Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process, the study of protostars and young stellar objects as its immediate products, it is related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations. A spiral galaxy like the Milky Way contains stars, stellar remnants, a diffuse interstellar medium of gas and dust; the interstellar medium consists of 10−4 to 106 particles per cm3 and is composed of 70% hydrogen by mass, with most of the remaining gas consisting of helium.
This medium has been chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, where star formation takes place. In contrast to spirals, an elliptical galaxy loses the cold component of its interstellar medium within a billion years, which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies. In the dense nebulae where stars are produced, much of the hydrogen is in the molecular form, so these nebulae are called molecular clouds. Observations indicate that the coldest clouds tend to form low-mass stars, observed first in the infrared inside the clouds in visible light at their surface when the clouds dissipate, while giant molecular clouds, which are warmer, produce stars of all masses; these giant molecular clouds have typical densities of 100 particles per cm3, diameters of 100 light-years, masses of up to 6 million solar masses, an average interior temperature of 10 K.
About half the total mass of the galactic ISM is found in molecular clouds and in the Milky Way there are an estimated 6,000 molecular clouds, each with more than 100,000 M☉. The nearest nebula to the Sun where massive stars are being formed is the Orion nebula, 1,300 ly away. However, lower mass star formation is occurring about 400–450 light years distant in the ρ Ophiuchi cloud complex. A more compact site of star formation is the opaque clouds of dense gas and dust known as Bok globules, so named after the astronomer Bart Bok; these can form in association with collapsing molecular clouds or independently. The Bok globules are up to a light year across and contain a few solar masses, they can be observed as dark clouds silhouetted against bright emission background stars. Over half the known Bok globules have been found to contain newly forming stars. An interstellar cloud of gas will remain in hydrostatic equilibrium as long as the kinetic energy of the gas pressure is in balance with the potential energy of the internal gravitational force.
Mathematically this is expressed using the virial theorem, which states that, to maintain equilibrium, the gravitational potential energy must equal twice the internal thermal energy. If a cloud is massive enough that the gas pressure is insufficient to support it, the cloud will undergo gravitational collapse; the mass above which a cloud will undergo such collapse is called the Jeans mass. The Jeans mass depends on the temperature and density of the cloud, but is thousands to tens of thousands of solar masses. During cloud collapse dozens to ten thousands of stars form more or less, observable in so-called embedded clusters; the end product of a core collapse is an open cluster of stars. In triggered star formation, one of several events might occur to compress a molecular cloud and initiate its gravitational collapse. Molecular clouds may collide with each other, or a nearby supernova explosion can be a trigger, sending shocked matter into the cloud at high speeds. Alternatively, galactic collisions can trigger massive starbursts of star formation as the gas clouds in each galaxy are compressed and agitated by tidal forces.
The latter mechanism may be responsible for the formation of globular clusters. A supermassive black hole at the core of a galaxy may serve to regulate the rate of star formation in a galactic nucleus. A black hole, accreting infalling matter can become active, emitting a strong wind through a collimated relativistic jet; this can limit further star formation. Massive black holes ejecting radio-frequency-emitting particles at near-light speed can block the formation of new stars in aging galaxies. However, the radio emissions around the jets may trigger star formation. A weaker jet may trigger star formation when it collides with a cloud; as it collapses, a molecular cloud breaks into smaller and smaller pieces in a hierarchical manner, until the fragments reach stellar mass. In each of these fragments, the collapsing gas radiates away the energy gained by the release of gravitational potential energy; as the density increases, the fragments become opaque and are thus less efficient at radiating away their energy.
This inhibits further fragmentation. The fragments now condense into rotating spheres of gas. Complicating this picture of a collapsing cloud are the effects of turbulence, macroscopic flows, magnetic f
A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth; the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way. For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and radiates into outer space. All occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, for some stars by supernova nucleosynthesis when it explodes.
Near the end of its life, a star can contain degenerate matter. Astronomers can determine the mass, age and many other properties of a star by observing its motion through space, its luminosity, spectrum respectively; the total mass of a star is the main factor. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined. A star's life begins with the gravitational collapse of a gaseous nebula of material composed of hydrogen, along with helium and trace amounts of heavier elements; when the stellar core is sufficiently dense, hydrogen becomes converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes.
The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements in shells around the core; as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled as new stars. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole. Binary and multi-star systems consist of two or more stars that are gravitationally bound and move around each other in stable orbits; when two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy. Stars have been important to civilizations throughout the world, they have used for celestial navigation and orientation.
Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun; the motion of the Sun against the background stars was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun; the oldest dated star chart was the result of ancient Egyptian astronomy in 1534 BC. The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period; the first star catalogue in Greek astronomy was created by Aristillus in 300 BC, with the help of Timocharis. The star catalog of Hipparchus included 1020 stars, was used to assemble Ptolemy's star catalogue.
Hipparchus is known for the discovery of the first recorded nova. Many of the constellations and star names in use today derive from Greek astronomy. In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear. In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185; the brightest stellar event in recorded history was the SN 1006 supernova, observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers. The SN 1054 supernova, which gave birth to the Crab Nebula, was observed by Chinese and Islamic astronomers. Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars, they built the first large observatory research institutes for the purpose of producing Zij star catalogues. Among these, the Book of Fixed Stars was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters and galaxies.
According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky
Mass is both a property of a physical body and a measure of its resistance to acceleration when a net force is applied. The object's mass determines the strength of its gravitational attraction to other bodies; the basic SI unit of mass is the kilogram. In physics, mass is not the same as weight though mass is determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass; this is because weight is a force, while mass is the property that determines the strength of this force. There are several distinct phenomena. Although some theorists have speculated that some of these phenomena could be independent of each other, current experiments have found no difference in results regardless of how it is measured: Inertial mass measures an object's resistance to being accelerated by a force. Active gravitational mass measures the gravitational force exerted by an object.
Passive gravitational mass measures the gravitational force exerted on an object in a known gravitational field. The mass of an object determines its acceleration in the presence of an applied force; the inertia and the inertial mass describe the same properties of physical bodies at the qualitative and quantitative level by other words, the mass quantitatively describes the inertia. According to Newton's second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A body's mass determines the degree to which it generates or is affected by a gravitational field. If a first body of mass mA is placed at a distance r from a second body of mass mB, each body is subject to an attractive force Fg = GmAmB/r2, where G = 6.67×10−11 N kg−2 m2 is the "universal gravitational constant". This is sometimes referred to as gravitational mass. Repeated experiments since the 17th century have demonstrated that inertial and gravitational mass are identical.
The standard International System of Units unit of mass is the kilogram. The kilogram is 1000 grams, first defined in 1795 as one cubic decimeter of water at the melting point of ice. However, because precise measurement of a decimeter of water at the proper temperature and pressure was difficult, in 1889 the kilogram was redefined as the mass of the international prototype kilogram of cast iron, thus became independent of the meter and the properties of water. However, the mass of the international prototype and its identical national copies have been found to be drifting over time, it is expected that the re-definition of the kilogram and several other units will occur on May 20, 2019, following a final vote by the CGPM in November 2018. The new definition will use only invariant quantities of nature: the speed of light, the caesium hyperfine frequency, the Planck constant. Other units are accepted for use in SI: the tonne is equal to 1000 kg. the electronvolt is a unit of energy, but because of the mass–energy equivalence it can be converted to a unit of mass, is used like one.
In this context, the mass has units of eV/c2. The electronvolt and its multiples, such as the MeV, are used in particle physics; the atomic mass unit is 1/12 of the mass of a carbon-12 atom 1.66×10−27 kg. The atomic mass unit is convenient for expressing the masses of molecules. Outside the SI system, other units of mass include: the slug is an Imperial unit of mass; the pound is a unit of both mass and force, used in the United States. In scientific contexts where pound and pound need to be distinguished, SI units are used instead; the Planck mass is the maximum mass of point particles. It is used in particle physics; the solar mass is defined as the mass of the Sun. It is used in astronomy to compare large masses such as stars or galaxies; the mass of a small particle may be identified by its inverse Compton wavelength. The mass of a large star or black hole may be identified with its Schwarzschild radius. In physical science, one may distinguish conceptually between at least seven different aspects of mass, or seven physical notions that involve the concept of mass.
Every experiment to date has shown these seven values to be proportional, in some cases equal, this proportionality gives rise to the abstract concept of mass. There are a number of ways mass can be measured or operationally defined: Inertial mass is a measure of an object's resistance to acceleration when a force is applied, it is determined by applying a force to an object and measuring the acceleration that results from that force. An object with small inertial mass will accelerate more than an object with large inertial mass when acted upon by the same force. One says. Active gravitational mass is a measure of the strength of an object's gravitational flux. Gravitational field can be measured by allowing a small "test object" to fall and measuring its free-fall acceleration. For example, an object in free fall near the Moon is subject to a smaller gravitational field, hence
Orion Molecular Cloud Complex
The Orion Molecular Cloud Complex is a star forming region with stellar ages ranging up to 12 Myr. Two giant molecular clouds are a part of it, Orion A and Orion B; the stars forming within the Complex are located within these clouds. A number of other somewhat older stars no longer associated with the molecular gas are part of the Complex, most notably the Orion's Belt, as well as the dispersed population north of it. Near the head of Orion there is a population of young stars, centered on Meissa; the Complex is between 1 000 and 1 400 light-years away, hundreds of light-years across. The Orion Complex is one of the most active regions of nearby stellar formation visible in the night sky, is home to both protoplanetary discs and young stars. Much of it is bright in infrared wavelengths due to the heat-intensive processes involved in stellar formation, though the complex contains dark nebulae, emission nebulae, reflection nebulae, H II regions; the presence of ripples on the surface of Orion's Molecular Clouds was discovered in 2010.
The ripples are a result of the expansion of the nebulae gas over pre-existing molecular gas. The Orion Complex includes a large group of dark clouds in the Orion constellation. Several nebulae can be observed through binoculars and small telescopes, some parts are visible to the naked eye; the following is a list of notable regions within the larger Complex: Orion A Molecular cloud The Orion Nebula known as M42 M43, part of the Orion Nebula Orion Molecular Cloud 1 with the Becklin–Neugebauer Object and the Kleinmann–Low Nebula Orion Molecular Cloud 2 Orion Molecular Cloud 3 Orion Molecular Cloud 4 NGC 1981 NGC 1980 L1641 Orion B Molecular cloud Flame Nebula IC 434, which contains the Horsehead Nebula The Horsehead Nebula M78, a reflection nebula L1622 Orion OB1 Association Orion's Belt Sigma Ori cluster 25 Ori cluster Lambda Orionis molecular ring Lambda Ori cluster B30 B35 Barnard's Loop Runaway star Rho Ophiuchi cloud complex Scorpius–Centaurus Association Orion Cloud Complex SEDS website Clickable table of Messier Objects Orion images ESO: Hidden Secrets of Orion’s Clouds incl.
Photos & Animations
H II region
An H II region or HII region is a region of interstellar atomic hydrogen, ionized. It is a cloud of ionized gas in which star formation has taken place, with a size ranging from one to hundreds of light years, density from a few to about a million particles per cubic cm; the Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered. They may be of any shape; the short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. H II regions—sometimes several hundred light-years across—are associated with giant molecular clouds, they appear clumpy and filamentary, sometimes showing intricate shapes such as the Horsehead Nebula. H II regions may give birth to thousands of stars over a period of several million years. In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H II region, leaving behind a cluster of stars which have formed, such as the Pleiades.
H II regions can be observed at considerable distances in the universe, the study of extragalactic H II regions is important in determining the distance and chemical composition of galaxies. Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are devoid of them. In spiral galaxies, including our Milky Way, H II regions are concentrated in the spiral arms, while in irregular galaxies they are distributed chaotically; some galaxies contain huge H II regions. Examples include the 30 Doradus region in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy; the term H II is pronounced "H two" by astronomers. "H" is the chemical symbol for hydrogen, "II" is the Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionised—H II is H+ in other sciences—III for doubly-ionised, e.g. O III is O++, etc. H II, or H+, consists of free protons. An H I region being neutral atomic hydrogen, a molecular cloud being molecular hydrogen, H2.
In spoken discussion with non-astronomers there is sometimes confusion between the identical spoken forms of "H II" and "H2". A few of the brightest H II regions are visible to the naked eye. However, none seem to have been noticed before the advent of the telescope in the early 17th century. Galileo did not notice the Orion Nebula when he first observed the star cluster within it; the French observer Nicolas-Claude Fabri de Peiresc is credited with the discovery of the Orion Nebula in 1610. Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies. William Herschel observed the Orion Nebula in 1774, described it as "an unformed fiery mist, the chaotic material of future suns". In early days astronomers distinguished between "diffuse nebulae", which retained their fuzzy appearance under magnification through a large telescope, nebulae that could be resolved into stars, now known to be galaxies external to our own. Confirmation of Herschel's hypothesis of star formation had to wait another hundred years, when William Huggins together with his wife Mary Huggins turned his spectroscope on various nebulae.
Some, such as the Andromeda Nebula, had spectra quite similar to those of stars, but turned out to be galaxies consisting of hundreds of millions of individual stars. Others looked different. Rather than a strong continuum with absorption lines superimposed, the Orion Nebula and other similar objects showed only a small number of emission lines. In planetary nebulae, the brightest of these spectral lines was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known chemical 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. However, 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. Interstellar matter, considered dense in an astronomical context, is at high vacuum by laboratory standards.
Physicists showed in the 1920s that in gas at low density, electrons can populate excited metastable energy levels in atoms and ions, which at higher densities are de-excited by collisions. Electron transitions from these levels in doubly ionized oxygen give rise to the 500.7 nm line. These spectral lines, which can only be seen in low density gases, are called forbidden lines. Spectroscopic observations thus showed that planetary nebulae consisted of rarefied ionised oxygen gas. During the 20th century, observations showed that H II regions contained hot, bright stars; these stars are many times more massive than the Sun, are the shortest-lived stars, with total lifetimes of only a few million years. Therefore, it was surmised. Over a period of several million years, a cluster of stars will form in an H II region, before radiation pressure from the hot young stars causes the nebula to disperse; the Pleiades are an example of a cluster which has'boiled away' the H II region from which it was formed.
Only a trace of reflection nebulosity remain
OB stars are hot, massive stars of spectral types O or early-type B that form in loosely organized groups called OB associations. They are short lived, thus do not move far from where they formed within their life. During their lifetime, they will emit much ultraviolet radiation; this radiation ionizes the surrounding interstellar gas of the giant molecular cloud, forming an H II region or Strömgren sphere. In lists of spectra the "spectrum of OB" refers to "unknown, but belonging to an OB association so thus of early type". O-type main-sequence star B-type main-sequence star Stellar kinematics B. Cameron Reed. "Catalog Of Galactic OB Stars". The Astronomical Journal. 125: 2531–2533. Bibcode:2003AJ....125.2531R. Doi:10.1086/374771. Bouy, Hervé and Alves, João: Cosmography of OB Stars in the Solar Neighborhood Astronomy & Astrophysics. A three-dimensional map of OB star density within 500 pc of the Sun. — Scientia Astrophysical Organization's star classification page Philippe Stee's homepage: Hot and Active Stars Research
The Gould Belt is a partial ring of stars in the Milky Way, about 3000 light years across, tilted toward the galactic plane by about 16 to 20 degrees. It contains many O- and B-type stars, may represent the local spiral arm to which the Sun belongs—currently the Sun is about 325 light years from the arm's center; the belt is thought to be from 30 to 50 million years old, of unknown origin. It is named after Benjamin Gould, who identified it in 1879; the belt contains bright stars in many constellations including Cepheus, Perseus, Canis Major, Vela, Crux, Centaurus and Scorpius. The Milky Way visible in the sky passes through most of these constellations, but a bit southeast of Lupus. Star-forming regions and OB associations that make up this region include the Orion Nebula and the Orion molecular clouds, the Scorpius-Centaurus OB Association, Cepheus OB2, Perseus OB2, the Taurus-Auriga Molecular Clouds; the Serpens Molecular Cloud containing star-forming regions W40 and Serpens south is included in Gould Belt surveys, but is not formally part of the Gould Belt due to its greater distance.
A theory proposed around 2009 suggests that the Gould Belt formed about 30 million years ago when a blob of dark matter collided with the molecular cloud in our region. There is evidence for similar Gould belts in other galaxies. Gould Belt - Astronoo Map of the Gould Belt 3D evolution of the Gould Belt The Spitzer Gould Belt Survey