Chandra X-ray Observatory
The Chandra X-ray Observatory known as the Advanced X-ray Astrophysics Facility, is a Flagship-class space telescope launched on STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, enabled by the high angular resolution of its mirrors. Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes. Chandra is an Earth satellite in a 64-hour orbit, its mission is ongoing as of 2019. Chandra is one of the Great Observatories, along with the Hubble Space Telescope, Compton Gamma Ray Observatory, the Spitzer Space Telescope; the telescope is named after the Nobel Prize-winning Indian-American astrophysicist Subrahmanyan Chandrasekhar. Its mission is similar to that of ESA's XMM-Newton spacecraft launched in 1999. In 1976 the Chandra X-ray Observatory was proposed to NASA by Riccardo Giacconi and Harvey Tananbaum. Preliminary work began the following year at Marshall Space Flight Center and the Smithsonian Astrophysical Observatory.
In the meantime, in 1978, NASA launched the first imaging X-ray telescope, into orbit. Work continued on the AXAF project throughout the 1990s. In 1992, to reduce costs, the spacecraft was redesigned. Four of the twelve planned mirrors were eliminated. AXAF's planned orbit was changed to an elliptical one, reaching one third of the way to the Moon's at its farthest point; this eliminated the possibility of improvement or repair by the space shuttle but put the observatory above the Earth's radiation belts for most of its orbit. AXAF was tested by TRW in Redondo Beach, California. AXAF was renamed Chandra as part of a contest held by NASA in 1998, which drew more than 6,000 submissions worldwide; the contest winners, Jatila van der Veen and Tyrel Johnson, suggested the name in honor of Nobel Prize–winning Indian-American astrophysicist Subrahmanyan Chandrasekhar. He is known for his work in determining the maximum mass of white dwarf stars, leading to greater understanding of high energy astronomical phenomena such as neutron stars and black holes.
Fittingly, the name Chandra means "moon" in Sanskrit. Scheduled to be launched in December 1998, the spacecraft was delayed several months being launched in July 23, 1999, at 04:31 UTC by Space Shuttle Columbia during STS-93. Chandra was deployed from Columbia at 11:47 UTC; the Inertial Upper Stage's first stage motor ignited at 12:48 UTC, after burning for 125 seconds and separating, the second stage ignited at 12:51 UTC and burned for 117 seconds. At 22,753 kilograms, it was the heaviest payload launched by the shuttle, a consequence of the two-stage Inertial Upper Stage booster rocket system needed to transport the spacecraft to its high orbit. Chandra has been returning data since the month, it is operated by the SAO at the Chandra X-ray Center in Cambridge, with assistance from MIT and Northrop Grumman Space Technology. The ACIS CCDs suffered particle damage during early radiation belt passages. To prevent further damage, the instrument is now removed from the telescope's focal plane during passages.
Although Chandra was given an expected lifetime of 5 years, on September 4, 2001, NASA extended its lifetime to 10 years "based on the observatory's outstanding results." Physically Chandra could last much longer. A 2004 study performed at the Chandra X-ray Center indicated that the observatory could last at least 15 years. In July 2008, the International X-ray Observatory, a joint project between ESA, NASA and JAXA, was proposed as the next major X-ray observatory but was cancelled. ESA resurrected the project as the Advanced Telescope for High Energy Astrophysics with a proposed launch in 2028. On October 10, 2018, Chandra entered safe mode operations, due to a gyroscope glitch. NASA reported. Within days, the 3-second error in data from one gyro was understood, plans were made to return Chandra to full service; the gyroscope that experienced the glitch is otherwise healthy. The data gathered by Chandra has advanced the field of X-ray astronomy. Here are some examples of discoveries supported by observations from Chandra: The first light image, of supernova remnant Cassiopeia A, gave astronomers their first glimpse of the compact object at the center of the remnant a neutron star or black hole.
In the Crab Nebula, another supernova remnant, Chandra showed a never-before-seen ring around the central pulsar and jets that had only been seen by earlier telescopes. The first X-ray emission was seen from the supermassive black hole, Sagittarius A*, at the center of the Milky Way. Chandra found much more cool gas than expected spiraling into the center of the Andromeda Galaxy. Pressure fronts were observed in detail for the first time in Abell 2142, where clusters of galaxies are merging; the earliest images in X-rays of the shock wave of a supernova were taken of SN 1987A. Chandra showed for the first time the shadow of a small galaxy as it is being cannibalized by a larger one, in an image of Perseus A. A new type of black hole was discovered in galaxy M82, mid-mass objects purported to be the missing link between stellar-sized black holes and super massive black holes. X-ray emission lines were associated for the first time with a gamma-ray burst
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
In physics, redshift is a phenomenon where electromagnetic radiation from an object undergoes an increase in wavelength. Whether or not the radiation is visible, "redshift" means an increase in wavelength, equivalent to a decrease in wave frequency and photon energy, in accordance with the wave and quantum theories of light. Neither the emitted nor perceived light is red. Examples of redshifting are a gamma ray perceived as an X-ray, or visible light perceived as radio waves; the opposite of a redshift is energy increases. However, redshift is a more common term and sometimes blueshift is referred to as negative redshift. There are three main causes of red in astronomy and cosmology: Objects move apart in space; this is an example of the Doppler effect. Space itself expands; this is known as cosmological redshift. All sufficiently distant light sources show redshift corresponding to the rate of increase in their distance from Earth, known as Hubble's Law. Gravitational redshift is a relativistic effect observed due to strong gravitational fields, which distort spacetime and exert a force on light and other particles.
Knowledge of redshifts and blueshifts has been used to develop several terrestrial technologies such as Doppler radar and radar guns. Redshifts are seen in the spectroscopic observations of astronomical objects, its value is represented by the letter z. A special relativistic redshift formula can be used to calculate the redshift of a nearby object when spacetime is flat. However, in many contexts, such as black holes and Big Bang cosmology, redshifts must be calculated using general relativity. Special relativistic and cosmological redshifts can be understood under the umbrella of frame transformation laws. There exist other physical processes that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; the history of the subject began with the development in the 19th century of wave mechanics and the exploration of phenomena associated with the Doppler effect. The effect is named after Christian Doppler, who offered the first known physical explanation for the phenomenon in 1842.
The hypothesis was tested and confirmed for sound waves by the Dutch scientist Christophorus Buys Ballot in 1845. Doppler predicted that the phenomenon should apply to all waves, in particular suggested that the varying colors of stars could be attributed to their motion with respect to the Earth. Before this was verified, however, it was found that stellar colors were due to a star's temperature, not motion. Only was Doppler vindicated by verified redshift observations; the first Doppler redshift was described by French physicist Hippolyte Fizeau in 1848, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. The effect is sometimes called the "Doppler–Fizeau effect". In 1868, British astronomer William Huggins was the first to determine the velocity of a star moving away from the Earth by this method. In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.1 Å in the red. In 1887, Vogel and Scheiner discovered the annual Doppler effect, the yearly change in the Doppler shift of stars located near the ecliptic due to the orbital velocity of the Earth.
In 1901, Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors. The earliest occurrence of the term red-shift in print appears to be by American astronomer Walter S. Adams in 1908, in which he mentions "Two methods of investigating that nature of the nebular red-shift"; the word does not appear unhyphenated until about 1934 by Willem de Sitter indicating that up to that point its German equivalent, was more used. Beginning with observations in 1912, Vesto Slipher discovered that most spiral galaxies mostly thought to be spiral nebulae, had considerable redshifts. Slipher first reports on his measurement in the inaugural volume of the Lowell Observatory Bulletin. Three years he wrote a review in the journal Popular Astronomy. In it he states that "the early discovery that the great Andromeda spiral had the quite exceptional velocity of –300 km showed the means available, capable of investigating not only the spectra of the spirals but their velocities as well."
Slipher reported the velocities for 15 spiral nebulae spread across the entire celestial sphere, all but three having observable "positive" velocities. Subsequently, Edwin Hubble discovered an approximate relationship between the redshifts of such "nebulae" and the distances to them with the formulation of his eponymous Hubble's law; these observations corroborated Alexander Friedmann's 1922 work, in which he derived the Friedmann-Lemaître equations. They are today considered strong evidence for the Big Bang theory; the spectrum of light that comes from a single source can be measured. To determine the redshift, one searches for features in the spectrum such as absorption lines, emission lines, or other variations in light intensity. If found, these featur
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dark matter. The word galaxy is derived from the Greek galaxias "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers; the Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, observed as it existed just 400 million years after the Big Bang. Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion to a suggested 2 trillion or more, containing more stars than all the grains of sand on planet Earth.
Most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. For comparison, the Milky Way has a diameter of at least 30,000 parsecs and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs; the space between galaxies is filled with a tenuous gas having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups and superclusters; the Milky Way is part of the Local Group, dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are arranged into sheets and filaments surrounded by immense voids; the largest structure of galaxies yet recognised is a cluster of superclusters, named Laniakea, which contains the Virgo supercluster. The origin of the word galaxy derives from the Greek term for the Milky Way, galaxias, or kyklos galaktikos due to its appearance as a "milky" band of light in the sky.
In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, it produces the faint band of light known as the Milky Way. In the astronomical literature, the capitalized word "Galaxy" is used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe; the English term Milky Way can be traced back to a story by Chaucer c. 1380: "See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt." Galaxies were discovered telescopically and were known as spiral nebulae. Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way.
For this reason they were popularly called island universes, but this term fell into disuse, as the word universe implied the entirety of existence. Instead, they became known as galaxies. Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, the MCG and UGC. All of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a spiral galaxy having the number 109 in the catalogue of Messier, having the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, PGC 37617; the realization that we live in a galaxy, one among many galaxies, parallels major discoveries that were made about the Milky Way and other nebulae. The Greek philosopher Democritus proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.
Aristotle, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World, continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger was critical of this view, arguing that if the Milky Way is sublunary it should appear different at different times and places on Earth, that it should have parallax, which it does not. In his view, the Milky Way is celestial. According to Mohani Mohamed, the Arabian astronomer Alhazen made the first attempt at observing and measuring the Milky Way's parallax, he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." The Persian astronomer al-Bīrūnī
A supercluster is a large group of smaller galaxy clusters or galaxy groups. The Milky Way is part of the Local Group galaxy group, which in turn is part of the Laniakea Supercluster; this supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years. The large size and low density of superclusters means they, unlike clusters, expand with the Hubble expansion; the number of superclusters in the observable universe is estimated to be 10 million. The existence of superclusters indicates that the galaxies in the Universe are not uniformly distributed; those groups and clusters and additional isolated galaxies in turn form larger structures called superclusters. Their existence was first postulated by George Abell in his 1958 Abell catalogue of galaxy clusters, he called them clusters of clusters. Superclusters form massive structures of galaxies, called "filaments", "supercluster complexes", "walls" or "sheets", that may span between several hundred million light-years to 10 billion light-years, covering more than 5% of the observable universe.
These are the largest known structures to date. Observations of superclusters can give information about the initial condition of the universe, when these superclusters were created; the directions of the rotational axes of galaxies within superclusters may give insight and information into the early formation process of galaxies in the history of the Universe. Interspersed among superclusters are large voids of space. Superclusters are subdivided into groups of clusters called galaxy groups and clusters. Freedman, Roger. "Galaxies". Universe. New York: W. H. Freedman. ISBN 978-1-319-04238-7. Media related to Superclusters of galaxies at Wikimedia Commons Overview of local superclusters The Nearest Superclusters Universe family tree: Supercluster Superclusters - Large Scale Structures
Dark matter is a hypothetical form of matter, thought to account for 85% of the matter in the universe and about a quarter of its total energy density. The majority of dark matter is thought to be non-baryonic in nature being composed of some as-yet undiscovered subatomic particles, its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained by accepted theories of gravity unless more matter is present than can be seen. For this reason, most experts think dark matter to be ubiquitous in the universe and to have had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with observable electromagnetic radiation, such as light, is thus invisible to the entire electromagnetic spectrum, making it difficult to detect using usual astronomical equipment; the primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of unseen matter.
Other lines of evidence include observations in gravitational lensing, from the cosmic microwave background, from astronomical observations of the observable universe's current structure, from the formation and evolution of galaxies, from mass location during galactic collisions, from the motion of galaxies within galaxy clusters. In the standard Lambda-CDM model of cosmology, the total mass–energy of the universe contains 5% ordinary matter and energy, 27% dark matter and 68% of an unknown form of energy known as dark energy. Thus, dark matter constitutes 85% of total mass, while dark energy plus dark matter constitute 95% of total mass–energy content; because dark matter has not yet been observed directly, if it exists, it must interact with ordinary baryonic matter and radiation, except through gravity. The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, in particular, weakly-interacting massive particles, or gravitationally-interacting massive particles.
Many experiments to directly detect and study dark matter particles are being undertaken, but none have yet succeeded. Dark matter is classified as warm, or hot according to its velocity. Current models favor a cold dark matter scenario, in which structures emerge by gradual accumulation of particles. Although the existence of dark matter is accepted by the scientific community, some astrophysicists, intrigued by certain observations that do not fit the dark matter theory, argue for various modifications of the standard laws of general relativity, such as modified Newtonian dynamics, tensor–vector–scalar gravity, or entropic gravity; these models attempt to account for all observations without invoking supplemental non-baryonic matter. The hypothesis of dark matter has an elaborate history. In a talk given in 1884, Lord Kelvin estimated the number of dark bodies in the Milky Way from the observed velocity dispersion of the stars orbiting around the center of the galaxy. By using these measurements, he estimated the mass of the galaxy, which he determined is different from the mass of visible stars.
Lord Kelvin thus concluded that "many of our stars a great majority of them, may be dark bodies". In 1906 Henri Poincaré in "The Milky Way and Theory of Gases" used "dark matter", or "matière obscure" in French, in discussing Kelvin's work; the first to suggest the existence of dark matter, using stellar velocities, was Dutch astronomer Jacobus Kapteyn in 1922. Fellow Dutchman and radio astronomy pioneer Jan Oort hypothesized the existence of dark matter in 1932. Oort was studying stellar motions in the local galactic neighborhood and found that the mass in the galactic plane must be greater than what was observed, but this measurement was determined to be erroneous. In 1933, Swiss astrophysicist Fritz Zwicky, who studied galaxy clusters while working at the California Institute of Technology, made a similar inference. Zwicky applied the virial theorem to the Coma Cluster and obtained evidence of unseen mass that he called dunkle Materie. Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies.
He estimated. The gravity effect of the visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred that some unseen matter provided the mass and associated gravitation attraction to hold the cluster together; this was the first formal inference about the existence of dark matter. Zwicky's estimates were off by more than an order of magnitude due to an obsolete value of the Hubble constant. However, Zwicky did infer that the bulk of the matter was dark. Further indications that the mass-to-light ratio was not unity came from measurements of galaxy rotation curves. In 1939, Horace W. Babcock reported the rotation curve for the Andromeda nebula, which suggested that the mass-to-luminosity ratio increases radially, he attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral and not to the missing matter that he had uncovered. Following Babcock's 1939 report of unexpectedly rapid rotation in the outskirts of the Andromeda galaxy and a mass-to-light ratio of 50, in 1940 Jan Oort discovered and wrote about the large non-visible halo of NGC 3115.
Vera Rubin, Kent Ford and Ken Freeman's work in the
Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy's disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision. A giant galaxy interacting with its satellites is common. A satellite's gravity could attract one of the primary's spiral arms, or the secondary satellite's path could coincide with the position of the primary satellite's and so would dive into the primary galaxy; that can trigger a small amount of star formation. Such orphaned clusters of stars were sometimes referred to as "blue blobs" before they were recognized as stars. Colliding galaxies are common during galaxy evolution; the tenuous distribution of matter in galaxies means these are not collisions in the traditional sense of the word, but rather gravitational interactions. Colliding may lead to merging if two galaxies collide and do not have enough momentum to continue traveling after the collision.
In that case, they fall back into each other and merge into one galaxy after many passes through each other. If one of the colliding galaxies is much larger than the other, it will remain intact after the merger; the larger galaxy will look much the same, while the smaller galaxy will be stripped apart and become part of the larger galaxy. When galaxies pass through each other, unlike during mergers, they retain their material and shape after the pass. Galactic collisions are now simulated on computers, which use realistic physics principles, including the simulation of gravitational forces, gas dissipation phenomena, star formation, feedback. Dynamical friction slows the relative motion galaxy pairs, which may merge at some point, according to the initial relative energy of the orbits. A library of simulated galaxy collisions can be found at the Paris Observatory website: GALMER Galactic cannibalism refers to the process in which a large galaxy, through tidal gravitational interactions with a companion, merges with that companion.
The most common result of the gravitational merger between two or more galaxies is an irregular galaxy, but elliptical galaxies may result. It has been suggested that galactic cannibalism is occurring between the Milky Way and the Large and Small Magellanic Clouds. Streams of gravitationally-attracted hydrogen arcing from these dwarf galaxies to the Milky Way is taken as evidence for the theory. Galaxy harassment is a type of interaction between a low-luminosity galaxy and a brighter one that takes place within rich galaxy clusters, such as Virgo and Coma, where galaxies are moving at high relative speeds and suffering frequent encounters with other systems of the cluster by the high galactic density of the latter. According to computer simulations, the interactions convert the affected galaxy disks into disturbed barred spiral galaxies and produces starbursts followed by, if more encounters occur, loss of angular momentum and heating of their gas; the result would be the conversion of low-luminosity spiral galaxies into dwarf spheroidals and dwarf ellipticals.
Evidence for the hypothesis had been claimed by studying early-type dwarf galaxies in the Virgo Cluster and finding structures, such as disks and spiral arms, which suggest they are former disk systems transformed by the above-mentioned interactions. However, the existence of similar structures in isolated early-type dwarf galaxies, such as LEDA 2108986, has undermined this hypothesis Astronomers have estimated the Milky Way galaxy, will collide with the Andromeda galaxy in about 4.5 billion years. It is thought that the two spiral galaxies will merge to become an elliptical galaxy or a large disk galaxy. NGC 7318 Galaxy Collisions Galactic cannibalism Galactic Collision Simulation GALMER: Galaxy Merger Simulations