The Astrophysical Journal, often abbreviated ApJ (pronounced "ap jay") in references and speech, is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and James Edward Keeler. The journal discontinued its print edition and became an electronic-only journal in 2015.
Since 1953 The Astrophysical Journal Supplement Series (ApJS) has been published in conjunction with The Astrophysical Journal, with generally longer articles to supplement the material in the journal. It publishes six volumes per year, with two 280-page issues per volume.
The Astrophysical Journal Letters (ApJL) is another section of The Astrophysical Journal which rapidly publishes short communications.
The journal and the supplement series were both published by the University of Chicago Press for the American Astronomical Society until, in January 2009, publication was transferred to IOP Publishing, following the move of the society's Astronomical Journal in 2008. The reason for the changes were given by the society as the increasing financial demands of the University of Chicago Press. Compared to journals in other scientific disciplines, The Astrophysical Journal has a larger (> 85%) acceptance rate, which, however, is similar to other journals covering astronomy and astrophysics.
|The Astrophysical Journal|
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The journal was founded in 1895 by George Ellery Hale and James E. Keeler as The Astrophysical Journal: An International Review of Spectroscopy and Astronomical Physics. In addition to the two founding editors, there was an international board of associate editors: M. A. Cornu, Paris; N. C. Dunér, Upsala; William Huggins, London; P. Tacchini, Rome; H. C. Vogel, Potsdam, C. S. Hastings, Yale; A. A. Michelson, Chicago; E. C. Pickering, Harvard; H. A. Rowland, Johns Hopkins; and C. A. Young, Princeton. It was intended that the journal would fill the gap between journals in astronomy and physics, providing a venue for publication of articles on astronomical applications of the spectroscope; on laboratory research closely allied to astronomical physics, including wavelength determinations of metallic and gaseous spectra and experiments on radiation and absorption; on theories of the Sun, Moon, planets, comets, meteors, and nebulae; and on instrumentation for telescopes and laboratories. The further development of ApJ up to 1995 was outlined by Helmut Abt in an article entitled "Some Statistical Highlights of the Astrophysical Journal" in 1995.
The following persons have been editors-in-chief of the journal:
A brown dwarf is a type of substellar object occupying the mass range between the heaviest gas giant planets and the lightest stars, having a mass between approximately 13 to 75–80 times that of Jupiter (MJ), or approximately 2.5×1028 kg to about 1.5×1029 kg. Below this range are the sub-brown dwarfs (sometimes referred to as rogue planets), and above it are the lightest red dwarfs (M9 V). Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.Unlike the stars in the main sequence, brown dwarfs are not massive enough to sustain nuclear fusion of ordinary hydrogen (1H) to helium in their cores. They are, however, thought to fuse deuterium (2H) and to fuse lithium (7Li) if their mass is above a debated threshold of 13 MJ and 65 MJ, respectively. It is also debated whether brown dwarfs would be better defined by their formation processes rather than by their supposed nuclear fusion reactions.Stars are categorized by spectral class, with brown dwarfs designated as types M, L, T, and Y. Despite their name, brown dwarfs are of different colors. Many brown dwarfs would likely appear magenta to the human eye, or possibly orange/red. Brown dwarfs are not very luminous at visible wavelengths.
There are planets known to orbit brown dwarfs: 2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144b.
At a distance of about 6.5 light years, the nearest known brown dwarf is Luhman 16, a binary system of brown dwarfs discovered in 2013. HR 2562 b is listed as the most-massive known exoplanet (as of December 2017) in NASA's exoplanet archive, despite having a mass (30±15 MJ) more than twice the 13-Jupiter-mass cutoff between planets and brown dwarfs.Circumbinary planet
A circumbinary planet is a planet that orbits two stars instead of one.
Because of the short orbits of some binary stars, the only stable orbits for planets are outside the orbit of the two stars. New studies showed that there is a strong hint that the planet and stars originate from a single disk.Exoplanet
An exoplanet () or extrasolar planet is a planet outside the Solar System. The first evidence of an exoplanet was noted in 1917, but was not recognized as such. The first scientific detection of an exoplanet was in 1988; it was confirmed to be an exoplanet in 2012. The first confirmed detection occurred in 1992. As of 23 March 2019, there are 4,016 confirmed planets in 2,999 systems, with 655 systems having more than one planet.There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone. In several cases, multiple planets have been observed around a star.
About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b, about 30 times the mass of Jupiter, although according to some definitions of a planet (based on the nuclear fusion of deuterium), it is too massive to be a planet and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit. Some are so far out that it is difficult to tell whether they are gravitationally bound to the star. Almost all of the planets detected so far are within the Milky Way. Nonetheless, evidence suggests that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist. The nearest exoplanet is Proxima Centauri b, located 4.2 light-years (1.3 parsecs) from Earth and orbiting Proxima Centauri, the closest star to the Sun.The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.Besides exoplanets, there are also rogue planets, which do not orbit any star. These tend to be considered as a separate category, especially if they are gas giants, in which case they are often counted as sub-brown dwarfs, like WISE 0855−0714. The rogue planets in the Milky Way possibly number in the billions (or more).Gliese 876
Gliese 876 is a red dwarf approximately 15 light-years away from Earth in the constellation of Aquarius. It is the one of the closest known stars to the Sun confirmed to possess a planetary system and the fifth closest such system known to consist of multiple planets (after Wolf 1061, Kapteyn's Star, Tau Ceti and Epsilon Eridani). As of 2018, four extrasolar planets have been found to orbit the star. The planetary system is also notable for the orbital properties of its planets. It is the only known system of orbital companions to exhibit a triple conjunction in the rare phenomenon of Laplace resonance (a type of resonance first noted in Jupiter's inner three Galilean moons). It is also the first extrasolar system around a normal star with measured coplanarity. Two of the middle planets are located in the system's habitable zone; however, they are giant planets believed to be analogous to Jupiter.HATNet Project
The Hungarian Automated Telescope Network (HATNet) project is a network of six small fully automated "HAT" telescopes. The scientific goal of the project is to detect and characterize extrasolar planets using the transit method. This network is used also to find and follow bright variable stars. The network is maintained by the Harvard-Smithsonian Center for Astrophysics.
The HAT acronym stands for Hungarian-made Automated Telescope, because it was developed by a small group of Hungarians who met through the Hungarian Astronomical Association. The project started in 1999 and has been fully operational since May 2001.Hot Jupiter
Hot Jupiters are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (P<10 days). The close proximity to their stars and high surface-atmosphere temperatures resulted in the moniker "hot Jupiters".Hot Jupiters are the easiest extrasolar planets to detect via the radial-velocity method, because the oscillations they induce in their parent stars' motion are relatively large and rapid compared to those of other known types of planets. One of the best-known hot Jupiters is 51 Pegasi b. Discovered in 1995, it was the first extrasolar planet found orbiting a Sun-like star. 51 Pegasi b has an orbital period of about 4 days.Large Magellanic Cloud
The Large Magellanic Cloud (LMC) is a satellite galaxy of the Milky Way. At a distance of about 50 kiloparsecs (≈163,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal (~16 kpc) and the possible dwarf irregular galaxy known as the Canis Major Overdensity. Based on readily visible stars and a mass of approximately 10 billion solar masses, the diameter of the LMC is about 14,000 light-years (4.3 kpc), making it roughly one one-hundredth as massive as the Milky Way. This makes the LMC the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy (M31), the Milky Way, and the Triangulum Galaxy (M33).
The LMC is classified as a Magellanic spiral. It contains a stellar bar that is geometrically off-center, suggesting that it was a barred dwarf spiral galaxy before its spiral arms were disrupted, likely by tidal interactions from the Small Magellanic Cloud (SMC), and the Milky Way's gravity.With a declination of about -70°, the LMC is visible as a faint "cloud" only in the southern celestial hemisphere and from latitudes south of 20° N, straddling the border between the constellations of Dorado and Mensa, and appears longer than 20 times the Moon's diameter (about 10° across) from dark sites away from light pollution.The Milky Way and the LMC are expected to collide in approximately 2.4 billion years.List of exoplanets detected by microlensing
This is the list of 19 extrasolar planets detected by microlensing, sorted by projected separations. To find planets using that method, the background star is temporarily magnified by a foreground star because of the gravity that bends light. If the foreground star has a planet, the light from background star would be slightly brighter than the star with no planet. Studying the brightness difference of background star between the foreground star with planets and foreground star with no planets, then mass can be estimated. The projected separation can be determined from how much the light bended.
The most massive planet detected by microlensing is MOA-bin-1b, which masses 3.7 MJ; the least massive is MOA-2007-BLG-192Lb, which masses 0.01 MJ or 3.3 M⊕. The widest separation between a planet and a star is MOA-bin-1b, which is 8.3 AU; the shortest separation is MOA-2007-BLG-192Lb, which is 0.66 AU.
There are 2 members of the multi-planet systems.
Yellow rows denote the members of the multi-planet systemList of largest stars
Below is an ordered list of the largest stars currently known by radius. The unit of measurement used is the radius of the Sun (approximately 695,700 km; 432,288 mi).
The exact order of this list is very incomplete, as great uncertainties currently remain, especially when deriving various important parameters used in calculations, such as stellar luminosity and effective temperature. Often stellar radii can only be expressed as an average or within a large range of values. Values for stellar radii vary significantly in sources and throughout the literature, mostly as the boundary of the very tenuous atmosphere (opacity) greatly differs depending on the wavelength of light in which the star is observed.
Radii of several stars can be directly obtained by stellar interferometry. Other methods can use lunar occultations or from eclipsing binaries, which can be used to test other indirect methods of finding true stellar size. Only a few useful supergiant stars can be occulted by the Moon, including Antares and Aldebaran. Examples of eclipsing binaries include Epsilon Aurigae, VV Cephei, and HR 5171.List of most massive black holes
This is an ordered list of the most massive black holes so far discovered (and probable candidates), measured in units of solar masses (M☉), or the mass of the Sun (approx. 2×1030 kilograms).List of supernova candidates
This is a list of supernova candidates, or stars that astronomers have suggested are supernova progenitors. Type II supernova progenitors include stars with at least 10 solar masses that are in the final stages of their evolution. Prominent examples of stars in this mass range include Antares, Spica, Gamma Velorum, Mu Cephei, and members of the Quintuplet Cluster. Type Ia supernova progenitors are white dwarf stars that are close to the Chandrasekhar limit of about 1.44 solar masses and are accreting matter from a binary companion star. The list includes massive Wolf–Rayet stars, which may become Type Ib/Ic supernovae.Messier 87
Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo. One of the most massive galaxies in the local Universe, it is notable for its large population of globular clusters—about 12,000 compared to the 150–200 orbiting the Milky Way—and its jet of energetic plasma that originates at the core and extends at least 1,500 parsecs (4,900 light-years), traveling at relativistic speed. It is one of the brightest radio sources in the sky, and a popular target for both amateur and professional astronomers.
The French astronomer Charles Messier discovered M87 in 1781, and catalogued it as a nebulous feature while searching for objects that would otherwise confuse comet hunters. M87 is located about 16.4 million parsecs (53 million light-years) from Earth and is the second-brightest galaxy within the northern Virgo Cluster, having many satellite galaxies. Unlike a disk-shaped spiral galaxy, M87 has no distinctive dust lanes. Instead, it has an almost featureless, ellipsoidal shape typical of most giant elliptical galaxies, diminishing in luminosity with distance from the center. Forming around one sixth of its mass, M87's stars have a nearly spherically symmetric distribution. Their population density decreases with increasing distance from the core. It has an active supermassive black hole at its core, which forms the primary component of an active galactic nucleus.
The galaxy is a strong source of multiwavelength radiation, particularly radio waves. Its galactic envelope extends to a radius of about 150 kiloparsecs (490 thousand light-years), where it is truncated—possibly by an encounter with another galaxy. Its interstellar medium consists of diffuse gas enriched by elements emitted from evolved stars.Metallicity
In astronomy, metallicity is used to describe the abundance of elements present in an object that are heavier than hydrogen or helium. Most of the physical matter in the Universe is in the form of hydrogen and helium, so astronomers use the word "metals" as a convenient short term for "all elements except hydrogen and helium". This usage is distinct from the usual physical definition of a solid metal. For example, stars and nebulae with relatively high abundances of carbon, nitrogen, oxygen, and neon are called "metal-rich" in astrophysical terms, even though those elements are non-metals in chemistry.
The presence of heavier elements hails from stellar nucleosynthesis, the theory that the majority of elements heavier than hydrogen and helium in the Universe ("metals", hereafter) are formed in the cores of stars as they evolve. Over time, stellar winds and supernovae deposit the metals into the surrounding environment, enriching the interstellar medium and providing recycling materials for the birth of new stars. It follows that older generations of stars, which formed in the metal-poor early Universe, generally have lower metallicities than those of younger generations, which formed in a more metal-rich Universe.
Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars.
These became commonly known as Population I (metal-rich) and Population II (metal-poor) stars. A third stellar population was introduced in 1978, known as Population III stars. These extremely metal-poor stars were theorised to have been the "first-born" stars created in the Universe.Milky Way
The Milky Way is the galaxy that contains our Solar System. The name describes the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, "milky circle"). From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
The Milky Way is a barred spiral galaxy with a diameter between 150,000 and 200,000 light-years (ly). It is estimated to contain 100–400 billion stars and more than 100 billion planets. The Solar System is located at a radius of 26,490 (± 100) light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, assumed to be a supermassive black hole of 4.100 (± 0.034) million solar masses.
Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much (about 90%) of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter". The rotational period is about 240 million years at the radius of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.NGC 4636
NGC 4636 is an elliptical galaxy located in the constellation Virgo. It is located at a distance of circa 55 million light years from Earth, which, given its apparent dimensions, means that NGC 4636 is about 105,000 light years across. It was discovered by William Herschel on February 23, 1784. NGC 4636 lies one and a half degrees southwest of Delta Virginis. It can be viewed through a telescope at a ×23 magnification as a bright oval glow. It is part of the Herschel 400 Catalogue.Red clump
The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude (MV) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a more dense region of the red giant branch or a bulge towards hotter temperatures. It is most distinct in many, but not all, galactic open clusters, but it is also noticeable in many intermediate-age globular clusters and in nearby field stars (e.g. the Hipparcos stars).
The red clump giants are cool horizontal branch stars, stars originally similar to the Sun which have undergone a helium flash and are now fusing helium in their cores.Supernova
A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is an event that occurs upon the death of certain types of stars.
Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous. The word supernova was coined by Walter Baade and Fritz Zwicky in 1931.Only three Milky Way, naked-eye supernova events have been observed during the last thousand years, though many have been seen in other galaxies. The most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but two more recent supernova remnants have also been found. Statistical observations of supernovae in other galaxies suggest they occur on average about three times every century in the Milky Way, and that any galactic supernova would almost certainly be observable with modern astronomical telescopes.
Supernovae may expel much, if not all, of the material away from a star at velocities up to 30,000 km/s or 10% of the speed of light. This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, and in turn, sweeping up an expanding shell of gas and dust, which is observed as a supernova remnant. Supernovae create, fuse and eject the bulk of the chemical elements produced by nucleosynthesis. Supernovae play a significant role in enriching the interstellar medium with the heavier atomic mass chemical elements. Furthermore, the expanding shock waves from supernovae can trigger the formation of new stars. Supernova remnants are expected to accelerate a large fraction of galactic primary cosmic rays, but direct evidence for cosmic ray production was found only in a few of them so far. They are also potentially strong galactic sources of gravitational waves.Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runaway nuclear fusion, completely disrupting the star. In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical collapse mechanics have been established and accepted by most astronomers for some time.
Owing to the wide range of astrophysical consequences of these events, astronomers now deem supernova research, across the fields of stellar and galactic evolution, as an especially important area for investigation.Void (astronomy)
Cosmic voids are vast spaces between filaments (the largest-scale structures in the universe), which contain very few or no galaxies. Voids typically have a diameter of 10 to 100 megaparsecs; particularly large voids, defined by the absence of rich superclusters, are sometimes called supervoids. They have less than one tenth of the average density of matter abundance that is considered typical for the observable universe. They were first discovered in 1978 in a pioneering study by Stephen Gregory and Laird A. Thompson at the Kitt Peak National Observatory.Voids are believed to have been formed by baryon acoustic oscillations in the Big Bang, collapses of mass followed by implosions of the compressed baryonic matter. Starting from initially small anisotropies from quantum fluctuations in the early universe, the anisotropies grew larger in scale over time. Regions of higher density collapsed more rapidly under gravity, eventually resulting in the large-scale, foam-like structure or "cosmic web" of voids and galaxy filaments seen today. Voids located in high-density environments are smaller than voids situated in low-density spaces of the universe.Voids appear to correlate with the observed temperature of the cosmic microwave background (CMB) because of the Sachs–Wolfe effect. Colder regions correlate with voids and hotter regions correlate with filaments because of gravitational redshifting. As the Sachs–Wolfe effect is only significant if the universe is dominated by radiation or dark energy, the existence of voids is significant in providing physical evidence for dark energy.White dwarf
A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth. A white dwarf's faint luminosity comes from the emission of stored thermal energy; no fusion takes place in a white dwarf wherein mass is converted to energy. The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910. The name white dwarf was coined by Willem Luyten in 1922.
White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star, that of about 10 solar masses. This includes over 97% of the other stars in the Milky Way., § 1. After the hydrogen-fusing period of a main-sequence star of low or medium mass ends, such a star will expand to a red giant during which it fuses helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon (around 1 billion K), an inert mass of carbon and oxygen will build up at its center. After such a star sheds its outer layers and forms a planetary nebula, it will leave behind a core, which is the remnant white dwarf. Usually, white dwarfs are composed of carbon and oxygen. If the mass of the progenitor is between 8 and 10.5 solar masses (M☉), the core temperature will be sufficient to fuse carbon but not neon, in which case an oxygen–neon–magnesium white dwarf may form. Stars of very low mass will not be able to fuse helium, hence, a helium white dwarf may form by mass loss in binary systems.
The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy. As a result, it cannot support itself by the heat generated by fusion against gravitational collapse, but is supported only by electron degeneracy pressure, causing it to be extremely dense. The physics of degeneracy yields a maximum mass for a non-rotating white dwarf, the Chandrasekhar limit—approximately 1.44 times of M☉—beyond which it cannot be supported by electron degeneracy pressure. A carbon-oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a type Ia supernova via a process known as carbon detonation; SN 1006 is thought to be a famous example.
A white dwarf is very hot when it forms, but because it has no source of energy, it will gradually cool as it radiates its energy. This means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very long time, a white dwarf will cool and its material will begin to crystallize, starting with the core. The star's low temperature means it will no longer emit significant heat or light, and it will become a cold black dwarf. Because the length of time it takes for a white dwarf to reach this state is calculated to be longer than the current age of the universe (approximately 13.8 billion years), it is thought that no black dwarfs yet exist. The oldest white dwarfs still radiate at temperatures of a few thousand kelvins.