A pulsar is a magnetized rotating neutron star that emits a beam of electromagnetic radiation. This radiation can be observed only when the beam of emission is pointing toward Earth, is responsible for the pulsed appearance of emission. Neutron stars are dense, have short, regular rotational periods; this produces a precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar. Pulsars are believed to be one of the candidates for the source of ultra-high-energy cosmic rays; the periods of pulsars make them useful tools. Observations of a pulsar in a binary neutron star system were used to indirectly confirm the existence of gravitational radiation; the first extrasolar planets were discovered around a pulsar, PSR B1257+12. Certain types of pulsars rival atomic clocks in their accuracy in keeping time; the first pulsar was observed on November 1967, by Jocelyn Bell Burnell and Antony Hewish. They observed pulses separated by 1.33 seconds that originated from the same location in the sky, kept to sidereal time.
In looking for explanations for the pulses, the short period of the pulses eliminated most astrophysical sources of radiation, such as stars, since the pulses followed sidereal time, it could not be man-made radio frequency interference. When observations with another telescope confirmed the emission, it eliminated any sort of instrumental effects. At this point, Bell Burnell said of herself and Hewish that "we did not believe that we had picked up signals from another civilization, but the idea had crossed our minds and we had no proof that it was an natural radio emission, it is an interesting problem—if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly?" So, they nicknamed the signal LGM-1, for "little green men". It was not until a second pulsating source was discovered in a different part of the sky that the "LGM hypothesis" was abandoned, their pulsar was dubbed CP 1919, is now known by a number of designators including PSR 1919+21 and PSR J1921+2153.
Although CP 1919 emits in radio wavelengths, pulsars have subsequently been found to emit in visible light, X-ray, gamma ray wavelengths. The word "pulsar" is a portmanteau of'pulsating' and'quasar', first appeared in print in 1968: The existence of neutron stars was first proposed by Walter Baade and Fritz Zwicky in 1934, when they argued that a small, dense star consisting of neutrons would result from a supernova. Based on the idea of magnetic flux conservation from magnetic main sequence stars, Lodewijk Woltjer proposed in 1964 that such neutron stars might contain magnetic fields as large as 10^14 to 10^16 G. In 1967, shortly before the discovery of pulsars, Franco Pacini suggested that a rotating neutron star with a magnetic field would emit radiation, noted that such energy could be pumped into a supernova remnant around a neutron star, such as the Crab Nebula. After the discovery of the first pulsar, Thomas Gold independently suggested a rotating neutron star model similar to that of Pacini, explicitly argued that this model could explain the pulsed radiation observed by Bell Burnell and Hewish.
The discovery of the Crab pulsar in 1968 seemed to provide confirmation of the rotating neutron star model of pulsars. The Crab pulsar has a 33-millisecond pulse period, too short to be consistent with other proposed models for pulsar emission. Moreover, the Crab pulsar is so named because it is located at the center of the Crab Nebula, consistent with the 1933 prediction of Baade and Zwicky. In 1974, Antony Hewish and Martin Ryle became the first astronomers to be awarded the Nobel Prize in Physics, with the Royal Swedish Academy of Sciences noting that Hewish played a "decisive role in the discovery of pulsars". Considerable controversy is associated with the fact that Hewish was awarded the prize while Bell, who made the initial discovery while she was his PhD student, was not. Bell claims no bitterness upon this point. In 1974, Joseph Hooton Taylor, Jr. and Russell Hulse discovered for the first time a pulsar in a binary system, PSR B1913+16. This pulsar orbits another neutron star with an orbital period of just eight hours.
Einstein's theory of general relativity predicts that this system should emit strong gravitational radiation, causing the orbit to continually contract as it loses orbital energy. Observations of the pulsar soon confirmed this prediction, providing the first evidence of the existence of gravitational waves; as of 2010, observations of this pulsar continue to agree with general relativity. In 1993, the Nobel Prize in Physics was awarded to Taylor and Hulse for the discovery of this pulsar. In 1982, Don Backer led a group which discovered PSR B1937+21, a pulsar with a rotation period of just 1.6 milliseconds. Observations soon revealed that its magnetic field was much weaker than ordinary pulsars, while further discoveries cemented the idea that a new class of object, the "millisecond pulsars" had been found. MSPs are believed to be the end product of X-ray binaries. Owing to their extraordinarily rapid and stable rotation, MSPs can be used by astronomers as clocks rivaling the stability of the best atomic clocks on Earth.
Factors affecting the arrival time of pulses at Earth by more than a few hundred nanoseconds can be detected and used to make precise measurements. Physical parameters accessible through pulsar timing include the 3D p
A white dwarf called a degenerate dwarf, is a stellar core remnant composed of electron-degenerate matter. A white dwarf is 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; the nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are 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, 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, the remnant white dwarf. White dwarfs are composed of carbon and oxygen. If the mass of the progenitor is between 8 and 10.5 solar masses, 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 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 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 by mass transfer from a companion star, may explode as a type Ia supernova via a process known as carbon detonation. A white dwarf is hot when it forms, but because it has no source of energy, it will cool as it radiates its energy; this means that its radiation, which has a high color temperature, will lessen and redden with time. Over a 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, 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, it is thought that no black dwarfs yet exist; the oldest white dwarfs still radiate at temperatures of a few thousand kelvins. The first white dwarf discovered was in the triple star system of 40 Eridani, which contains the bright main sequence star 40 Eridani A, orbited at a distance by the closer binary system of the white dwarf 40 Eridani B and the main sequence red dwarf 40 Eridani C.
The pair 40 Eridani B/C was discovered by William Herschel on 31 January 1783. In 1910, Henry Norris Russell, Edward Charles Pickering and Williamina Fleming discovered that, despite being a dim star, 40 Eridani B was of spectral type A, or white. In 1939, Russell looked back on the discovery:, p. 1 I was visiting my friend and generous benefactor, Prof. Edward C. Pickering. With characteristic kindness, he had volunteered to have the spectra observed for all the stars—including comparison stars—which had been observed in the observations for stellar parallax which Hinks and I made at Cambridge, I discussed; this piece of routine work proved fruitful—it led to the discovery that all the stars of faint absolute magnitude were of spectral class M. In conversation on this subject, I asked Pickering about certain other faint stars, not on my list, mentioning in particular 40 Eridani B. Characteristically, he sent a note to the Observatory office and before long the answer came that the spectrum of this star was A.
I knew enough about it in these paleozoic days, to realize at once that there was an extreme inconsistency between what we would have called "possible" values of the surface brightness and density. I must have shown that I was not only puzzled but crestfallen, at this exception to what looked like a pretty rule of stellar characteristics; the spectral type of 40 Eridani B was described in 1914 by Walter Adams. The white dwarf companion of Sirius, Sirius B, was next to be discovered. During the nineteenth century, positional measurements of some stars became precise enough to measure small changes in their location. Friedrich Bessel used position measurements to determine that the stars Sirius and Procyon were changing their positions periodically. In 1844 he predicted that both stars had unseen companions: If we were to regard Sirius and Procyon as double stars, the change of their motions would not surprise us.
Alvan Graham Clark
Alvan Graham Clark was an American astronomer and telescope-maker. Born in Fall River, Massachusetts, he was the son of founder of Alvan Clark & Sons. On January 31, 1862, while testing a new 18.5-inch aperture great refractor telescope in Cambridgeport, Clark made the first observation of a white dwarf star. This discovery of Sirius B, or affectionately "the Pup", proved an earlier hypotheses that Sirius, the brightest star in the night sky with an apparent magnitude of −1.46, had an unseen companion disturbing its motion. Clark used the largest refracting telescope lens in existence at the time, the largest telescope in the United States, to observe the magnitude 8 companion. Clark's 18.5 inch refracting telescope was delivered to his customer, the landmark Dearborn Observatory of Northwestern University in Evanston, where it is still being used today. List of astronomical instrument makers "Alvan Clark, Biographies". AllRefer.com. Archived from the original on 2004-06-23; the Dearborn Telescope Sirius A & B: A Double Star System In The Constellation Canis Major Northwestern University Astronomy and Astrophysics – History of Dearborn Observatory Look south to see winter's brightest constellations Portraits of Alvan Graham Clark from the Lick Observatory Records Digital Archive, UC Santa Cruz Library's Digital Collections
A nova or classical nova is a transient astronomical event that causes the sudden appearance of a bright "new" star, that fades over several weeks or many months. Novae involve an interaction between two stars that cause the flareup, perceived as a new entity, much brighter than the stars involved. Causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. All observed novae involve located binary stars, either a pair of red dwarfs in the process of merging, or a white dwarf and another star; the main sub-classes of novae are classical novae, recurrent novae, dwarf novae. They are all considered to be cataclysmic variable stars. Luminous red novae share the name and are cataclysmic variables, but are a different type of event caused by a stellar merger. With similar names are the much more energetic supernovae and kilonovae. Classical nova eruptions are the most common type of nova, they are created in a close binary star system consisting of a white dwarf and either a main sequence, sub-giant, or red giant star.
When the orbital period falls in the range of several days to one day, the white dwarf is close enough to its companion star to start drawing accreted matter onto the surface of the white dwarf, which creates a dense but shallow atmosphere. This atmosphere is hydrogen and is thermally heated by the hot white dwarf, which reaches a critical temperature causing rapid runaway ignition by fusion. From the dramatic and sudden energies created, the now hydrogen-burnt atmosphere is dramatically expelled into interstellar space, its brightened envelope is seen as the visible light created from the nova event, was mistaken as a "new" star. A few novae produce short-lived nova remnants, lasting for several centuries. Recurrent nova processes are the same as the classical nova, except that the fusion ignition may be repetitive because the companion star can again feed the dense atmosphere of the white dwarf. Novae most occur in the sky along the path of the Milky Way near the observed galactic centre in Sagittarius.
They occur far more than galactic supernovae, averaging about ten per year. Most are found telescopically only one every year to eighteen months reaching naked-eye visibility. Novae reaching first or second magnitude occur only several times per century; the last bright nova was V1369 Centauri reaching 3.3 magnitude on 14 December 2013. During the sixteenth century, astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia, he described it in his book De nova stella. In this work he argued that a nearby object should be seen to move relative to the fixed stars, that the nova had to be far away. Although this event was a supernova and not a nova, the terms were considered interchangeable until the 1930s. After this, novae were classified as classical novae to distinguish them from supernovae, as their causes and energies were thought to be different, based in the observational evidence. Despite the term "stella nova" meaning "new star", novae most take place as a result of white dwarfs: remnants of old stars.
Evolution of potential novae begins with two main sequence stars in a binary system. One of the two evolves into a red giant, leaving its remnant white dwarf core in orbit with the remaining star; the second star—which may be either a main sequence star or an aging giant—begins to shed its envelope onto its white dwarf companion when it overflows its Roche lobe. As a result, the white dwarf captures matter from the companion's outer atmosphere in an accretion disk, in turn, the accreted matter falls into the atmosphere; as the white dwarf consists of degenerate matter, the accreted hydrogen does not inflate, but its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 million K, initiating nuclear burning, via the CNO cycle. Hydrogen fusion may occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, giving rise to a super soft X-ray source, but for most binary system parameters, the hydrogen burning is unstable thermally and converts a large amount of the hydrogen into other, heavier chemical elements in a runaway reaction, liberating an enormous amount of energy.
This blows the remaining gases away from the surface of the white dwarf surface and produces an bright outburst of light. The rise to peak brightness may be rapid, or gradual; this is related to the speed class of the nova. The time taken for a nova to decay by around 2 or 3 magnitudes from maximum optical brightness is used for classification, via its speed class. Fast novae will take fewer than 25 days to decay by 2 magnitudes, while slow novae will take more than 80 days. In spite of their violence the amount of material ejected in novae is only about 1⁄10,000 of a solar mass, quite small relative to the mass of the white dwarf. Furthermore, only five percent of the accreted mass is fused during the power outburst. Nonetheless, this is enough energy to accelerate nova ejecta to velocities as high as several thousand kilometers per second—higher for fast novae than slow ones—with a concurrent rise in luminosity from a few times solar to 50,000–100,000 times solar. In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope discovered that a nova can emit gamma-rays.
A white dwarf can generate multiple novae over t
SN 1006 was a supernova, the brightest observed stellar event in recorded history, reaching an estimated −7.5 visual magnitude, exceeding sixteen times the brightness of Venus. Appearing between April 30 and May 1, 1006 AD in the constellation of Lupus, this "guest star" was described by observers across China, Iraq and Europe, recorded in North American petroglyphs; some reports state it was visible in the daytime. Modern astronomers now consider its distance from us to be about 7,200 light-years. Egyptian astrologer and astronomer Ali ibn Ridwan, writing in a commentary on Ptolemy's Tetrabiblos, stated that the "spectacle was a large circular body, 2½ to 3 times as large as Venus; the sky was shining because of its light. The intensity of its light was a little more than a quarter that of Moon light". Like all other observers, Ali ibn Ridwan noted; some astrologers interpreted the event as a portent of famine. The most northerly sighting is recorded in the annals of the Abbey of Saint Gall in Switzerland, at a latitude of 47.5° North.
Monks at St Gall provide independent data as to its magnitude and location in the sky, writing that "n a wonderful manner this was sometimes contracted, sometimes diffused, moreover sometimes extinguished… It was seen for three months in the inmost limits of the south, beyond all the constellations which are seen in the sky". This description is taken as probable evidence that the supernova was of Type Ia; some sources state. According to Songshi, the official history of the Song Dynasty, the star seen on 1 May 1006 appeared to the south of constellation Di, east of Lupus and one degree to the west of Centaurus, it shone so brightly. By December, it was again sighted in the constellation Di; the Chinese astrologer Zhou Keming, on his return to Kaifeng from his duty in Guangdong, interpreted the star to the emperor on May 30 as an auspicious star, yellow in color and brilliant in its brightness, that would bring great prosperity to the state over which it appeared. The reported color yellow should be taken with some suspicion however, because Zhou may have chosen a favorable color for political reasons.
There appear to have been two distinct phases in the early evolution of this supernova. There was first a three-month period. A petroglyph by the Hohokam in White Tank Mountain Regional Park, has been interpreted as the first known North American representation of the supernova, though other researchers remain skeptical. Earlier observations discovered from Yemen may have seen SN 1006 on April 17, two weeks before its assumed earliest observation. SN 1006's associated supernova remnant from this event was not identified until 1965, when Doug Milne and Frank Gardner used the Parkes radio telescope to demonstrate a connection to known radio source, PKS 1459-41; this is located near the star Beta Lupi. X-ray and optical emission from this remnant have been detected, during 2010 the H. E. S. S. Gamma-ray observatory announced the detection of very-high-energy gamma-ray emission from the remnant. No associated neutron star or black hole has been found, the situation expected for the remnant of a Type Ia supernova.
A survey in 2012 to find any surviving companions of the SN 1006 progenitor found no subgiant or giant companion stars, indicating that SN 1006 was most a double degenerate progenitor, that is, the merging of two white dwarf stars. Remnant SNR G327.6+14.6 has an estimated distance of 2.2 kpc. from Earth, making the true linear diameter 20 parsecs. Research has suggested that Type Ia supernovae can irradiate the Earth with significant amounts of gamma-ray flux, compared with the typical flux from the Sun, up to distances on the order of 1 kiloparsec; the greatest risk is to producing effects on life and climate. While SN 1006 did not appear to have such significant effects, a signal of its outburst can be found in nitrate deposits in Antarctic ice. History of supernova observation List of supernova candidates List of supernova remnants List of supernovae Near-Earth supernova Cause of Supernova SN 1006 Revealed Stories of SN 1006 in Chinese literature National Optical Observatory Press Release for March 2003 Simulation of SN 1006 as it appeared in the southern sky at midnight, May 1, 1006 Entry for supernova remnant of SN 1006 from the Galactic Supernova Remnant Catalogue X-ray image of supernova remnant of SN 1006, as seen with the Chandra X-ray Observatory Ancient rock art may depict exploding star Astronomy Picture of the Day, March 17, 2003 Astronomy Picture of the Day, July 4, 2008 Margaret Donsbach: The Scholar's Supernova SN 1006 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
Thomas Gold was an Austrian-born astrophysicist, a professor of astronomy at Cornell University, a member of the U. S. National Academy of Sciences, a Fellow of the Royal Society. Gold was one of three young Cambridge scientists who in 1948 proposed the now abandoned "steady state" hypothesis of the universe. Gold's work crossed academic and scientific boundaries, into biophysics, aerospace engineering, geophysics. Gold was born on May 22, 1920 in Vienna, Austria to Max Gold, a wealthy Jewish industrialist who ran one of Austria's largest mining and metal fabrication companies, German former actress Josefine Martin. Following the economic downfall of the European mining industry in the late 1920s, Max Gold moved his family to Berlin, where he had taken a job as director of a metal trading company. Following the start of Nazi leader Adolf Hitler's anti-Jewish campaigns in 1933, Gold and his family left Germany because of his father's heritage; the family travelled through Europe for the next few years.
Gold attended boarding school at the Lyceum Alpinum Zuoz in Zuoz, where he proved to be a clever and physically and mentally aggressive individual. Gold finished his schooling at Zuoz in 1938, fled with his family to England after the German invasion of Austria in early 1938. Gold began studying mechanical sciences. In May 1940, just as Hitler was commencing his advance in Belgium and France, Gold was sent into internment as an enemy alien by the British government, it was on the first night of internment, at an army barracks in Bury St Edmunds, that he met his future collaborator and close friend, Hermann Bondi. Gold spent most of his nearly 15 months of internment in a camp in Canada, after which he returned to England and reentered Cambridge University, where he abandoned his study of mechanical sciences for physics. After graduating with a pass degree in June 1942, Gold worked as an agricultural labourer and lumberjack in northern England before joining Bondi and Fred Hoyle on naval research into radar ground clutter near Dunsfold, Surrey.
The three men would spend their off-duty hours in "intense and wide-ranging scientific discussion" on topics such as cosmology and astrophysics. Within months, Gold was placed in charge of constructing new radar systems. Gold determined how landing craft could use radar to navigate to the appropriate landing spot on D-Day and discovered that the German navy had fitted snorkels to its U-boats, making them operable underwater while still taking in air from above the surface. After the war and Bondi returned to Cambridge, while Gold stayed with naval research until 1947, he began working at Cambridge's Cavendish Laboratory to help construct the world's largest magnetron, a device invented by two British scientists in 1940 that generated intense microwaves for radar. Soon after, Gold joined R. J. Pumphrey, a zoologist at the Cambridge Zoology Laboratory who had served as the deputy head of radar naval research during the war, to study the effect of resonance on the human ear, he found that the degree of resonance observed in the cochlea was not in accordance with the level of damping that would be expected from the viscosity of the watery liquid that fills the inner ear.
In 1948, Gold hypothesized that the ear operates by "regeneration", in that electromechanical action occurs when electrical energy is used to counteract the effects of damping. Although Gold won a prize fellowship from Trinity College for his thesis on the regeneration and obtained a junior lectureship at the Cavendish Laboratory, his theory was ignored by ear specialists and physiologists, such as future Nobel Prize winner Georg von Békésy, who did not believe the cochlea operated under a feedback system. In the 1970s, researchers discovered that Gold's hypothesis had been correct – the ear contained microscopic hair cells that operated on a feedback mechanism to generate resonance. Gold began discussing problems in physics with Hoyle and Bondi again, centering on the issues over redshift and Hubble's law; this led the three to all start questioning the Big Bang theory proposed by Georges Lemaître in 1931 and advanced by George Gamow, which suggested that the universe expanded from an dense and hot state and continues to expand today.
As recounted in a 1978 interview with physicist and historian Spencer R. Weart, Gold believed that there was reason to think that the creation of matter was "done all the time and none of the problems about fleeting moments arise, it can be just in a steady state with the expansion taking things apart as fast as new matter comes into being and condenses into new galaxies". Two papers were published in 1948 discussing the "steady-state theory" as an alternative to the Big Bang: one by Gold and Bondi, the other by Hoyle. In their seminal paper and Bondi asserted that although the universe is expanding, it does not change its look over time, they proposed the perfect cosmological principle as the underpinning of their theory, which held that the universe is homogeneous and isotropic in space and time. On the large scale, they argued that there "is nothing outstanding about any place in the universe, that those differences which do exist are only of local significance. However, since the universe was not characterized by a lack of evolution, distinguishing features or recognizable direction of time, they postulated that there had to be large-scale motions in the universe.
They highlighted two possible types of motion: large-scale expansion and its reverse, large-scale contraction. They estim
A timeline is a display of a list of events in chronological order. It is a graphic design showing a long bar labelled with dates paralleling it, contemporaneous events. Timelines can use any suitable scale representing time, suiting data; this timescale is dependent on the events in the timeline. A timeline of evolution can be over millions of years, whereas a timeline for the day of the September 11 attacks can take place over minutes, that of an explosion over milliseconds. While many timelines use a linear timescale -- where large or small timespans are relevant -- logarithmic timelines entail a logarithmic scale of time. There are different types of timelines Text timelines, labeled as text Number timelines, the labels are numbers line graphs Interactive, zoomableThere are many methods of visualizations for timelines. Timelines were static images and drawn or printed on paper. Timelines relied on graphic design, the ability of the artist to visualize the data. Timelines, no longer constrained by previous space and functional limitations, are now digital and interactive created with computer software.
ChronoZoom is an example of computer-aided interactive timeline software. Timelines are used in education to help students and researchers with understanding the order or chronology of historical events and trends for a subject; when showing time on a specific scale on an axis, a timeline can be used to visualize time lapses between events and the simultaneity or overlap of spans and events. Timelines are useful for studying history, as they convey a sense of change over time. Wars and social movements are shown as timelines. Timelines are useful for biographies. Examples include: Timeline of the civil rights movement Timeline of European exploration Timeline of imperialism Timeline of Solar System exploration Timeline of United States history Timeline of World War I Timeline of religion Timelines are used in the natural world and sciences, for subjects such as astronomy and geology: 2009 flu pandemic timeline Chronology of the universe Geologic time scale Timeline of evolutionary history of life Another type of timeline is used for project management.
In these cases, timelines are used to help team members to know what milestones need to be achieved and under what time schedule. For example, in the case of establishing a project timeline in the implementation phase of the life cycle of a computer system. British Library interactive timeline Port Royal des Champs museum timeline