to have similar redshifts), they are termed a "quasar pair", or as a "binary quasar" if they are close enough that their host galaxies are likely to be physically interacting. As quasars are overall rare objects in the universe, the probability of three or more separate quasars being found near the same physical location is very low, and determining whether the system is closely separated physically requires significant observational effort. The first true triple quasar was found in 2007 by observations at the W. M. Keck Observatory in Mauna Kea, Hawaii. LBQS 1429-008 (or QQQ J1432-0106) was first observed in 1989 and at the time was found to be a double quasar. When astronomers discovered the third member, they confirmed that the sources were separate and not the result of gravitational lensing. This triple quasar has a redshift of z = 2.076. The components are separated by an estimated 30–50 kiloparsecs (roughly 97,000–160,000 light-years), which is typical for interacting galaxies. In 2013, the second true triplet of quasars, QQQ J1519+0627, was found with a redshift z = 1.51, the whole system fitting within a physical separation of 25 kpc (about 80,000 light-years). The first true quadruple quasar system was discovered in 2015 at a redshift z = 2.0412 and has an overall physical scale of about 200 kpc (roughly 650,000 light-years). A multiple-image quasar is a quasar whose light undergoes gravitational lensing, resulting in double, triple or quadruple images of the same quasar. The first such gravitational lens to be discovered was the double-imaged quasar Q0957+561 (or Twin Quasar) in 1979. An example of a triply lensed quasar is PG1115+08. Several quadruple-image quasars are known, including the Einstein Cross and the Cloverleaf Quasar, with the first such discoveries happening in the mid-1980s. Gallery See also Galaxy formation and evolution Large quasar group List of quasars List of microquasars Microquasar Quasi-star References External links 3C 273: Variable Star Of The Season SKY-MAP.ORG SDSS image of quasar 3C 273 Expanding Gallery of Hires Quasar Images Gallery of Quasar Spectra from SDSS SDSS Advanced Student Projects: Quasars Black Holes: Gravity's Relentless Pull Award-winning interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute Audio: Fraser Cain/Pamela