Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8; the temperature range of supergiant stars spans from about 3,450 K to over 20,000 K. The title supergiant, as applied to a star, does not have a single concrete definition; the term giant star was first coined by Hertzsprung when it became apparent that the majority of stars fell into two distinct regions of the Hertzsprung–Russell diagram. One region contained larger and more luminous stars of spectral types A to M and received the name giant. Subsequently, as they lacked any measurable parallax, it became apparent that some of these stars were larger and more luminous than the bulk, the term super-giant arose adopted as supergiant. Supergiant stars can be identified on the basis of their spectra, with distinctive lines sensitive to high luminosity and low surface gravity. In 1897, Antonia C. Maury had divided stars based on the widths of their spectral lines, with her class "c" identifying stars with the narrowest lines.
Although it was not known at the time, these were the most luminous stars. In 1943 Morgan and Keenan formalised the definition of spectral luminosity classes, with class I referring to supergiant stars; the same system of MK luminosity classes is still used today, with refinements based on the increased resolution of modern spectra. Supergiants occur in every spectral class from young blue class O supergiants to evolved red class M supergiants; because they are enlarged compared to main-sequence and giant stars of the same spectral type, they have lower surface gravities, changes can be observed in their line profiles. Supergiants are evolved stars with higher levels of heavy elements than main-sequence stars; this is the basis of the MK luminosity system which assigns stars to luminosity classes purely from observing their spectra. In addition to the line changes due to low surface gravity and fusion products, the most luminous stars have high mass-loss rates and resulting clouds of expelled circumstellar materials which can produce emission lines, P Cygni profiles, or forbidden lines.
The MK system assigns stars to luminosity classes: Ib for supergiants. In reality there is much more of a continuum than well defined bands for these classifications, classifications such as Iab are used for intermediate luminosity supergiants. Supergiant spectra are annotated to indicate spectral peculiarities, for example B2 Iae or F5 Ipec. Supergiants can be defined as a specific phase in the evolutionary history of certain stars. Stars with initial masses above 8-10 M☉ and smoothly initiate helium core fusion after they have exhausted their hydrogen, continue fusing heavier elements after helium exhaustion until they develop an iron core, at which point the core collapses to produce a Type 2 supernova. Once these massive stars leave the main sequence, their atmospheres inflate, they are described as supergiants. Stars under 10 M☉ will never form an iron core and in evolutionary terms do not become supergiants, although they can reach luminosities thousands of times the sun's, they cannot fuse carbon and heavier elements after the helium is exhausted, so they just lose their outer layers, leaving the core of a white dwarf.
The phase where these stars have both hydrogen and helium burning shells is referred to as the asymptotic giant branch, as stars become more and more luminous class M stars. Stars of 8-10 M☉ may fuse sufficient carbon on the AGB to produce an oxygen-neon core and an electron-capture supernova, but astrophysicists categorise these as super-AGB stars rather than supergiants. There are several categories of evolved stars which are not supergiants in evolutionary terms but may show supergiant spectral features or have luminosities comparable to supergiants. Asymptotic-giant-branch and post-AGB stars are evolved lower-mass red giants with luminosities that can be comparable to more massive red supergiants, but because of their low mass, being in a different stage of development, their lives ending in a different way, astrophysicists prefer to keep them separate; the dividing line becomes blurred at around 7–10 M☉ where stars start to undergo limited fusion of elements heavier than helium. Specialists studying these stars refer to them as super AGB stars, since they have many properties in common with AGB such as thermal pulsing.
Others describe them as low-mass supergiants since they start to burn elements heavier than helium and can explode as supernovae. Many post-AGB stars receive spectral types with supergiant luminosity classes. For example, RV Tauri has an Ia luminosity class despite being less massive than the sun; some AGB stars receive a supergiant luminosity class, most notably W Virginis variables such as W Virginis itself, stars that are executing a blue loop triggered by thermal pulsing. A small number of Mira variables and other late AGB stars have supergiant luminosity classes, for example α Herculis. Classical Cepheid variables have supergiant luminosity classes, although only the most luminous and massive will go on to develop an iron core; the majority of them are intermediate mass stars fusing helium in their cores and will transition to the asymptotic giant branch. Δ Cephei itself is an example with a luminosity of 2,000 L☉ and a mass of 4.5 M☉. Wolf–Rayet stars are high-mass luminous evolved stars, hotter than most supergiants and smaller, visually less
A planetary nebula, abbreviated as PN or plural PNe, is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is arguably a misnomer because they are unrelated to planets or exoplanets; the true origin of the term was derived from the planet-like round shape of these nebulae as observed by astronomers through early telescopes, although the terminology is inaccurate, it is still used by astronomers today. The first usage may have occurred during the 1780s with the English astronomer William Herschel who described these nebulae as resembling planets, they are a short-lived phenomenon, lasting a few tens of thousands of years, compared to longer phases of stellar evolution. Once all of the red giant's atmosphere has been dissipated, energetic ultraviolet radiation from the exposed hot luminous core, called a planetary nebula nucleus, ionizes the ejected material. Absorbed ultraviolet light energises the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula.
Planetary nebulae play a crucial role in the chemical evolution of the Milky Way by expelling elements into the interstellar medium from stars where those elements were created. Planetary nebulae are observed in more distant galaxies, yielding useful information about their chemical abundances. Starting from the 1990s, Hubble Space Telescope images revealed that many planetary nebulae have complex and varied morphologies. About one-fifth are spherical, but the majority are not spherically symmetric; the mechanisms that produce such a wide variety of shapes and features are not yet well understood, but binary central stars, stellar winds and magnetic fields may play a role. The first planetary nebula discovered was the Dumbbell Nebula in the constellation of Vulpecula, it was listed as M27 in his catalogue of nebulous objects. To early observers with low-resolution telescopes, M27 and subsequently discovered planetary nebulae resembled the giant planets like Uranus. William Herschel, discoverer of Uranus coined the term "planetary nebula".
However, in as early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula, "a dull nebula, but outlined. Whatever the true origin of the term, the label "planetary nebula" became ingrained in the terminology used by astronomers to categorize these types of nebulae, is still in use by astronomers today; the true nature of these objects was uncertain, Herschel first thought the objects were stars surrounded by material, condensing into planets rather than what is now known to be evidence of dead stars that have incinerated any orbiting planets. In 1782, William Herschel had discovered the object now known as NGC 7009, upon which he used the term "planetary nebula". In 1785, Herschel wrote to Jerome Lalande: "These are celestial bodies of which as yet we have no clear idea and which are of a type quite different from those that we are familiar with in the heavens. I have found four that have a visible diameter of between 15 and 30 seconds.
These bodies appear to have a disk, rather like a planet, to say, of equal brightness all over, round or somewhat oval, about as well defined in outline as the disk of the planets, of a light strong enough to be visible with an ordinary telescope of only one foot, yet they have only the appearance of a star of about ninth magnitude." Herschel assigned these to Class IV of his catalogue of "nebulae" listing 78 "planetary nebulae", most of which are in fact galaxies. The nature of planetary nebulae remained unknown until the first spectroscopic observations were made in the mid-19th century. Using a prism to disperse their light, William Huggins was one of the earliest astronomers to study the optical spectra of astronomical objects. On August 29, 1864, Huggins was the first to analyze the spectrum of a planetary nebula when he observed Cat's Eye Nebula, his observations of stars had shown that their spectra consisted of a continuum of radiation with many dark lines superimposed. He found that many nebulous objects such as the Andromeda Nebula had spectra that were quite similar.
However, when Huggins looked at the Cat's Eye Nebula, he found a different spectrum. Rather than a strong continuum with absorption lines superimposed, the Cat's Eye Nebula and other similar objects showed a number of emission lines. Brightest of these was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known element. At first, it was hypothesized that the line might be due to an unknown element, named nebulium. A similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868. While helium was isolated on Earth soon after its discovery in the spectrum of the Sun, "nebulium" was not. In the early 20th century, Henry Norris Russell proposed that, rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions. Physicists showed in the 1920s that in gas at low densities, electrons can occupy excited metastable energy levels in atoms and ions that would otherwise be de-excited by collisions that would occur
Dark matter is a hypothetical form of matter, thought to account for 85% of the matter in the universe and about a quarter of its total energy density. The majority of dark matter is thought to be non-baryonic in nature being composed of some as-yet undiscovered subatomic particles, its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained by accepted theories of gravity unless more matter is present than can be seen. For this reason, most experts think dark matter to be ubiquitous in the universe and to have had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with observable electromagnetic radiation, such as light, is thus invisible to the entire electromagnetic spectrum, making it difficult to detect using usual astronomical equipment; the primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of unseen matter.
Other lines of evidence include observations in gravitational lensing, from the cosmic microwave background, from astronomical observations of the observable universe's current structure, from the formation and evolution of galaxies, from mass location during galactic collisions, from the motion of galaxies within galaxy clusters. In the standard Lambda-CDM model of cosmology, the total mass–energy of the universe contains 5% ordinary matter and energy, 27% dark matter and 68% of an unknown form of energy known as dark energy. Thus, dark matter constitutes 85% of total mass, while dark energy plus dark matter constitute 95% of total mass–energy content; because dark matter has not yet been observed directly, if it exists, it must interact with ordinary baryonic matter and radiation, except through gravity. The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, in particular, weakly-interacting massive particles, or gravitationally-interacting massive particles.
Many experiments to directly detect and study dark matter particles are being undertaken, but none have yet succeeded. Dark matter is classified as warm, or hot according to its velocity. Current models favor a cold dark matter scenario, in which structures emerge by gradual accumulation of particles. Although the existence of dark matter is accepted by the scientific community, some astrophysicists, intrigued by certain observations that do not fit the dark matter theory, argue for various modifications of the standard laws of general relativity, such as modified Newtonian dynamics, tensor–vector–scalar gravity, or entropic gravity; these models attempt to account for all observations without invoking supplemental non-baryonic matter. The hypothesis of dark matter has an elaborate history. In a talk given in 1884, Lord Kelvin estimated the number of dark bodies in the Milky Way from the observed velocity dispersion of the stars orbiting around the center of the galaxy. By using these measurements, he estimated the mass of the galaxy, which he determined is different from the mass of visible stars.
Lord Kelvin thus concluded that "many of our stars a great majority of them, may be dark bodies". In 1906 Henri Poincaré in "The Milky Way and Theory of Gases" used "dark matter", or "matière obscure" in French, in discussing Kelvin's work; the first to suggest the existence of dark matter, using stellar velocities, was Dutch astronomer Jacobus Kapteyn in 1922. Fellow Dutchman and radio astronomy pioneer Jan Oort hypothesized the existence of dark matter in 1932. Oort was studying stellar motions in the local galactic neighborhood and found that the mass in the galactic plane must be greater than what was observed, but this measurement was determined to be erroneous. In 1933, Swiss astrophysicist Fritz Zwicky, who studied galaxy clusters while working at the California Institute of Technology, made a similar inference. Zwicky applied the virial theorem to the Coma Cluster and obtained evidence of unseen mass that he called dunkle Materie. Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies.
He estimated. The gravity effect of the visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred that some unseen matter provided the mass and associated gravitation attraction to hold the cluster together; this was the first formal inference about the existence of dark matter. Zwicky's estimates were off by more than an order of magnitude due to an obsolete value of the Hubble constant. However, Zwicky did infer that the bulk of the matter was dark. Further indications that the mass-to-light ratio was not unity came from measurements of galaxy rotation curves. In 1939, Horace W. Babcock reported the rotation curve for the Andromeda nebula, which suggested that the mass-to-luminosity ratio increases radially, he attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral and not to the missing matter that he had uncovered. Following Babcock's 1939 report of unexpectedly rapid rotation in the outskirts of the Andromeda galaxy and a mass-to-light ratio of 50, in 1940 Jan Oort discovered and wrote about the large non-visible halo of NGC 3115.
Vera Rubin, Kent Ford and Ken Freeman's work in the
A hypergiant is among the rare kinds of stars that show tremendous luminosities and high rates of mass loss by stellar winds. The term hypergiant is defined as luminosity class 0 in the MKK system. However, this is seen in the literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG, or blue B supergiants with emission spectra. More hypergiants may be classed as Ia-0 or Ia+, but red supergiants are assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution with star formation and their expected demise as supernovae. In 1956, the astronomers Feast and Thackeray used the term super-supergiant for stars with an absolute magnitude brighter than MV = −7. In 1971, Keenan suggested that the term would be used only for supergiants showing at least one broad emission component in Hα, indicating an extended stellar atmosphere or a large mass loss rate; the Keenan criterion is the one most used by scientists today.
To be classified as a hypergiant, a star must be luminous and have spectral signatures showing atmospheric instability and high mass loss. Hence it is possible for a non-hypergiant, supergiant star to have the same or higher luminosity as a hypergiant of the same spectral class. Hypergiants are expected to have a characteristic broadening and red-shifting of their spectral lines, producing a distinctive spectral shape known as a P Cygni profile; the use of hydrogen emission lines is not helpful for defining the coolest hypergiants, these are classified by luminosity since mass loss is inevitable for the class. Stars with an initial mass above about 25 M☉ move away from the main sequence and increase somewhat in luminosity to become blue supergiants, they cool and enlarge at constant luminosity to become a red supergiant contract and increase in temperature as the outer layers are blown away. They may "bounce" backwards and forwards executing one or more "blue loops", still at a steady luminosity, until they explode as a supernova or shed their outer layers to become a Wolf–Rayet star.
Stars with an initial mass above about 40 M☉ are too luminous to develop a stable extended atmosphere and so they never cool sufficiently to become red supergiants. The most massive stars rapidly rotating stars with enhanced convection and mixing, may skip these steps and move directly to the Wolf–Rayet stage; this means that stars at the top of the Hertzsprung–Russell diagram where hypergiants are found may be newly evolved from the main sequence and still with high mass, or much more evolved post-red supergiant stars that have lost a significant fraction of their initial mass, these objects cannot be distinguished on the basis of their luminosity and temperature. High-mass stars with a high proportion of remaining hydrogen are more stable, while older stars with lower masses and a higher proportion of heavy elements have less stable atmospheres due to increased radiation pressure and decreased gravitational attraction; these are thought to be the hypergiants, near the Eddington limit and losing mass.
The yellow hypergiants are thought to be post-red supergiant stars that have lost most of their atmospheres and hydrogen. A few more stable high mass yellow supergiants with the same luminosity are known and thought to be evolving towards the red supergiant phase, but these are rare as this is expected to be a rapid transition; because yellow hypergiants are post-red supergiant stars, there is a hard upper limit to their luminosity at around 500,000–750,000 L☉, but blue hypergiants can be much more luminous, sometimes several million L☉. All hypergiants exhibit variations in luminosity over time due to instabilities within their interiors, but these are small except for two distinct instability regions where luminous blue variables and yellow hypergiants are found; because of their high masses, the lifetime of a hypergiant is short in astronomical timescales: only a few million years compared to around 10 billion years for stars like the Sun. Hypergiants are only created in the largest and densest areas of star formation and because of their short lives, only a small number are known despite their extreme luminosity that allows them to be identified in neighbouring galaxies.
The time spent in some phases such as LBVs can be as short as a few thousand years. As the luminosity of stars increases with mass, the luminosity of hypergiants lies close to the Eddington limit, the luminosity at which the radiation pressure expanding the star outward equals the force of the star's gravity collapsing the star inward; this means that the radiative flux passing through the photosphere of a hypergiant may be nearly strong enough to lift off the photosphere. Above the Eddington limit, the star would generate so much radiation that parts of its outer layers would be thrown off in massive outbursts. A good candidate for hosting a continuum-driven wind is Eta Carinae, one of the most massive stars observed. With an estimated mass of around 130 solar masses and a luminosity four million times that of the Sun, astrophysicists speculate that Eta Carinae may exceed the Eddington limit; the last time might have been a series of outbursts observed in 1840–1860, reaching mass loss rates much higher than
A substellar object, sometimes called a substar, is an astronomical object whose mass is smaller than the smallest mass at which hydrogen fusion can be sustained. This definition includes brown dwarfs and former stars similar to EF Eridani B, can include objects of planetary mass, regardless of their formation mechanism and whether or not they are associated with a primary star. Assuming that a substellar object has a composition similar to the Sun's and at least the mass of Jupiter, its radius will be comparable to that of Jupiter regardless of the mass of the substellar object; this is because the center of such a substellar object at the top range of the mass is quite degenerate, with a density of ≈103 g/cm3, but this degeneracy lessens with decreasing mass until, at the mass of Jupiter, a substellar object has a central density less than 10 g/cm3. The density decrease balances the mass decrease, keeping the radius constant. Substellar objects like brown dwarfs can live forever though they do not have enough mass to fuse hydrogen and helium.
A substellar object with a mass just below the hydrogen-fusing limit may ignite hydrogen fusion temporarily at its center. Although this will provide some energy, it will not be enough to overcome the object's ongoing gravitational contraction. Although an object with mass above 0.013 solar masses will be able to fuse deuterium for a time, this source of energy will be exhausted in 106 to 108 years. Apart from these sources, the radiation of an isolated substellar object comes only from the release of its gravitational potential energy, which causes it to cool and shrink. A substellar object in orbit about a star will shrink more as it is kept warm by the star, evolving towards an equilibrium state where it emits as much energy as it receives from the star. Substellar objects are cool enough to have water vapor in their atmosphere. Infrared spectroscopy can detect the distinctive color of water in gas giant size substellar objects if they are not in orbit about a star. William Duncan MacMillan proposed in 1918 the classification of substellar objects into three categories based on their density and phase state: solid and dark gaseous.
Solid objects include smaller terrestrial planets and moons. Saturn and large gas giant planets are in a "gaseous" state. Brown dwarf Planet Sub-brown dwarf Substellar companion Quoted as Chabrier and Baraffe: Chabrier, Gilles. "Theory of Low-Mass Stars and Substellar Objects". Annual Review of Astronomy and Astrophysics. 38: 337–377. ArXiv:astro-ph/0006383. Bibcode:2000ARA&A..38..337C. doi:10.1146/annurev.astro.38.1.337
A chemical element is a species of atom having the same number of protons in their atomic nuclei. For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have 8 protons. 118 elements have been identified, of which the first 94 occur on Earth with the remaining 24 being synthetic elements. There are 80 elements that have at least one stable isotope and 38 that have radionuclides, which decay over time into other elements. Iron is the most abundant element making up Earth, while oxygen is the most common element in the Earth's crust. Chemical elements constitute all of the ordinary matter of the universe; however astronomical observations suggest that ordinary observable matter makes up only about 15% of the matter in the universe: the remainder is dark matter. The two lightest elements and helium, were formed in the Big Bang and are the most common elements in the universe; the next three elements were formed by cosmic ray spallation, are thus rarer than heavier elements.
Formation of elements with from 6 to 26 protons occurred and continues to occur in main sequence stars via stellar nucleosynthesis. The high abundance of oxygen and iron on Earth reflects their common production in such stars. Elements with greater than 26 protons are formed by supernova nucleosynthesis in supernovae, when they explode, blast these elements as supernova remnants far into space, where they may become incorporated into planets when they are formed; the term "element" is used for atoms with a given number of protons as well as for a pure chemical substance consisting of a single element. For the second meaning, the terms "elementary substance" and "simple substance" have been suggested, but they have not gained much acceptance in English chemical literature, whereas in some other languages their equivalent is used. A single element can form multiple substances differing in their structure; when different elements are chemically combined, with the atoms held together by chemical bonds, they form chemical compounds.
Only a minority of elements are found uncombined as pure minerals. Among the more common of such native elements are copper, gold and sulfur. All but a few of the most inert elements, such as noble gases and noble metals, are found on Earth in chemically combined form, as chemical compounds. While about 32 of the chemical elements occur on Earth in native uncombined forms, most of these occur as mixtures. For example, atmospheric air is a mixture of nitrogen and argon, native solid elements occur in alloys, such as that of iron and nickel; the history of the discovery and use of the elements began with primitive human societies that found native elements like carbon, sulfur and gold. Civilizations extracted elemental copper, tin and iron from their ores by smelting, using charcoal. Alchemists and chemists subsequently identified many more; the properties of the chemical elements are summarized in the periodic table, which organizes the elements by increasing atomic number into rows in which the columns share recurring physical and chemical properties.
Save for unstable radioactive elements with short half-lives, all of the elements are available industrially, most of them in low degrees of impurities. The lightest chemical elements are hydrogen and helium, both created by Big Bang nucleosynthesis during the first 20 minutes of the universe in a ratio of around 3:1 by mass, along with tiny traces of the next two elements and beryllium. All other elements found in nature were made by various natural methods of nucleosynthesis. On Earth, small amounts of new atoms are produced in nucleogenic reactions, or in cosmogenic processes, such as cosmic ray spallation. New atoms are naturally produced on Earth as radiogenic daughter isotopes of ongoing radioactive decay processes such as alpha decay, beta decay, spontaneous fission, cluster decay, other rarer modes of decay. Of the 94 occurring elements, those with atomic numbers 1 through 82 each have at least one stable isotope. Isotopes considered stable are those. Elements with atomic numbers 83 through 94 are unstable to the point that radioactive decay of all isotopes can be detected.
Some of these elements, notably bismuth and uranium, have one or more isotopes with half-lives long enough to survive as remnants of the explosive stellar nucleosynthesis that produced the heavy metals before the formation of our Solar System. At over 1.9×1019 years, over a billion times longer than the current estimated age of the universe, bismuth-209 has the longest known alpha decay half-life of any occurring element, is always considered on par with the 80 stable elements. The heaviest elements undergo radioactive decay with half-lives so short that they are not found in nature and must be synthesized; as of 2010, there are 118 known elements (in this context, "known" means observed well enough from just a few de
Gravity, or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars and light—are brought toward one another. On Earth, gravity gives weight to physical objects, the Moon's gravity causes the ocean tides; the gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing, forming stars – and for the stars to group together into galaxies – so gravity is responsible for many of the large-scale structures in the Universe. Gravity has an infinite range, although its effects become weaker on farther objects. Gravity is most described by the general theory of relativity which describes gravity not as a force, but as a consequence of the curvature of spacetime caused by the uneven distribution of mass; the most extreme example of this curvature of spacetime is a black hole, from which nothing—not light—can escape once past the black hole's event horizon. However, for most applications, gravity is well approximated by Newton's law of universal gravitation, which describes gravity as a force which causes any two bodies to be attracted to each other, with the force proportional to the product of their masses and inversely proportional to the square of the distance between them.
Gravity is the weakest of the four fundamental forces of physics 1038 times weaker than the strong force, 1036 times weaker than the electromagnetic force and 1029 times weaker than the weak force. As a consequence, it has no significant influence at the level of subatomic particles. In contrast, it is the dominant force at the macroscopic scale, is the cause of the formation and trajectory of astronomical bodies. For example, gravity causes the Earth and the other planets to orbit the Sun, it causes the Moon to orbit the Earth, causes the formation of tides, the formation and evolution of the Solar System and galaxies; the earliest instance of gravity in the Universe in the form of quantum gravity, supergravity or a gravitational singularity, along with ordinary space and time, developed during the Planck epoch from a primeval state, such as a false vacuum, quantum vacuum or virtual particle, in a unknown manner. Attempts to develop a theory of gravity consistent with quantum mechanics, a quantum gravity theory, which would allow gravity to be united in a common mathematical framework with the other three forces of physics, are a current area of research.
Archimedes discovered the center of gravity of a triangle. He postulated that if the centers of gravity of two equal weights wasn't the same, it would be located in the middle of the line that joins them; the Roman architect and engineer Vitruvius in De Architectura postulated that gravity of an object didn't depend on weight but its "nature". Aryabhata first identified the force to explain why objects are not thrown out when the earth rotates. Brahmagupta described gravity as an attractive force and used the term "gruhtvaakarshan" for gravity. Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and early 17th centuries. In his famous experiment dropping balls from the Tower of Pisa, with careful measurements of balls rolling down inclines, Galileo showed that gravitational acceleration is the same for all objects; this was a major departure from Aristotle's belief that heavier objects have a higher gravitational acceleration. Galileo postulated air resistance as the reason that objects with less mass fall more in an atmosphere.
Galileo's work set the stage for the formulation of Newton's theory of gravity. In 1687, English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation. In his own words, "I deduced that the forces which keep the planets in their orbs must reciprocally as the squares of their distances from the centers about which they revolve: and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth; the equation is the following: F = G m 1 m 2 r 2 Where F is the force, m1 and m2 are the masses of the objects interacting, r is the distance between the centers of the masses and G is the gravitational constant. Newton's theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted for by the actions of the other planets. Calculations by both John Couch Adams and Urbain Le Verrier predicted the general position of the planet, Le Verrier's calculations are what led Johann Gottfried Galle to the discovery of Neptune.
A discrepancy in Mercury's orbit pointed out flaws in Newton's theory. By the end of the 19th century, it was known that its orbit showed slight perturbations that could not be accounted for under Newton's theory, but all searches for another perturbing body had been fruitless; the issue was resolved in 1915 by Albert Einstein's new theory of general relativity, which accounted for the small discrepancy in Mercury's orbit. This discrepancy was the advance in the perihelion of Mercury of 42.98 arcseconds per century. Although Newton's theory has been superseded by Einstein's general relativity, most modern non-relativistic gravitational calculations are still made using Newton