A quasar is an extremely luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by a supermassive black hole with a mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. Gas in the disc falling towards the black hole heats up and releases energy in the form of electromagnetic radiation. The radiant energy of quasars is enormous; the most powerful quasars have luminosities thousands of times greater than that of a galaxy such as the Milky Way. Quasars are usually categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation. The opposite change, a decrease in wavelength and increase in frequency and energy, is known as a blueshift, or negative redshift. The terms derive from the colours red and blue which form the extremes of the visible light spectrum. The main causes of electromagnetic redshift in astronomy and cosmology are the relative motions of radiation sources, which give rise to the relativistic Doppler effect, and gravitational potentials, which gravitationally redshift escaping radiation. All sufficiently distant light sources show cosmological redshift corresponding to recession speeds proportional to their distances from Earth, a fact known as Hubble's law that implies the universe is expanding.
A gravitational lens is matter, such as a cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's general theory of relativity. If light is treated as corpuscles travelling at the speed of light, Newtonian physics also predicts the bending of light, but only half of that predicted by general relativity.
A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-largest known gravitationally bound structures in the universe after some superclusters (of which only one, the Shapley Supercluster, is known to be bound). They were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. One of the key features of clusters is the intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with galactic clusters (also known as open clusters), which are star clusters within galaxies, or with globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. The galaxy groups and clusters can themselves cluster together to form superclusters.
Messier 87 is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars. One of the largest and most massive galaxies in the local universe, it has a large population of globular clusters—about 15,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.
A supermassive black hole is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, including light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way galaxy has a supermassive black hole at its center, corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei (AGNs) and quasars.
A blazar is an active galactic nucleus (AGN) with a relativistic jet directed very nearly towards an observer. Relativistic beaming of electromagnetic radiation from the jet makes blazars appear much brighter than they would be if the jet were pointed in a direction away from Earth. Blazars are powerful sources of emission across the electromagnetic spectrum and are observed to be sources of high-energy gamma ray photons. Blazars are highly variable sources, often undergoing rapid and dramatic fluctuations in brightness on short timescales. Some blazar jets appear to exhibit superluminal motion, another consequence of material in the jet traveling toward the observer at nearly the speed of light.
An Einstein ring, also known as an Einstein–Chwolson ring or Chwolson ring, is created when light from a galaxy or star passes by a massive object en route to the Earth. Due to gravitational lensing, the light is diverted, making it seem to come from different places. If source, lens, and observer are all in perfect alignment (syzygy), the light appears as a ring.
An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as extended beams along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets become relativistic jets as they show effects from special relativity.
Gravitational microlensing is an astronomical phenomenon caused by the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light (stars) or large objects that block background light. These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light.
APM 08279+5255 is a very distant, broad absorption line quasar located in the constellation Lynx. It is magnified and split into multiple images by the gravitational lensing effect of a foreground galaxy through which its light passes. It appears to be a giant elliptical galaxy with a supermassive black hole and associated accretion disk. It possesses large regions of hot dust and molecular gas, as well as regions with starburst activity.
The Andromeda–Milky Way collision is a galactic collision predicted to occur in about 4.5 billion years between the two largest galaxies in the Local Group—the Milky Way and the Andromeda Galaxy. The stars involved are sufficiently far apart that it is improbable that any of them will individually collide, though some stars will be ejected.
The Great Observatories Origins Deep Survey, or GOODS, is an astronomical survey combining deep observations from three of NASA's Great Observatories: the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory, along with data from other space-based telescopes, such as XMM Newton, and some of the world's most powerful ground-based telescopes.
The Cloverleaf quasar is a bright, gravitationally lensed quasar.
An extragalactic planet, also known as an extragalactic exoplanet or an extroplanet, is a star-bound planet or rogue planet located outside of the Milky Way Galaxy. Due to the immense distances to such worlds, they would be very hard to detect directly. However, indirect evidence suggests that such planets exist. Nonetheless, the most distant known planets are SWEEPS-11 and SWEEPS-04, located in Sagittarius, approximately 27,710 light-years from the Sun, while the Milky Way is about 87,400 light-years in diameter. This means that even galactic planets located further than that distance have not been detected.
Georges Meylan is a Swiss astronomer, born on July 31, 1950, in Lausanne, Switzerland. He was the director of the Laboratory of Astrophysics of the Swiss Federal Institute of Technology (EPFL) in Lausanne, Switzerland, and now a professor emeritus of astrophysics and cosmology at EPFL. He is still active in both research and teaching.
MACS J1149 Lensed Star 1, also known as Icarus, is a blue supergiant star observed through a gravitational lens. It is the seventh most distant individual star to have been detected so far, at approximately 14 billion light-years from Earth. Light from the star was emitted 4.4 billion years after the Big Bang. According to co-discoverer Patrick Kelly, the star is at least a hundred times more distant than the next-farthest non-supernova star observed, SDSS J1229+1122, and is the first magnified individual star seen.
UHZ1 is a background galaxy containing a quasar. At a redshift of approximately 10.1, UHZ1 is at a distance of 13.2 billion light-years, seen when our universe was about 3 percent of its current age. This redshift made it the most distant, and therefore earliest known quasar in the observable universe as of 2023. To detect this object, astronomers working at the Chandra X-ray Observatory used the Abell 2744's cluster mass as a gravitational lens in order to magnify distant objects directly behind it. At the time of discovery, it exceeded the distance record of QSO J0313−1806.
James Webb photo of the quasar gravitationally lensed by a galaxy
Combined Hubble and Chandra photo
Combined Hubble and Chandra photo, with a different coloring
