A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member stars, all orbiting in a stable, compact formation. Globular clusters are similar in form to dwarf spheroidal galaxies, and the distinction between the two is not always clear. Their name is derived from Latin globulus. Globular clusters are occasionally known simply as "globulars".
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 with much greater accuracy than Newtonian physics, which treats light as corpuscles travelling at the speed of light.
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 galaxy filaments and 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.
Abell 2218 is a large cluster of galaxies over 2 billion light-years away in the constellation Draco.
Omega Centauri is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. Located at a distance of 17,090 light-years, it is the largest-known globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars, and a total mass equivalent to 4 million solar masses, making it the most massive-known globular cluster in the Milky Way.
NGC 6397 is a globular cluster in the constellation Ara. It is located about 7,800 light-years from Earth, making it one of the two nearest globular clusters to Earth. The cluster contains around 400,000 stars, and can be seen with the naked eye under good observing conditions.
A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.
The Perseus cluster is a cluster of galaxies in the constellation Perseus. It has a recession speed of 5,366 km/s and a diameter of 863′. It is one of the most massive objects in the known universe, containing thousands of galaxies immersed in a vast cloud of multimillion-degree gas.
Abell 2667 is a galaxy cluster. It is one of the most luminous galaxy clusters in the X-ray waveband known at a redshift about 0.2.
NGC 1427A is an irregular galaxy in the constellation Fornax. Its distance modulus has been estimated using the globular cluster luminosity function to be 31.01 ± 0.21 which is about 52 Mly. It is the brightest dwarf irregular member of the Fornax cluster and is in the foreground of the cluster's central galaxy NGC 1399.
Abell 520 is a galaxy cluster in the Orion constellation, located at a co-moving radial distance of 811 Mpc (2,645 Mly) and subtends 25 arcminutes on the Earth sky.
A1689-zD1 is a galaxy in the Virgo constellation. It was a candidate for the most distant and therefore earliest-observed galaxy discovered as of February 2008, based on a photometric redshift.
Abell 370 is a galaxy cluster located nearly 5 billion light-years away from the Earth, in the constellation Cetus. Its core is made up of several hundred galaxies. It was catalogued by George Abell, and is the most distant of the clusters he catalogued.
Abell 2744, nicknamed Pandora's Cluster, is a giant galaxy cluster resulting from the simultaneous pile-up of at least four separate, smaller galaxy clusters that took place over a span of 350 million years, and is located approximately 4 billion light years from Earth. The galaxies in the cluster make up less than five percent of its mass. The gas is so hot that it shines only in X-rays. Dark matter makes up around 75 percent of the cluster's mass.
MACS0647-JD is a galaxy with a redshift of about z = 10.7, equivalent to a light travel distance of 13.26 billion light-years. If the distance estimate is correct, it formed about 427 million years after the Big Bang.
Abell 2261 is one of 25 galaxy clusters being studied as part of the Cluster Lensing And Supernova survey with Hubble (CLASH) program, a major project to build a library of scientific data on lensing clusters.
The galaxy cluster IRC 0218 hosts the most distant strong gravitational lensing galaxy currently known at a redshift of z = 1.62. The lens is one of the two brightest cluster galaxies and is lensing a background star-forming galaxy at a redshift of z = 2.26 into a bright arc and a faint counterimage. The lens was discovered through a combination of Hubble Space Telescope and Keck telescope imaging and spectroscopy. The discovery and subsequent analysis of the lens was published in the Astrophysical Journal Letters on June 23, 2014 by an international team of astronomers led by Dr. Kim-Vy Tran from Texas A&M University in College Station, Texas and team members Dr. Kenneth Wong and Dr. Sherry Suyu from the Academia Sinica Institute of Astronomy and Astrophysics in Taipei, Taiwan.
MACS J0416.1-2403 is a cluster of galaxies at a redshift of z=0.397 with a mass 160 trillion times the mass of the Sun inside 200 kpc (650 kly). Its mass extends out to a radius of 950 kpc (3,100 kly) and was measured as 1.15 × 1015 solar masses. The system was discovered in images taken by the Hubble Space Telescope during the Massive Cluster Survey, MACS. This cluster causes gravitational lensing of distant galaxies producing multiple images. Based on the distribution of the multiple image copies, scientists have been able to deduce and map the distribution of dark matter. The images, released in 2014, were used in the Cluster Lensing And Supernova survey with Hubble (CLASH) to help scientists peer back in time at the early Universe and to discover the distribution of dark matter.
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.
SMACS J0723.3–7327, commonly referred to as SMACS 0723, is a galaxy cluster about 4 billion light years from Earth, within the southern constellation of Volans. It is a patch of sky visible from the Southern Hemisphere on Earth and often observed by the Hubble Space Telescope and other telescopes in search of the deep past. It was the target of the first full-color image to be unveiled by the James Webb Space Telescope (JWST), imaged using NIRCam, with spectra included, showing objects lensed by the cluster with redshifts implying they are 13.1 billion years old. The cluster has been previously observed by the Hubble Space Telescope (HST) as part of the Southern MAssive Cluster Survey (SMACS), as well as Planck and Chandra.
