In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space. The subject of stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well as the measurement of the internal kinematics of more distant galaxies. Measurement of the kinematics of stars in different subcomponents of the Milky Way including the thin disk, the thick disk, the bulge, the stellar halo provides important information about the formation and evolutionary history of our Galaxy. Kinematic measurements can identify exotic phenomena such as hypervelocity stars escaping from the Milky Way, which are interpreted as the result of gravitational encounters of binary stars with the supermassive black hole at the Galactic Center. Stellar kinematics is related to but distinct from the subject of stellar dynamics, which involves the theoretical study or modeling of the motions of stars under the influence of gravity.
Stellar-dynamical models of systems such as galaxies or star clusters are compared with or tested against stellar-kinematic data to study their evolutionary history and mass distributions, to detect the presence of dark matter or supermassive black holes through their gravitational influence on stellar orbits. The component of stellar motion toward or away from the Sun, known as radial velocity, can be measured from the spectrum shift caused by the Doppler effect; the transverse, or proper motion must be found by taking a series of positional determinations against more distant objects. Once the distance to a star is determined through astrometric means such as parallax, the space velocity can be computed; this is the local standard of rest. The latter is taken as a position at the Sun's present location, following a circular orbit around the Galactic Center at the mean velocity of those nearby stars with low velocity dispersion; the Sun's motion with respect to the LSR is called the "peculiar solar motion".
The components of space velocity in the Milky Way's Galactic coordinate system are designated U, V, W, given in km/s, with U positive in the direction of the Galactic Center, V positive in the direction of galactic rotation, W positive in the direction of the North Galactic Pole. The peculiar motion of the Sun with respect to the LSR is = km/s,with statistical uncertainty km/s and systematic uncertainty km/s. Stellar kinematics yields important astrophysical information about stars, the galaxies in which they reside. Stellar kinematics data combined with astrophysical modeling produces important information about the galactic system as a whole. Measured stellar velocities in the innermost regions of galaxies including the Milky Way have provided evidence that many galaxies host supermassive black holes at their center. In farther out regions of galaxies such as within the galactic halo, velocity measurements of globular clusters orbiting in these halo regions of galaxies provides evidence for dark matter.
Both of these cases derive from the key fact that stellar kinematics can be related to the overall potential in which the stars are bound. This means that if accurate stellar kinematics measurements are made for a star or group of stars orbiting in a certain region of a galaxy, the gravitational potential and mass distribution can be inferred given that the gravitational potential in which the star is bound produces its orbit and serves as the impetus for its stellar motion. Examples of using kinematics combined with modeling to construct an astrophysical system include: Rotation of the Milky Way's Disc From the proper motions and radial velocities of stars within the Milky way disc one can show that there is differential rotation; when combining these measurements of stars' proper motions and their radial velocities, along with careful modeling, it is possible to obtain a picture of the rotation of the Milky Way disc. The local character of galactic rotation in the solar neighborhood is encapsulated in the Oort constants.
Structural Components of The Milky Way Using stellar kinematics, astronomers construct models which seek to explain the overall galactic structure in terms of distinct kinematic populations of stars. This is possible because these distinct populations are located in specific regions of galaxies. For example, within the Milky Way, there are three primary components, each with its own distinct stellar kinematics: the disc and bulge or bar; these kinematic groups are related to the stellar populations in the Milky Way, forming a strong correlation between the motion and chemical composition, thus indicating different formation mechanisms. For the Milky Way, the speed of disk stars is V = 220 k m s − 1 and an RMS velocity relative to this speed of V R M S = 50 k m s − 1. For bulge population stars, the velocities are randomly oriented with a larger relative RMS velocity of V R M S = 150 k m s − 1 and no net circular velocity; the Galactic stellar halo consists of stars with orbits that extend to the outer regions of the galaxy.
Some of these stars will
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, moons and galaxies – in either observational or theoretical astronomy. Examples of topics or fields astronomers study include planetary science, solar astronomy, the origin or evolution of stars, or the formation of galaxies. Related but distinct subjects like physical cosmology. Astronomers fall under either of two main types: observational and theoretical. Observational astronomers analyze the data. In contrast, theoretical astronomers create and investigate models of things that cannot be observed; because it takes millions to billions of years for a system of stars or a galaxy to complete a life cycle, astronomers must observe snapshots of different systems at unique points in their evolution to determine how they form and die. They use these data to create models or simulations to theorize how different celestial objects work.
Further subcategories under these two main branches of astronomy include planetary astronomy, galactic astronomy, or physical cosmology. Astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using physical laws. Today, that distinction has disappeared and the terms "astronomer" and "astrophysicist" are interchangeable. Professional astronomers are educated individuals who have a Ph. D. in physics or astronomy and are employed by research institutions or universities. They spend the majority of their time working on research, although they quite have other duties such as teaching, building instruments, or aiding in the operation of an observatory; the number of professional astronomers in the United States is quite small. The American Astronomical Society, the major organization of professional astronomers in North America, has 7,000 members; this number includes scientists from other fields such as physics and engineering, whose research interests are related to astronomy.
The International Astronomical Union comprises 10,145 members from 70 different countries who are involved in astronomical research at the Ph. D. beyond. Contrary to the classical image of an old astronomer peering through a telescope through the dark hours of the night, it is far more common to use a charge-coupled device camera to record a long, deep exposure, allowing a more sensitive image to be created because the light is added over time. Before CCDs, photographic plates were a common method of observation. Modern astronomers spend little time at telescopes just a few weeks per year. Analysis of observed phenomena, along with making predictions as to the causes of what they observe, takes the majority of observational astronomers' time. Astronomers who serve as faculty spend much of their time teaching undergraduate and graduate classes. Most universities have outreach programs including public telescope time and sometimes planetariums as a public service to encourage interest in the field.
Those who become astronomers have a broad background in maths and computing in high school. Taking courses that teach how to research and present papers are invaluable. In college/university most astronomers get a Ph. D. in astronomy or physics. While there is a low number of professional astronomers, the field is popular among amateurs. Most cities have amateur astronomy clubs that meet on a regular basis and host star parties; the Astronomical Society of the Pacific is the largest general astronomical society in the world, comprising both professional and amateur astronomers as well as educators from 70 different nations. Like any hobby, most people who think of themselves as amateur astronomers may devote a few hours a month to stargazing and reading the latest developments in research. However, amateurs span the range from so-called "armchair astronomers" to the ambitious, who own science-grade telescopes and instruments with which they are able to make their own discoveries and assist professional astronomers in research.
List of astronomers List of women astronomers List of Muslim astronomers List of French astronomers List of Hungarian astronomers List of Russian astronomers and astrophysicists List of Slovenian astronomers Dallal, Ahmad. "Science and Technology". In Esposito, John; the Oxford History of Islam. Oxford University Press, New York. ISBN 0-300-15911-0. Kennedy, E. S.. "A Survey of Islamic Astronomical Tables. 46. Philadelphia: American Philosophical Society. Toomer, Gerald. "Al-Khwārizmī, Abu Jaʿfar Muḥammad ibn Mūsā". In Gillispie, Charles Coulston. Dictionary of Scientific Biography. 7. New York: Charles Scribner's Sons. ISBN 0-684-16962-2. American Astronomical Society European Astronomical Society International Astronomical Union Astronomical Society of the Pacific Space's astronomy news
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
Glossary of astronomy
This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth; the field of astronomy features a significant amount of jargon. Absolute magnitude A measure of a star's absolute brightness, it is defined as the apparent magnitude the star would show if it were located at a distance of 10 parsecs, or 32.6 light-years. Accretion disk A circular mass of diffuse material in orbit around a central object, such as a star or black hole; the material is acquired from a source external to the central object, friction causes it to spiral inward towards the object. Active galactic nucleus A compact region in the center of a galaxy displaying a much higher than normal luminosity over some part of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars.
A galaxy hosting an AGN is called an active galaxy. Albedo albedo feature A large area on the surface of a reflecting object that shows a significant contrast in brightness or darkness with adjacent areas. Am star A chemically peculiar star belonging to the more general class of A-type stars; the spectrum of the Am stars shows abnormal deficiencies of certain metals. See metallicity. Anticenter shell aphelion The point. Apoapsis The point of furthest excursion, or separation, between two orbiting objects. Apparent magnitude Also called visual brightness. A measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere; the brighter the object appears. Appulse The closest approach of one celestial object to another. Argument of periapsis Also called the argument of perifocus or argument of pericenter. Artificial satellite asterism Any pattern of stars recognizable in Earth's night sky, it may form part of an official constellation or it may be composed of stars from more than one constellation.
Asteroid asteroid belt The circumstellar disc in the Solar System located between the orbits of Mars and Jupiter, occupied by numerous irregularly shaped small Solar System bodies ranging in size from dust particles to asteroids and minor planets. The asteroid belt is called the main asteroid belt or main belt to distinguish it from other asteroid populations in other parts of the Solar System. Astrometric binary A type of binary system where evidence for an unseen orbiting companion is revealed by its periodic gravitational perturbation of the visible component. See spectroscopic binary. Astrometry astronomical body Also called a celestial body. A type of occurring physical entity, association, or structure within the observable universe, a single bound, contiguous structure, such as a star, moon, or asteroid. Though the terms astronomical "body" and astronomical "object" are used interchangeably, there are technical distinctions. Astronomical catalogue Also spelled astronomical catalog. Astronomical object Also called a celestial object.
A type of occurring physical entity, association, or structure that exists within the observable universe but is a more complex, less cohesively bound structure than an astronomical body, consisting of multiple bodies or other objects with substructures, such as a planetary system, star cluster, nebula, or galaxy. Though the terms astronomical "object" and astronomical "body" are used interchangeably, there are technical distinctions. Astronomical symbol astronomical unit A unit of length used for measuring astronomical distances within the Solar System or between the Earth and distant stars. Conceived as the approximate average distance between the midpoints of the Earth and the Sun, the astronomical unit is now more rigidly defined as 149,597,870.7 kilometres. Astronomy The scientific study of celestial objects and phenomena, the origins of those objects and phenomena, their evolution. Astrophysics atmosphere autumnal equinox The precise time of year on Earth when the Sun appears to cross the celestial equator, while trending southward at each zenith passage.
It represents the moment when the North Pole of the Earth begins to tilt away from the Sun, occurs on or near September 22 each year. Axial precession axial tilt Also called obliquity. Azimuth An angular measurement of an object's orientation along the horizon of the observer, relative to the direction of true north; when combined with the altitude above the horizon, it defines an object's current position in the spherical coordinate system. Barycenter baryogenesis The process by which the class of subatomic particles known as baryons were generated in the early Universe, including the means by which baryons outnumber antibaryons. Big Bang The prevailing cosmological model for the origin of the observable universe, it depicts a starting condition of high density and temperature, followed by an ongoing expansion that led to the current conditions. Binary star black hole A concentration of mass so compact that it creates a region of space from which not light can escape; the outer boundary of this region is called the event horizon.
Break-up velocity Also called critical rotation. The surface velocity at which the centrifugal force generated by a spinning star matches the force of Newtonian gravity. Beyond this point, the star would begin to eject matter from its surface. B
Observational astronomy is a division of astronomy, concerned with recording data about the observable universe, in contrast with theoretical astronomy, concerned with calculating the measurable implications of physical models. It is the practice and study of observing celestial objects with the use of telescopes and other astronomical instruments; as a science, the study of astronomy is somewhat hindered in that direct experiments with the properties of the distant universe are not possible. However, this is compensated by the fact that astronomers have a vast number of visible examples of stellar phenomena that can be examined; this allows for observational data to be plotted on graphs, general trends recorded. Nearby examples of specific phenomena, such as variable stars, can be used to infer the behavior of more distant representatives; those distant yardsticks can be employed to measure other phenomena in that neighborhood, including the distance to a galaxy. Galileo Galilei recorded what he saw.
Since that time, observational astronomy has made steady advances with each improvement in telescope technology. A traditional division of observational astronomy is based on the region of the electromagnetic spectrum observed: Optical astronomy is the part of astronomy that uses optical instruments to observe light from near-infrared to near-ultraviolet wavelengths. Visible-light astronomy, using wavelengths detectable with the human eyes, falls in the middle of this spectrum. Infrared astronomy deals with the analysis of infrared radiation; the most common tool is the reflecting telescope, but with a detector sensitive to infrared wavelengths. Space telescopes are used at certain wavelengths where the atmosphere is opaque, or to eliminate noise. Radio astronomy detects radiation of millimetre to decametre wavelength; the receivers are similar to those used in radio broadcast transmission but much more sensitive. See Radio telescopes. High-energy astronomy includes X-ray astronomy, gamma-ray astronomy, extreme UV astronomy.
Occultation astronomy is the observation of the instant one celestial object occults or eclipses another. Multi-chord asteroid occultation observations measure the profile of the asteroid to the kilometre level. In addition to using electromagnetic radiation, modern astrophysicists can make observations using neutrinos, cosmic rays or gravitational waves. Observing a source using multiple methods is known as multi-messenger astronomy. Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is transparent at the wavelengths being detected. Observatories are located at high altitudes so as to minimise the absorption and distortion caused by the Earth's atmosphere; some wavelengths of infrared light are absorbed by water vapor, so many infrared observatories are located in dry places at high altitude, or in space. The atmosphere is opaque at the wavelengths used by X-ray astronomy, gamma-ray astronomy, UV astronomy and far infrared astronomy, so observations must be carried out from balloons or space observatories.
Powerful gamma rays can, however be detected by the large air showers they produce, the study of cosmic rays is a expanding branch of astronomy. For much of the history of observational astronomy all observation was performed in the visual spectrum with optical telescopes. While the Earth's atmosphere is transparent in this portion of the electromagnetic spectrum, most telescope work is still dependent on seeing conditions and air transparency, is restricted to the night time; the seeing conditions depend on thermal variations in the air. Locations that are cloudy or suffer from atmospheric turbulence limit the resolution of observations; the presence of the full Moon can brighten up the sky with scattered light, hindering observation of faint objects. For observation purposes, the optimal location for an optical telescope is undoubtedly in outer space. There the telescope can make observations without being affected by the atmosphere. However, at present it remains costly to lift telescopes into orbit.
Thus the next best locations are certain mountain peaks that have a high number of cloudless days and possess good atmospheric conditions. The peaks of the islands of Mauna Kea, Hawaii and La Palma possess these properties, as to a lesser extent do inland sites such as Llano de Chajnantor, Cerro Tololo and La Silla in Chile; these observatory locations have attracted an assemblage of powerful telescopes, totalling many billion US dollars of investment. The darkness of the night sky is an important factor in optical astronomy. With the size of cities and human populated areas expanding, the amount of artificial light at night has increased; these artificial lights produce a diffuse background illumination that makes observation of faint astronomical features difficult without special filters. In a few locations such as the state of Arizona and in the United Kingdom, this has led to campaigns for the reduction of light pollution; the use of hoods around street lights not only improves the amount of light directed toward the ground, but helps reduce the light directed toward the sky.
Atmospheric effects can hinder the resolution of a telescope. Without some means of correcting for the blurring effect of the shifting atmosphere, teles
Gamma-ray astronomy is the astronomical observation of gamma rays, the most energetic form of electromagnetic radiation, with photon energies above 100 keV. Radiation is the subject of X-ray astronomy. In most known cases, gamma rays from solar flares and Earth's atmosphere are generated in the MeV range, but it is now known that gamma rays in the GeV range can be generated by solar flares, it had been believed. As GeV gamma rays are important in the study of extra-solar, extra-galactic, new observations may complicate some prior models and findings."Strange gamma rays from the sun may help decipher its magnetic fields"."NASA's Fermi Sees Gamma Rays from'Hidden' Solar Flares". The mechanisms emitting gamma rays are diverse identical with those emitting X-rays but at higher energies, including electron-positron annihilation, the inverse Compton effect, in some cases the decay of radioactive material in space reflecting extreme events such as supernovae and hypernovae, the behaviour of matter under extreme conditions, as in pulsars and blazars.
The highest photon energies measured to date are in the TeV range, the record being held by the Crab Pulsar in 2004, yielding photons with as much as 80 TeV. Observation of gamma rays first became possible in the 1960s, their observation is much more problematic than that of X-rays or of visible light, because gamma-rays are comparatively rare a "bright" source needing an observation time of several minutes before it is detected, because gamma rays are difficult to focus, resulting in a low resolution. The most recent generation of gamma-ray telescopes have a resolution of the order of 6 arc minutes in the GeV range, compared to 0.5 arc seconds seen in the low energy X-ray range by the Chandra X-ray Observatory, about 1.5 arc minutes in the high energy X-ray range seen by High-Energy Focusing Telescope. Energetic gamma rays, with photon energies over ~30 GeV, can be detected by ground-based experiments; the low photon fluxes at such high energies require detector effective areas that are impractically large for current space-based instruments.
Such high-energy photons produce extensive showers of secondary particles in the atmosphere that can be observed on the ground, both directly by radiation counters and optically via the Cherenkov light which the ultra-relativistic shower particles emit. The Imaging Atmospheric Cherenkov Telescope technique achieves the highest sensitivity. Gamma radiation in the TeV range emanating from the Crab Nebula was first detected in 1989 by the Fred Lawrence Whipple Observatory at Mt. Hopkins, in Arizona in the USA. Modern Cherenkov telescope experiments like H. E. S. S. VERITAS, MAGIC, CANGAROO III can detect the Crab Nebula in a few minutes; the most energetic photons observed from an extragalactic object originate from the blazar, Markarian 501. These measurements were done by the High-Energy-Gamma-Ray Astronomy air Cherenkov telescopes. Gamma-ray astronomy observations are still limited by non-gamma-ray backgrounds at lower energies, and, at higher energy, by the number of photons that can be detected.
Larger area detectors and better background suppression are essential for progress in the field. A discovery in 2012 may allow focusing gamma-ray telescopes. At photon energies greater than 700 keV, the index of refraction starts to increase again. Long before experiments could detect gamma rays emitted by cosmic sources, scientists had known that the universe should be producing them. Work by Eugene Feenberg and Henry Primakoff in 1948, Sachio Hayakawa and I. B. Hutchinson in 1952, Philip Morrison in 1958 had led scientists to believe that a number of different processes which were occurring in the universe would result in gamma-ray emission; these processes included cosmic ray interactions with interstellar gas, supernova explosions, interactions of energetic electrons with magnetic fields. However, it was not until the 1960s that our ability to detect these emissions came to pass. Most gamma rays coming from space are absorbed by the Earth's atmosphere, so gamma-ray astronomy could not develop until it was possible to get detectors above all or most of the atmosphere using balloons and spacecraft.
The first gamma-ray telescope carried into orbit, on the Explorer 11 satellite in 1961, picked up fewer than 100 cosmic gamma-ray photons. They appeared to come from all directions in the Universe, implying some sort of uniform "gamma-ray background"; such a background would be expected from the interaction of cosmic rays with interstellar gas. The first true astrophysical gamma-ray sources were solar flares, which revealed the strong 2.223 MeV line predicted by Morrison. This line results from the formation of deuterium via the union of a proton; these first gamma-ray line observations were from OSO-3, OSO-7, the Solar Maximum Mission, the latter spacecraft launched in 1980. The solar observations inspired theoretical work by others. Significant gamma-ray emission from our galaxy was first detected in 1967 by the detector aboard the OSO-3 satellite, it detected 621 events attributable to cosmic gamma rays. However, the field of gamma-ray astronomy took great leaps forward with the SAS-2 and the COS-B satellites.
These two satellites provided an exciting view into the high-energy universe (sometimes called the'violent' universe