In astronomy, the interstellar medium is the matter and radiation that exists in the space between the star systems in a galaxy. This matter includes gas in ionic and molecular form, as well as dust and cosmic rays, it fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field; the interstellar medium is composed of multiple phases, distinguished by whether matter is ionic, atomic, or molecular, the temperature and density of the matter. The interstellar medium is composed of hydrogen followed by helium with trace amounts of carbon and nitrogen comparatively to hydrogen; the thermal pressures of these phases are in rough equilibrium with one another. Magnetic fields and turbulent motions provide pressure in the ISM, are more important dynamically than the thermal pressure is. In all phases, the interstellar medium is tenuous by terrestrial standards.
In cool, dense regions of the ISM, matter is in molecular form, reaches number densities of 106 molecules per cm3. In hot, diffuse regions of the ISM, matter is ionized, the density may be as low as 10−4 ions per cm3. Compare this with a number density of 1019 molecules per cm3 for air at sea level, 1010 molecules per cm3 for a laboratory high-vacuum chamber. By mass, 99% of the ISM is gas in any form, 1% is dust. Of the gas in the ISM, by number 91% of atoms are hydrogen and 8.9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium, known as "metals" in astronomical parlance. By mass this amounts to 70% hydrogen, 28% helium, 1.5% heavier elements. The hydrogen and helium are a result of primordial nucleosynthesis, while the heavier elements in the ISM are a result of enrichment in the process of stellar evolution; the ISM plays a crucial role in astrophysics because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, which contributes to molecular clouds and replenishes the ISM with matter and energy through planetary nebulae, stellar winds, supernovae.
This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, therefore its lifespan of active star formation. Voyager 1 reached the ISM on August 25, 2012, making it the first artificial object from Earth to do so. Interstellar plasma and dust will be studied until the mission's end in 2025, its twin, Voyager 2 entered the ISM in November 2018. Table 1 shows a breakdown of the properties of the components of the ISM of the Milky Way. Field, Goldsmith & Habing put forward the static two phase equilibrium model to explain the observed properties of the ISM, their modeled ISM consisted of a cold dense phase, consisting of clouds of neutral and molecular hydrogen, a warm intercloud phase, consisting of rarefied neutral and ionized gas. McKee & Ostriker added a dynamic third phase that represented the hot gas, shock heated by supernovae and constituted most of the volume of the ISM; these phases are the temperatures where cooling can reach a stable equilibrium.
Their paper formed the basis for further study over the past three decades. However, the relative proportions of the phases and their subdivisions are still not well known; this model takes into account only atomic hydrogen: Temperature larger than 3000 K breaks molecules, lower than 50 000 K leaves atoms in their ground state. It is assumed. Pressure is assumed low, so that durations of free paths of atoms are larger than the ~ 1 nanosecond duration of light pulses which make ordinary, temporally incoherent light. In this collisionless gas, Einstein’s theory of coherent light-matter interactions applies, all gas-light interactions are spatially coherent. Suppose that a monochromatic light is pulsed scattered by molecules having a quadrupole resonance frequency. If “length of light pulses is shorter than all involved time constants”, an “impulsive stimulated Raman scattering ” works: While light generated by incoherent Raman at a shifted frequency has a phase independent on phase of exciting light, thus generates a new spectral line, coherence between incident and scattered light allows their interference into a single frequency, thus shifts incident frequency.
Assume that a star radiates a continuous light spectrum up to X rays. Lyman frequencies are absorbed in this light and pump atoms to first excited state. In this state, hyperfine periods are longer than 1 ns, so that an ISRS “may” redshift light frequency, populating high hyperfine levels. An other ISRS “may” transfer energy from hyperfine levels to thermal electromagnetic waves, so that redshift is permanent. Temperature of a light beam is defined from spectral radiance by Planck's formula; as entropy must increase, “may” becomes “does”. However, where a absorbed line reaches Lyman alpha frequency, redshifting process stops and all hydrogen lines are absorbed, but the stop is not perfect if there is energy at frequency shifted to Lyman beta frequency, which produces a slow redshift. Successive redshifts separated by Lyman absorptions generate many absorption lines, frequencies of which, deduced from absorption process, obey a law more dependable than Karlsson’s formula; the previous process excites more and more atoms because a de-excitation obeys Einstein’s law of coherent interactions: Variation dI of radiance
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
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters its semi-major axis. Planetary migration is the most explanation for hot Jupiters: exoplanets with jovian masses but orbits of only a few days; the accepted theory of planet formation from a protoplanetary disk predicts such planets cannot form so close to their stars, as there is insufficient mass at such small radii and the temperature is too high to allow the formation of rocky or icy planetesimals. It has become clear that terrestrial-mass planets may be subject to rapid inward migration if they form while the gas disk is still present; this may affect the formation of the cores of the giant planets, if those planets form via the core-accretion mechanism. Protoplanetary gas disks around young stars are observed to have lifetimes of a few million years. If planets with masses of around an Earth mass or greater form while the gas is still present, the planets can exchange angular momentum with the surrounding gas in the protoplanetary disk so that their orbits change gradually.
Although the sense of migration is inwards in locally isothermal disks, outward migration may occur in disks that possess entropy gradients. During the late phase of planetary system formation, massive protoplanets and planetesimals gravitationally interact in a chaotic manner causing many planetesimals to be thrown into new orbits; this results in angular-momentum exchange between the planets and the planetesimals, leads to migration. Outward migration of Neptune is believed to be responsible for the resonant capture of Pluto and other Plutinos into the 3:2 resonance with Neptune; this type of orbital migration arises from the gravitational force exerted by a sufficiently massive body embedded in a disk on the surrounding disk's gas, which perturbs its density distribution. By the reaction principle of classical mechanics, the gas exerts an equal and opposite gravitational force on the body, which can be expressed in terms of a torque; this torque alters the angular momentum of the planet's orbit, resulting in a variation of the orbital elements, such as the semi-major axis.
An increase over time of the semi-major axis leads to outward migration, i.e. away from the star, whereas the opposite behavior leads to inward migration. Small planets undergo Type I migration driven by torques arising from waves launched at the locations of the Lindblad and from co-rotation resonances. Lindblad resonances excite spiral density waves in the surrounding gas and exterior of the planet's orbit. In most cases, the outer spiral wave exerts a greater torque than does the inner wave, causing the planet to lose angular momentum, hence migrate toward the star; the migration rate due to these torques is proportional to the mass of the planet and to the local gas density, results in a migration timescale that tends to be short relative to the million-year lifetime of the gaseous disk. Additional co-rotation torques are exerted by gas orbiting with a period similar to that of the planet. In a reference frame attached to the planet, this gas follows horseshoe orbits, reversing direction when it approaches the planet from ahead or from behind.
The gas reversing course ahead of the planet originates from a larger semi-major axis and may be cooler and denser than the gas reversing course behind the planet. This may result in a region of excess density ahead of the planet and of lesser density behind the planet, causing the planet to gain angular momentum; the planet mass for which migration can be approximated to Type I depends on the local gas pressure scale-height and, to a lesser extent, the kinematic viscosity of the gas. In warm and viscous disks, Type I migration may apply to larger mass planets. In locally isothermal disks and far from steep density and temperature gradients, co-rotation torques are overpowered by the Lindblad torques. Regions of outward migration may exist for some planetary mass ranges and disk conditions in both local isothermal and non-isothermal disks; the locations of these regions may vary during the evolution of the disk, in the local-isothermal case are restricted to regions with large density and/or temperature radial gradients over several pressure scale-heights.
Type I migration in a local isothermal disk was shown to be compatible with the formation and long-term evolution of some of the observed Kepler planets. The rapid accretion of solid material by the planet may produce a "heating torque" that causes the planet to gain angular momentum. A planet massive enough to open a gap in a gaseous disk undergoes a regime of migration referred to as Type II; when the mass of a perturbing planet is large enough, the tidal torque it exerts on the gas transfers angular momentum to the gas exterior of the planet's orbit, does the opposite interior to the planet, thereby repelling gas from around the orbit. In a Type I regime, viscous torques can efficiently counter this effect by resupplying gas and smoothing out sharp density gradients, but when the torques become strong enough to overcome the viscous torques in the vicinity of the planet's orbit, a lower density annular gap is created. The depth of this gap depends on the planet mass. In the simple scenario in which no gas crosses the gap, the migration of the planet follows the viscous evolution of the disk's gas.
In the inner disk, the planet spirals inward on the viscous timescale, following the accretion of gas onto the star. In this case, the migration rate is slower than would be the migration of t
HOPS 383 is a Class 0 protostar. It is the first class-0 protostar discovered to have had an outburst, as of 2015, the youngest protostar known to have had an outburst; the protostar was discovered by the Herschel Orion Protostar Survey team. "HOPS 383: An Outbursting Class 0 Protostar in Orion". L5, 6 pp. February 2015.
Young stellar object
Young stellar object denotes a star in its early stage of evolution. This class consists of two groups of objects: pre-main-sequence stars. A star forms by accumulation of material that falls in to a protostar from a circumstellar disk or envelope. Material in the disk is cooler than the surface of the protostar, so it radiates at longer wavelengths of light producing excess infrared emission; as material in the disk is depleted, the infrared excess decreases. Thus, YSOs are classified into evolutionary stages based on the slope of their spectral energy distribution in the mid-infrared, using a scheme introduced by Lada, he proposed three classes, based on the values of intervals of spectral index α: α = d log d log . Here λ is wavelength, F λ is flux density; the α is calculated in the wavelength interval of 2.2–20 μ m. Andre et al. discovered a class 0: objects with strong submillimeter emission, but faint at λ < 10 μ m. Greene et al. added a fifth class of "flat spectrum" sources. Class 0 sources – undetectable at λ < 20 μ m Class I sources have α > 0.3 Flat spectrum sources have 0.3 > α > − 0.3 Class II sources have − 0.3 > α > − 1.6 Class III sources have α < − 1.6 This classification schema reflects evolutionary sequence.
It is believed that most embedded Class 0 sources evolve towards Class I stage, dissipating their circumstellar envelopes. They become optically visible on the stellar birthline as pre-main-sequence stars. Class II objects have circumstellar disks and correspond to classical T Tauri stars, while Class III stars have lost their disks and correspond to weak-line T Tauri stars. An intermediate stage where disks can only be detected at longer wavelengths are known as transition-disk objects. YSOs are associated with early star evolution phenomena: jets and bipolar outflows, Herbig–Haro objects, protoplanetary disks; these stars may be differentiated by mass: Massive YSOs, intermediate-mass YSOs, brown dwarfs. Bok globule Media related to Young stellar objects at Wikimedia Commons
In physics, angular momentum is the rotational equivalent of linear momentum. It is an important quantity in physics because it is a conserved quantity—the total angular momentum of a closed system remains constant. In three dimensions, the angular momentum for a point particle is a pseudovector r × p, the cross product of the particle's position vector r and its momentum vector p = mv; this definition can be applied to each point in physical fields. Unlike momentum, angular momentum does depend on where the origin is chosen, since the particle's position is measured from it. Just like for angular velocity, there are two special types of angular momentum: the spin angular momentum and the orbital angular momentum; the spin angular momentum of an object is defined as the angular momentum about its centre of mass coordinate. The orbital angular momentum of an object about a chosen origin is defined as the angular momentum of the centre of mass about the origin; the total angular momentum of an object is the sum of orbital angular momenta.
The orbital angular momentum vector of a particle is always parallel and directly proportional to the orbital angular velocity vector ω of the particle, where the constant of proportionality depends on both the mass of the particle and its distance from origin. However, the spin angular momentum of the object is proportional but not always parallel to the spin angular velocity Ω, making the constant of proportionality a second-rank tensor rather than a scalar. Angular momentum is additive. For a continuous rigid body, the total angular momentum is the volume integral of angular momentum density over the entire body. Torque can be defined as the rate of change of angular momentum, analogous to force; the net external torque on any system is always equal to the total torque on the system. Therefore, for a closed system, the total torque on the system must be 0, which means that the total angular momentum of the system is constant; the conservation of angular momentum helps explain many observed phenomena, for example the increase in rotational speed of a spinning figure skater as the skater's arms are contracted, the high rotational rates of neutron stars, the Coriolis effect, the precession of gyroscopes.
In general, conservation does limit the possible motion of a system, but does not uniquely determine what the exact motion is. In quantum mechanics, angular momentum is an operator with quantized eigenvalues. Angular momentum is subject to the Heisenberg uncertainty principle, meaning that at any time, only one component can be measured with definite precision; because of this, it turns out that the notion of an elementary particle "spinning" about an axis does not exist. For technical reasons, elementary particles still possess a spin angular momentum, but this angular momentum does not correspond to spinning motion in the ordinary sense. Angular momentum is a vector quantity that represents the product of a body's rotational inertia and rotational velocity about a particular axis. However, if the particle's trajectory lies in a single plane, it is sufficient to discard the vector nature of angular momentum, treat it as a scalar. Angular momentum can be considered a rotational analog of linear momentum.
Thus, where linear momentum p is proportional to mass m and linear speed v, p = m v, angular momentum L is proportional to moment of inertia I and angular speed ω, L = I ω. Unlike mass, which depends only on amount of matter, moment of inertia is dependent on the position of the axis of rotation and the shape of the matter. Unlike linear speed, which does not depend upon the choice of origin, angular velocity is always measured with respect to a fixed origin; therefore speaking, L should be referred to as the angular momentum relative to that center. Because I = r 2 m for a single particle and ω = v r for circular motion, angular momentum can be expanded, L = r 2 m ⋅ v r, reduced to, L = r m v, the product of the radius of rotation r and the linear momentum of the particle p = m v, where v in this case is the equivalent linear speed at the radius; this simple analysis can apply to non-circular motion if only the component of the motion, perpendicular to the radius vector is considered. In that case, L