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Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
Innovation, patents, science, technology and world-changing contributions.
Pioneers, champions, Negro Leagues, records, activism and excellence.
Meet the people whose lives, choices and achievements shaped the journey.
Black towns, communities, institutions and places where history happened.
Moments that changed communities, movements, institutions and the nation.
Katherine Johnson’s mathematical calculations helped guide some of America’s most important early space missions while she confronted the racial and gender barriers faced by Black women in twentieth-century America.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
A thin disk is a structural component of spiral and S0-type galaxies, composed of stars, gas and dust. It is the main non-centre (e.g. galactic bulge) density of such matter. The thin disk of the Milky Way is thought to have a scale height of around 300–400 parsecs (980–1,300 ly) in the vertical axis perpendicular to the disk,[1] and a scale length of around 2.5–4.5 kiloparsecs (8.2–14.7 kly) in the horizontal axis, in the direction of the radius.[2] However, new data from the Gaia DR3 studies suggest a smaller scale height of 280 ± 12.5 pc (912 ± 40.75 ly).[3]
For comparison, the Sun is 8 kiloparsecs (26 kly) out from the center. The thin disk contributes about 85% of the stars in the Galactic plane[4] and 95% of the total stars in the disk.[2] It can be set apart from the thick disk of a galaxy since the latter is composed of older population stars created at an earlier stage of galaxy formation and thus has fewer heavy elements. Stars in the thin disk, on the other hand, are created as a result of gas accretion at the later stages of galaxy formation and are on average more metal-rich.[5]
The thin disk contains stars with a wide range of ages and may be divided into a series of sub-populations of increasing age. Nevertheless, it is considered to be considerably younger than the thick disk.[6]
Based upon the emerging science of nucleocosmochronology, the galactic thin disk of the Milky Way is estimated to have been formed 8.8 ± 1.7 billion years ago.[7] It may have collided with a smaller satellite galaxy, causing the stars in the thin disk to be shaken up and creating the thick disk,[8] while the gas would have settled into the galactic plane and reformed the thin disk.[2]
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Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
A thin disk is a structural component of spiral and S0-type galaxies, composed of stars, gas and dust. It is the main non-centre (e.g. galactic bulge) density of such matter. The thin disk of the Milky Way is thought to have a scale height of around 300–400 parsecs (980–1,300 ly) in the vertical axis perpendicular to the disk, and a scale length of around 2.5–4.5 kiloparsecs (8.2–14.7 kly) in the horizontal axis, in the direction of the radius. However, new data from the Gaia DR3 studies suggest a smaller scale height of 280 ± 12.5 pc (912 ± 40.75 ly). For comparison, the Sun is 8 kiloparsecs (26 kly) out from the center. The thin disk contributes about 85% of the stars in the Galactic plane and 95% of the total stars in the disk. It can be set apart from the thick disk of a galaxy since the latter is composed of older population stars created at an earlier stage of galaxy formation and thus has fewer heavy elements. Stars in the thin disk, on the other hand, are created as a result of gas accretion at the later stages of galaxy formation and are on average more metal-rich. The thin disk contains stars with a wide range of ages and may be divided into a series of sub-populations of increasing age. Nevertheless, it is considered to be considerably younger than the thick disk. Based upon the emerging science of nucleocosmochronology, the galactic thin disk of the Milky Way is estimated to have been formed 8.8 ± 1.7 billion years ago. It may have collided with a smaller satellite galaxy, causing the stars in the thin disk to be shaken up and creating the thick disk, while the gas would have settled into the galactic plane and reformed the thin disk.
A thick disk is one of the structural components of about 2/3 of all disk galaxies, including the Milky Way. It was discovered first in external edge-on galaxies. Soon after, it was proposed as a distinct galactic structure in the Milky Way, different from the thin disk and the halo. It is thought to dominate the stellar number density between 1 and 5 kiloparsecs (3.3 and 16.3 kly) above the galactic plane and, in the solar neighborhood, is composed almost exclusively of older stars. For the Milky Way, the thick disk has a scale height of around 0.6–1.1 kiloparsecs (2.0–3.6 kly) in the axis perpendicular to the disk, which is 3-4 times larger than the thin disk, and a scale length of around 1.9–2.3 kiloparsecs (6.2–7.5 kly) in the horizontal axis, in the direction of the radius. Its stellar chemistry and stellar kinematics (composition and motion of its stars) also set it apart from the thin disk. Compared to the thin disk, thick disk stars typically have significantly lower levels of metals—that is, the abundance of elements other than hydrogen and helium. The thick disk is a source of early kinematic and chemical evidence for a galaxy's composition and thus is regarded as a very significant component for understanding galaxy formation. With the availability of observations at larger distances away from the Sun, it has become apparent that the Milky Way thick disk does not have the same chemical and age composition at all distances from the galactic centre. Instead, it is metal-poor inside the solar radius, but becomes more metal-rich outside it. Additionally, observations have revealed that the average stellar age of thick disk stars quickly decreases as one moves from the inner to the outer disk.
In general relativity, the relativistic disk expression refers to a class of axi-symmetric self-consistent solutions to Einstein's field equations corresponding to the gravitational field generated by axi-symmetric isolated sources. To find such solutions, one has to pose correctly and solve together the 'outer' problem, a boundary value problem for vacuum Einstein's field equations whose solution determines the external field, and the 'inner' problem, whose solution determines the structure and the dynamics of the matter source in its own gravitational field. Physically reasonable solutions must satisfy some additional conditions such as finiteness and positiveness of mass, physically reasonable kind of matter and finite geometrical size. Exact solutions describing relativistic static thin disks as their sources were first studied by Bonnor and Sackfield and Morgan and Morgan. Subsequently, several classes of exact solutions corresponding to static and stationary thin disks have been obtained by different authors.
3I/ATLAS, also known as C/2025 N1 (ATLAS) and previously as A11pl3Z, is an interstellar comet discovered on 1 July 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station. The comet follows an unbound, hyperbolic trajectory past the Sun, and passed by Earth at 1.8 AU, posing no threat. The prefix "3I" designates it as the third confirmed interstellar object passing through the Solar System, after 1I/ʻOumuamua and 2I/Borisov. 3I/ATLAS is an active comet consisting of a solid icy nucleus and a coma, which is a cloud of gas and icy dust escaping from the nucleus. The Sun is responsible for the comet's activity because it heats up the comet's nucleus to sublimate its ice into gas, which outgasses and lifts up dust from the comet's surface to form its coma. Images taken by the Hubble Space Telescope and various interplanetary spacecraft suggest that the diameter of 3I/ATLAS's nucleus is less than 1 km (0.62 mi). Observations by the James Webb Space Telescope (JWST) have shown that 3I/ATLAS is unusually rich in carbon dioxide, and contains a small amount of water ice, water vapor, carbon monoxide, carbonyl sulfide, and methane. Observations by the Very Large Telescope have also shown that 3I/ATLAS is emitting cyanide gas and atomic nickel vapor, at concentrations similar to those seen in Solar System comets. The comet came to solar conjunction on 21 October 2025, and it came closest to the Sun on 29 October 2025, at a distance of 1.36 AU (203 million km; 126 million mi) from the Sun, which is between the orbits of Earth and Mars. The comet appears to have originated from either the Milky Way's thin disk, or thick disk; if 3I/ATLAS originated from the thick disk, the comet could be at least 7 billion years old—older than the Solar System.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →The Greenwood District.
