SIMBAD is an astronomical database of objects beyond the Solar System. It is maintained by the Centre de données astronomiques de France. SIMBAD was created by merging the Catalog of Stellar Identifications and the Bibliographic Star Index as they existed at the Meudon Computer Centre until 1979, expanded by additional source data from other catalogues and the academic literature; the first on-line interactive version, known as Version 2, was made available in 1981. Version 3, developed in the C language and running on UNIX stations at the Strasbourg Observatory, was released in 1990. Fall of 2006 saw the release of Version 4 of the database, now stored in PostgreSQL, the supporting software, now written in Java; as of 10 February 2017, SIMBAD contains information for 9,099,070 objects under 24,529,080 different names, with 327,634 bibliographical references and 15,511,733 bibliographic citations. The minor planet 4692 SIMBAD was named in its honour. Planetary Data System – NASA's database of information on SSSB, maintained by JPL and Caltech.
NASA/IPAC Extragalactic Database – a database of information on objects outside the Milky Way maintained by JPL. NASA Exoplanet Archive – an online astronomical exoplanet catalog and data service Bibcode SIMBAD, Strasbourg SIMBAD, Harvard
Astronomy & Astrophysics
Astronomy & Astrophysics is a peer-reviewed scientific journal covering theoretical and instrumental astronomy and astrophysics. It is one of the premier journals for astronomy in the world; the journal is published by EDP Sciences in 16 issues per year. The editor-in-chief is Thierry Forveille. Previous editors in chief include Claude Bertout, James Lequeux, Michael Grewing, Catherine Cesarsky and George Contopoulos. Astronomy & Astrophysics was formed in 1969 by the merging of several national journals of individual European countries into one comprehensive publication; these journals, with their ISSN and date of first publication are as follows: Annales d'Astrophysique ISSN 0365-0499, established in 1938 Arkiv för Astronomi ISSN 0004-2048, established in 1948 Bulletin of the Astronomical Institutes of the Netherlands ISSN 0365-8910, established in 1921 Bulletin Astronomique ISSN 0245-9787, established in 1884 Journal des Observateurs ISSN 0368-3389, established in 1915 Zeitschrift für Astrophysik ISSN 0372-8331, established in 1930The publishing of Astronomy & Astrophysics was further extended in 1992 by the incorporation of Bulletin of the Astronomical Institutes of Czechoslovakia, established in 1947.
Astronomy & Astrophysics published articles in either English, French, or German, but articles in French and German were always few. They were discontinued, in part due to difficulties in finding adequately specialized independent referees who were fluent in those languages; the original sponsoring countries were the four countries whose journals merged to form Astronomy & Astrophysics, together with Belgium, Denmark and Norway. The European Southern Observatory participated as a "member country". Norway withdrew, but Austria, Italy and Switzerland all joined; the Czech Republic, Hungary and Slovakia all joined as new members in the 1990s. In 2001 the words "A European Journal" were removed from the front cover in recognition of the fact that the journal was becoming global in scope, in 2002 Argentina was admitted as an "observer". In 2004 the Board of Directors decided that the journal "will henceforth consider applications for sponsoring membership from any country in the world with well-documented active and excellent astronomical research".
Argentina became the first non-European country to gain full membership in 2005. Brazil and Portugal all gained "observer" status at this time and have since progressed to full membership; this journal is listed in the following databases: All letters to the editor and all articles published in the online sections of the journal are open access upon publication. Articles in the other sections of the journal are made available 12 months after publication, through the publisher's site and via the Astrophysics Data System. Authors have the option to pay for immediate open access; the Astrophysical Journal The Astronomical Journal Monthly Notices of the Royal Astronomical Society History and purpose of Astronomy & Astrophysics journal. S. R. Pottasch. EDP Sciences. 2012
The zodiac is an area of the sky that extends 8° north or south of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. The paths of the Moon and visible planets are within the belt of the zodiac. In Western astrology, astronomy, the zodiac is divided into twelve signs, each occupying 30° of celestial longitude and corresponding to the constellations Aries, Gemini, Leo, Libra, Sagittarius, Capricorn and Pisces; the twelve astrological signs form a celestial coordinate system, or more an ecliptic coordinate system, which takes the ecliptic as the origin of latitude and the Sun's position at vernal equinox as the origin of longitude. The English word zodiac derives from zōdiacus, the Latinized form of the Ancient Greek zōidiakòs kýklos, meaning "cycle or circle of little animals". Zōidion is the diminutive of zōion; the name reflects the prominence of animals among the twelve signs. The zodiac was in use by the Roman era, based on concepts inherited by Hellenistic astronomy from Babylonian astronomy of the Chaldean period, which, in turn, derived from an earlier system of lists of stars along the ecliptic.
The construction of the zodiac is described in the Almagest. Although the zodiac remains the basis of the ecliptic coordinate system in use in astronomy besides the equatorial one, the term and the names of the twelve signs are today associated with horoscopic astrology; the term "zodiac" may refer to the region of the celestial sphere encompassing the paths of the planets corresponding to the band of about eight arc degrees above and below the ecliptic. The zodiac of a given planet is the band. By extension, the "zodiac of the comets" may refer to the band encompassing most short-period comets; the division of the ecliptic into the zodiacal signs originates in Babylonian astronomy during the first half of the 1st millennium BC. The zodiac draws on stars in earlier Babylonian star catalogues, such as the MUL. APIN catalogue, compiled around 1000 BC; some of the constellations can be traced further back, to Bronze Age sources, including Gemini "The Twins", from MAŠ. TAB. BA. GAL. GAL "The Great Twins", Cancer "The Crab", from AL.
LUL "The Crayfish", among others. Around the end of the 5th century BC, Babylonian astronomers divided the ecliptic into twelve equal "signs", by analogy to twelve schematic months of thirty days each; each sign contained thirty degrees of celestial longitude, thus creating the first known celestial coordinate system. According to calculations by modern astrophysics, the zodiac was introduced between 409 and 398 BC and within a few years of 401 BC Unlike modern astronomers, who place the beginning of the sign of Aries at the place of the Sun at the vernal equinox; the divisions do not correspond to where the constellations started and ended in the sky. The Sun in fact passed through at least 13, not 12 Babylonian constellations. In order to align with the number of months in a year, designers of the system omitted the major constellation Ophiuchus. Including smaller figures, astronomers have counted up to 21 eligible zodiac constellations. Changes in the orientation of the Earth's axis of rotation means that the time of year the sun is in a given constellation has changed since Babylonian times.
Because the division was made into equal arcs, 30° each, they constituted an ideal system of reference for making predictions about a planet's longitude. However, Babylonian techniques of observational measurements were in a rudimentary stage of evolution and they measured the position of a planet in reference to a set of "normal stars" close to the ecliptic as observational reference points to help positioning a planet within this ecliptic coordinate system. In Babylonian astronomical diaries, a planet position was given with respect to a zodiacal sign alone, less in specific degrees within a sign; when the degrees of longitude were given, they were expressed with reference to the 30° of the zodiacal sign, i.e. not with a reference to the continuous 360° ecliptic. In astronomical ephemerides, the positions of significant astronomical phenomena were computed in sexagesimal fractions of a degree. For daily ephemerides, the daily positions of a planet were not as important as the astrologically significant dates when the planet crossed from one zodiacal sign to the next.
Knowledge of the Babylonian zodiac is reflected in the Hebrew Bible. Some authors have linked the twelve tribes of Israel with the twelve signs and/or the lunar Hebrew calendar having 12 lunar months in a lunar year. Martin and others have argued that the arrangement of the tribes around the Tabernacle corresponded to the order of the Zodiac, with Judah, Reuben and Dan representing the middle signs of Leo, Aquarius and Scorpio, respectively; such connectio
Proper motion is the astronomical measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more distant stars. The components for proper motion in the equatorial coordinate system are given in the direction of right ascension and of declination, their combined value is computed as the total proper motion. It has dimensions of angle per time arcseconds per year or milliarcseconds per year. Knowledge of the proper motion and radial velocity allows calculations of true stellar motion or velocity in space in respect to the Sun, by coordinate transformation, the motion in respect to the Milky Way. Proper motion is not "proper", because it includes a component due to the motion of the Solar System itself. Over the course of centuries, stars appear to maintain nearly fixed positions with respect to each other, so that they form the same constellations over historical time.
Ursa Major or Crux, for example, looks nearly the same now. However, precise long-term observations show that the constellations change shape, albeit slowly, that each star has an independent motion; this motion is caused by the movement of the stars relative to the Solar System. The Sun travels in a nearly circular orbit about the center of the Milky Way at a speed of about 220 km/s at a radius of 8 kPc from the center, which can be taken as the rate of rotation of the Milky Way itself at this radius; the proper motion is a two-dimensional vector and is thus defined by two quantities: its position angle and its magnitude. The first quantity indicates the direction of the proper motion on the celestial sphere, the second quantity is the motion's magnitude expressed in arcseconds per year or milliarcsecond per year. Proper motion may alternatively be defined by the angular changes per year in the star's right ascension and declination, using a constant epoch in defining these; the components of proper motion by convention are arrived at.
Suppose an object moves from coordinates to coordinates in a time Δt. The proper motions are given by: μ α = α 2 − α 1 Δ t, μ δ = δ 2 − δ 1 Δ t; the magnitude of the proper motion μ is given by the Pythagorean theorem: μ 2 = μ δ 2 + μ α 2 ⋅ cos 2 δ, μ 2 = μ δ 2 + μ α ∗ 2, where δ is the declination. The factor in cos2δ accounts for the fact that the radius from the axis of the sphere to its surface varies as cosδ, for example, zero at the pole. Thus, the component of velocity parallel to the equator corresponding to a given angular change in α is smaller the further north the object's location; the change μα, which must be multiplied by cosδ to become a component of the proper motion, is sometimes called the "proper motion in right ascension", μδ the "proper motion in declination". If the proper motion in right ascension has been converted by cosδ, the result is designated μα*. For example, the proper motion results in right ascension in the Hipparcos Catalogue have been converted. Hence, the individual proper motions in right ascension and declination are made equivalent for straightforward calculations of various other stellar motions.
The position angle θ is related to these components by: μ sin θ = μ α cos δ = μ α ∗, μ cos θ = μ δ. Motions in equatorial coordinates can be converted to motions in galactic coordinates. For the majority of stars seen in the sky, the observed proper motions are small and unremarkable; such stars are either faint or are distant, have changes of below 10 milliarcseconds per year, do not appear to move appreciably over many millennia. A few do have significant motions, are called high-proper motion stars. Motions can be in seemingly random directions. Two or more stars, double stars or open star clusters, which are moving in similar directions, exhibit so-called shared or common proper motion, suggesting they may be gravitationally attached or share similar motion in space. Barnard's Star has the largest proper motion of all stars, moving at 10.3 seconds of arc per year. L
In astronomy, luminosity is the total amount of energy emitted per unit of time by a star, galaxy, or other astronomical object. As a term for energy emitted per unit time, luminosity is synonymous with power. In SI units luminosity is measured in joules per second or watts. Values for luminosity are given in the terms of the luminosity of the Sun, L⊙. Luminosity can be given in terms of the astronomical magnitude system: the absolute bolometric magnitude of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band. In contrast, the term brightness in astronomy is used to refer to an object's apparent brightness: that is, how bright an object appears to an observer. Apparent brightness depends on both the luminosity of the object and the distance between the object and observer, on any absorption of light along the path from object to observer. Apparent magnitude is a logarithmic measure of apparent brightness.
The distance determined by luminosity measures can be somewhat ambiguous, is thus sometimes called the luminosity distance. In astronomy, luminosity is the amount of electromagnetic energy; when not qualified, the term "luminosity" means bolometric luminosity, measured either in the SI units, watts, or in terms of solar luminosities. A bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. A star radiates neutrinos, which carry off some energy, contributing to the star's total luminosity; the IAU has defined a nominal solar luminosity of 3.828×1026 W to promote publication of consistent and comparable values in units of the solar luminosity. While bolometers do exist, they cannot be used to measure the apparent brightness of a star because they are insufficiently sensitive across the electromagnetic spectrum and because most wavelengths do not reach the surface of the Earth. In practice bolometric magnitudes are measured by taking measurements at certain wavelengths and constructing a model of the total spectrum, most to match those measurements.
In some cases, the process of estimation is extreme, with luminosities being calculated when less than 1% of the energy output is observed, for example with a hot Wolf-Rayet star observed only in the infra-red. Bolometric luminosities can be calculated using a bolometric correction to a luminosity in a particular passband; the term luminosity is used in relation to particular passbands such as a visual luminosity of K-band luminosity. These are not luminosities in the strict sense of an absolute measure of radiated power, but absolute magnitudes defined for a given filter in a photometric system. Several different photometric systems exist; some such as the UBV or Johnson system are defined against photometric standard stars, while others such as the AB system are defined in terms of a spectral flux density. A star's luminosity can be determined from two stellar characteristics: size and effective temperature; the former is represented in terms of solar radii, R⊙, while the latter is represented in kelvins, but in most cases neither can be measured directly.
To determine a star's radius, two other metrics are needed: the star's angular diameter and its distance from Earth. Both can be measured with great accuracy in certain cases, with cool supergiants having large angular diameters, some cool evolved stars having masers in their atmospheres that can be used to measure the parallax using VLBI. However, for most stars the angular diameter or parallax, or both, are far below our ability to measure with any certainty. Since the effective temperature is a number that represents the temperature of a black body that would reproduce the luminosity, it cannot be measured directly, but it can be estimated from the spectrum. An alternative way to measure stellar luminosity is to measure the star's apparent brightness and distance. A third component needed to derive the luminosity is the degree of interstellar extinction, present, a condition that arises because of gas and dust present in the interstellar medium, the Earth's atmosphere, circumstellar matter.
One of astronomy's central challenges in determining a star's luminosity is to derive accurate measurements for each of these components, without which an accurate luminosity figure remains elusive. Extinction can only be measured directly if the actual and observed luminosities are both known, but it can be estimated from the observed colour of a star, using models of the expected level of reddening from the interstellar medium. In the current system of stellar classification, stars are grouped according to temperature, with the massive young and energetic Class O stars boasting temperatures in excess of 30,000 K while the less massive older Class M stars exhibit temperatures less than 3,500 K; because luminosity is proportional to temperature to the fourth power, the large variation in stellar temperatures produces an vaster variation in stellar luminosity. Because the luminosity depends on a high power of the stellar mass, high mass luminous stars have much shorter lifetimes; the most luminous stars are always young stars, no more than a few million years for the most extreme.
In the Hertzsprung–Russell diagram, the x-axis represents temperature or spectral type while the y-axis represents luminosity or magnitude. The vast majority of stars are found along the main sequence with blue Class O stars found at the top left of the chart while red Class M stars fall to the bottom right. Certain stars like Deneb and Betelgeuse are
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, longer than the age of the universe; the table shows the lifetimes of stars as a function of their masses. All stars are born from collapsing clouds of gas and dust called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star. Nuclear fusion powers a star for most of its life; the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. As the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core; this process causes the star to grow in size, passing through the subgiant stage until it reaches the red giant phase. Stars with at least half the mass of the Sun can begin to generate energy through the fusion of helium at their core, whereas more-massive stars can fuse heavier elements along a series of concentric shells.
Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an dense neutron star or black hole. Although the universe is not old enough for any of the smallest red dwarfs to have reached the end of their lives, stellar models suggest they will become brighter and hotter before running out of hydrogen fuel and becoming low-mass white dwarfs. Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too to be detected over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, by simulating stellar structure using computer models. Stellar evolution starts with the gravitational collapse of a giant molecular cloud. Typical giant molecular clouds are 100 light-years across and contain up to 6,000,000 solar masses.
As it collapses, a giant molecular cloud breaks into smaller pieces. In each of these fragments, the collapsing gas releases gravitational potential energy as heat; as its temperature and pressure increase, a fragment condenses into a rotating sphere of superhot gas known as a protostar. A protostar continues to grow by accretion of gas and dust from the molecular cloud, becoming a pre-main-sequence star as it reaches its final mass. Further development is determined by its mass. Mass is compared to the mass of the Sun: 1.0 M☉ means 1 solar mass. Protostars are encompassed in dust, are thus more visible at infrared wavelengths. Observations from the Wide-field Infrared Survey Explorer have been important for unveiling numerous Galactic protostars and their parent star clusters. Protostars with masses less than 0.08 M☉ never reach temperatures high enough for nuclear fusion of hydrogen to begin. These are known as brown dwarfs; the International Astronomical Union defines brown dwarfs as stars massive enough to fuse deuterium at some point in their lives.
Objects smaller than 13 MJ are classified as sub-brown dwarfs. Both types, deuterium-burning and not, shine dimly and die away cooling over hundreds of millions of years. For a more-massive protostar, the core temperature will reach 10 million kelvin, initiating the proton–proton chain reaction and allowing hydrogen to fuse, first to deuterium and to helium. In stars of over 1 M☉, the carbon–nitrogen–oxygen fusion reaction contributes a large portion of the energy generation; the onset of nuclear fusion leads quickly to a hydrostatic equilibrium in which energy released by the core maintains a high gas pressure, balancing the weight of the star's matter and preventing further gravitational collapse. The star thus evolves to a stable state, beginning the main-sequence phase of its evolution. A new star will sit at a specific point on the main sequence of the Hertzsprung–Russell diagram, with the main-sequence spectral type depending upon the mass of the star. Small cold, low-mass red dwarfs fuse hydrogen and will remain on the main sequence for hundreds of billions of years or longer, whereas massive, hot O-type stars will leave the main sequence after just a few million years.
A mid-sized yellow dwarf star, like the Sun, will remain on the main sequence for about 10 billion years. The Sun is thought to be in the middle of its main sequence lifespan; the core exhausts its supply of hydrogen and the star begins to evolve off of the main sequence. Without the outward pressure generated by the fusion of hydrogen to counteract the force of gravity the core contracts until either electron degeneracy pressure becomes sufficient to oppose gravity or the core becomes hot enough for helium fusion to begin. Which of these happens first depends upon the star's mass. What happens after a low-mass star ceases to produce energy through fusion has not been directly observed. Recent astrophysical models suggest that red dwarfs of 0.1 M☉ may stay on the main sequence for some six to twelve tril
The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to it being a spherical surface, perceived to emit light, it extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of 2/3, or equivalently, a depth from which 50% of light will escape without being scattered. In other words, a photosphere is the deepest region of a luminous object a star, transparent to photons of certain wavelengths; the surface of a star is defined to have a temperature given by the effective temperature in the Stefan–Boltzmann law. Stars, except neutron stars, have no liquid surface. Therefore, the photosphere is used to describe the Sun's or another star's visual surface; the Sun is composed of the chemical elements hydrogen and helium. All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen, carbon and iron being the most abundant.
The Sun's photosphere has a temperature between 4,500 and 6,000 K and a density somewhere around 1×10−3 to 1×10−6 kg/m3. The Sun's photosphere is around 100 kilometers thick, is composed of convection cells called granules—cells of plasma each 1000 kilometers in diameter with hot rising plasma in the center and cooler plasma falling in the narrow spaces between them, flowing at velocities of 7 kilometer per second; each granule has a lifespan of only about twenty minutes, resulting in a continually shifting "boiling" pattern. Grouping the typical granules are super granules up to 30,000 kilometers in diameter with lifespans of up to 24 hours and flow speeds of about 500 meter per second, carrying magnetic field bundles to the edges of the cells. Other magnetically-related phenomena include sunspots and solar faculae dispersed between the granules; these details are too fine to be seen. The Sun's visible atmosphere has other layers above the photosphere: the 2,000 kilometer-deep chromosphere lies just between the photosphere and the much hotter but more tenuous corona.
Other "surface features" on the photosphere are solar sunspots. Animated explanation of the Photosphere. Animated explanation of the temperature of the Photosphere. Solar Lower Atmosphere and Magnetism