Misplaced Pages

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88-403: The name also emphasizes two main features of the installation, namely the simultaneous observation of air showers with several telescopes, under different viewing angles, and the combination of telescopes to a large system to increase the effective detection area for gamma rays. H.E.S.S. permits the exploration of gamma-ray sources with intensities at a level of a few thousandth parts of the flux of

176-500: A catalogue of celestial objects of a cloudy nature, but fixed in the sky, to avoid incorrectly cataloguing them as comets. This realization led him to compile the " Messier catalogue ". William Herschel observed the Crab Nebula numerous times between 1783 and 1809, but it is not known whether he was aware of its existence in 1783, or if he discovered it independently of Messier and Bevis. After several observations, he concluded that it

264-403: A left-hand sense. Right circularly polarized radio waves consist of photons spinning in a right hand sense. Left circularly polarized radio waves consist of photons spinning in a left hand sense. Plane polarized radio waves consist of photons in a quantum superposition of right and left hand spin states. The electric field consists of a superposition of right and left rotating fields, resulting in

352-605: A plane oscillation. Radio waves are more widely used for communication than other electromagnetic waves mainly because of their desirable propagation properties, stemming from their large wavelength . Radio waves have the ability to pass through the atmosphere in any weather, foliage, and through most building materials. By diffraction , longer wavelengths can bend around obstructions, and unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength. The study of radio propagation , how radio waves move in free space and over

440-402: A plane perpendicular to the direction of motion. In a horizontally polarized radio wave the electric field oscillates in a horizontal direction. In a vertically polarized wave the electric field oscillates in a vertical direction. In a circularly polarized wave the electric field at any point rotates about the direction of travel, once per cycle. A right circularly polarized wave rotates in

528-404: A right-hand sense about the direction of travel, while a left circularly polarized wave rotates in the opposite sense. The wave's magnetic field is perpendicular to the electric field, and the electric and magnetic field are oriented in a right-hand sense with respect to the direction of radiation. An antenna emits polarized radio waves, with the polarization determined by the direction of

616-555: A shell, none has yet been found. The Crab Nebula lies roughly 1.5 degrees away from the ecliptic —the plane of Earth's orbit around the Sun. This means that the Moon—and occasionally, planets—can transit or occult the nebula. Although the Sun does not transit the nebula, its corona passes in front of it. These transits and occultations can be used to analyse both the nebula and the object passing in front of it, by observing how radiation from

704-434: A source of petaelectronvolt galactic cosmic rays. Crab Nebula The Crab Nebula (catalogue designations M1 , NGC 1952 , Taurus A ) is a supernova remnant and pulsar wind nebula in the constellation of Taurus . The common name comes from a drawing that somewhat resembled a crab with arms produced by William Parsons, 3rd Earl of Rosse , in 1842 or 1843 using a 36-inch (91 cm) telescope . The nebula

792-423: A spin rate of 30.2 times per second, lies at the center of the Crab Nebula. The star emits pulses of radiation from gamma rays to radio waves . At X-ray and gamma ray energies above 30 keV , the Crab Nebula is generally the brightest persistent gamma-ray source in the sky, with measured flux extending to above 10 TeV . The nebula's radiation allows detailed study of celestial bodies that occult it. In

880-520: A supernova is referred to as the supernova's progenitor star . Two types of stars explode as supernovae: white dwarfs and massive stars . In the so-called Type Ia supernovae , gases falling onto a 'dead' white dwarf raise its mass until it nears a critical level, the Chandrasekhar limit , resulting in a runaway nuclear fusion explosion that obliterates the star; in Type Ib/c and Type II supernovae,

968-518: A visible image, or other devices. A digital data signal is applied to a computer or microprocessor , which interacts with a human user. The radio waves from many transmitters pass through the air simultaneously without interfering with each other. They can be separated in the receiver because each transmitter's radio waves oscillate at a different rate, in other words each transmitter has a different frequency , measured in kilohertz (kHz), megahertz (MHz) or gigahertz (GHz). The bandpass filter in

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1056-502: Is estimated to be between 1.4 and 2  M ☉ . The predominant theory to account for the missing mass of the Crab Nebula is that a substantial proportion of the mass of the progenitor was carried away before the supernova explosion in a fast stellar wind , a phenomenon commonly seen in Wolf–Rayet stars . However, this would have created a shell around the nebula. Although attempts have been made at several wavelengths to observe

1144-438: Is expanding outward at about 1,500 km/s (930 mi/s). Images taken several years apart reveal the slow expansion of the nebula, and by comparing this angular expansion with its spectroscopically determined expansion velocity, the nebula's distance can be estimated. In 1973, an analysis of many methods used to compute the distance to the nebula had reached a conclusion of about 1.9 kpc (6,300 ly), consistent with

1232-442: Is extremely small, from 10 to 10   joules . So the antenna of even a very low power transmitter emits an enormous number of photons every second. Therefore, except for certain molecular electron transition processes such as atoms in a maser emitting microwave photons, radio wave emission and absorption is usually regarded as a continuous classical process, governed by Maxwell's equations . Radio waves in vacuum travel at

1320-426: Is the wavelength of a 1  hertz radio signal. A 1  megahertz radio wave (mid- AM band ) has a wavelength of 299.79 meters (983.6 ft). Like other electromagnetic waves, a radio wave has a property called polarization , which is defined as the direction of the wave's oscillating electric field perpendicular to the direction of motion. A plane-polarized radio wave has an electric field that oscillates in

1408-639: Is to heat them, similarly to the infrared waves radiated by sources of heat such as a space heater or wood fire. The oscillating electric field of the wave causes polar molecules to vibrate back and forth, increasing the temperature; this is how a microwave oven cooks food. Radio waves have been applied to the body for 100 years in the medical therapy of diathermy for deep heating of body tissue, to promote increased blood flow and healing. More recently they have been used to create higher temperatures in hyperthermia therapy and to kill cancer cells. However, unlike infrared waves, which are mainly absorbed at

1496-400: Is weak mechanistic evidence of cancer risk via personal exposure to RF-EMF from mobile telephones. Radio waves can be shielded against by a conductive metal sheet or screen, an enclosure of sheet or screen is called a Faraday cage . A metal screen shields against radio waves as well as a solid sheet as long as the holes in the screen are smaller than about 1 ⁄ 20 of wavelength of

1584-642: The Arecibo Radio Observatory . This discovery also proved that pulsars are rotating neutron stars (not pulsating white dwarfs, as many scientists suggested). Soon after the discovery of the Crab Pulsar , David Richards discovered (using the Arecibo Observatory) that the Crab Pulsar spins down and, therefore, the pulsar loses its rotational energy. Thomas Gold has shown that the spin-down power of

1672-511: The Crab Nebula . H.E.S.S. consists of five telescopes: four with mirrors just under 12 m in diameter, arranged as a square with 120 m sides, and one larger telescope with a 28 m mirror, located at the centre of the array. The four 12 m telescopes began operation in 2004, with the 28 m telescope added as an upgrade (called H.E.S.S. II) in 2012. As with other gamma-ray telescopes, H.E.S.S. observes high energy processes in

1760-530: The Perseus Arm of the Milky Way galaxy, at a distance of about 2.0 kiloparsecs (6,500  ly ) from Earth. It has a diameter of 3.4 parsecs (11 ly), corresponding to an apparent diameter of some 7  arcminutes , and is expanding at a rate of about 1,500 kilometres per second (930 mi/s), or 0.5% of the speed of light . The Crab Pulsar , a neutron star 28–30 kilometres (17–19 mi) across with

1848-637: The blackbody radiation emitted by all warm objects. Radio waves are generated artificially by an electronic device called a transmitter , which is connected to an antenna , which radiates the waves. They are received by another antenna connected to a radio receiver , which processes the received signal. Radio waves are very widely used in modern technology for fixed and mobile radio communication , broadcasting , radar and radio navigation systems, communications satellites , wireless computer networks and many other applications. Different frequencies of radio waves have different propagation characteristics in

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1936-402: The electromagnetic spectrum , from radio waves to X-rays. Like all isolated pulsars, its period is slowing very gradually. Occasionally, its rotational period shows sharp changes, known as 'glitches', which are believed to be caused by a sudden realignment inside the neutron star. The energy released as the pulsar slows down is enormous, and it powers the emission of the synchrotron radiation of

2024-679: The full moon is 30 arcminutes across) surrounding a diffuse blue central region. In three dimensions, the nebula is thought to be shaped either like an oblate spheroid (estimated as 1,380 pc/4,500 ly away) or a prolate spheroid (estimated as 2,020 pc/6,600 ly away). The filaments are the remnants of the progenitor star's atmosphere, and consist largely of ionised helium and hydrogen , along with carbon , oxygen , nitrogen , iron , neon and sulfur . The filaments' temperatures are typically between 11,000 and 18,000  K , and their densities are about 1,300 particles per cm . In 1953, Iosif Shklovsky proposed that

2112-463: The speed of light c {\displaystyle c} . When passing through a material medium, they are slowed depending on the medium's permeability and permittivity . Air is tenuous enough that in the Earth's atmosphere radio waves travel at very nearly the speed of light. The wavelength λ {\displaystyle \lambda } is the distance from one peak (crest) of

2200-482: The "near field" zone, the physical relationships between the electric and magnetic components of the field can be complex, and it is best to use the field strength units discussed above. Power density is measured in terms of power per unit area, for example, with the unit milliwatt per square centimeter (mW/cm ). When speaking of frequencies in the microwave range and higher, power density is usually used to express intensity since exposures that might occur would likely be in

2288-467: The 1909 Nobel Prize in physics for his radio work. Radio communication began to be used commercially around 1900. The modern term " radio wave " replaced the original name " Hertzian wave " around 1912. Radio waves are radiated by charged particles when they are accelerated . Natural sources of radio waves include radio noise produced by lightning and other natural processes in the Earth's atmosphere, and astronomical radio sources in space such as

2376-423: The 1909 Nobel Prize in physics for his radio work. Radio communication began to be used commercially around 1900. The modern term " radio wave " replaced the original name " Hertzian wave " around 1912. stronger, then finally extracts the information-bearing modulation signal in a demodulator . The recovered signal is sent to a loudspeaker or earphone to produce sound, or a television display screen to produce

2464-472: The 1950s and 1960s, the Sun's corona was mapped from observations of the Crab Nebula's radio waves passing through it, and in 2003, the thickness of the atmosphere of Saturn's moon Titan was measured as it blocked out X-rays from the nebula. The earliest recorded documentation of observation of astronomical object SN 1054 was as it was occurring in 1054, by Chinese astrononomers and Japanese observers, hence its numerical identification. Modern understanding that

2552-400: The Crab Nebula is a helium-rich torus which is visible as an east–west band crossing the pulsar region. The torus composes about 25% of the visible ejecta. However, it is suggested by calculation that about 95% of the torus is helium. As yet, there has been no plausible explanation put forth for the structure of the torus. At the center of the Crab Nebula are two faint stars, one of which is

2640-463: The Crab Nebula is dominated by a pulsar wind nebula enveloping the pulsar. Some sources consider the Crab Nebula to be an example of both a pulsar wind nebula as well as a supernova remnant, while others separate the two phenomena based on the different sources of energy production and behaviour. The Crab Nebula was the first astrophysical object confirmed to emit gamma rays in the very-high-energy (VHE) band above 100 GeV in energy. The VHE detection

2728-503: The Crab Nebula was created by a supernova traces back to 1921, when Carl Otto Lampland announced he had seen changes in the nebula's structure. This eventually led to the conclusion that the creation of the Crab Nebula corresponds to the bright SN 1054 supernova recorded by medieval astronomers in AD 1054. The Crab Nebula was first identified in 1731 by John Bevis . The nebula was independently rediscovered in 1758 by Charles Messier as he

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2816-457: The Crab Nebula, a lunar occultation was used to determine the exact location of their source. The Sun's corona passes in front of the Crab Nebula every June. Variations in the radio waves received from the Crab Nebula at this time can be used to infer details about the corona's density and structure. Early observations established that the corona extended out to much greater distances than had previously been thought; later observations found that

2904-427: The Crab Nebula, which has a total luminosity about 75,000 times greater than that of the Sun. The pulsar's extreme energy output creates an unusually dynamic region at the centre of the Crab Nebula. While most astronomical objects evolve so slowly that changes are visible only over timescales of many years, the inner parts of the Crab Nebula show changes over timescales of only a few days. The most dynamic feature in

2992-411: The Crab Nebula. Recent studies, however, suggest the progenitor could have been a super-asymptotic giant branch star in the 8 to 10  M ☉ range that would have exploded in an electron-capture supernova . In June 2021 a paper in the journal Nature Astronomy reported that the 2018 supernova SN 2018zd (in the galaxy NGC 2146 , about 31 million light-years from Earth) appeared to be

3080-447: The Earth's atmosphere; long waves can diffract around obstacles like mountains and follow the contour of the Earth ( ground waves ), shorter waves can reflect off the ionosphere and return to Earth beyond the horizon ( skywaves ), while much shorter wavelengths bend or diffract very little and travel on a line of sight , so their propagation distances are limited to the visual horizon. To prevent interference between different users,

3168-583: The Gamsberg mountain, an area well known for its excellent optical quality. The first of the four telescopes of Phase I of the H.E.S.S. project went into operation in Summer 2002; all four were operational in December 2003. In 2004 H.E.S.S. was the first IACT experiment to spatially resolve a source of cosmic gamma rays . In 2005, it was announced that H.E.S.S. had detected eight new high-energy gamma ray sources, doubling

3256-414: The Sun, galaxies and nebulas. All warm objects radiate high frequency radio waves ( microwaves ) as part of their black body radiation . Radio waves are produced artificially by time-varying electric currents , consisting of electrons flowing back and forth in a specially shaped metal conductor called an antenna . An electronic device called a radio transmitter applies oscillating electric current to

3344-540: The antenna, and the antenna radiates the power as radio waves. Radio waves are received by another antenna attached to a radio receiver . When radio waves strike the receiving antenna they push the electrons in the metal back and forth, creating tiny oscillating currents which are detected by the receiver. From quantum mechanics , like other electromagnetic radiation such as light, radio waves can alternatively be regarded as streams of uncharged elementary particles called photons . In an antenna transmitting radio waves,

3432-642: The artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunication Union (ITU), which defines radio waves as " electromagnetic waves of frequencies arbitrarily lower than 3000  GHz , propagated in space without artificial guide". The radio spectrum is divided into a number of radio bands on the basis of frequency, allocated to different uses. Higher-frequency, shorter-wavelength radio waves are called microwaves . Radio waves were first predicted by

3520-445: The centre of the Crab Nebula was strong evidence that pulsars were formed by supernova explosions. They now are understood to be rapidly rotating neutron stars , whose powerful magnetic fields concentrates their radiation emissions into narrow beams. The Crab Pulsar is believed to be about 28–30 km (17–19 mi) in diameter; it emits pulses of radiation every 33  milliseconds . Pulses are emitted at wavelengths across

3608-516: The cloud. In late 1968, David H. Staelin and Edward C. Reifenstein III reported the discovery of two rapidly variable radio sources in the area of the Crab Nebula using the Green Bank Telescope . They named them NP 0527 and NP 0532. The period of 33 milliseconds and precise location of the Crab Nebula pulsar NP 0532 was discovered by Richard V. E. Lovelace and collaborators on 10 November 1968 at

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3696-399: The comet should appear in the constellation of Taurus . It was in searching in vain for the comet that Charles Messier found the Crab Nebula, which he at first thought to be Halley's comet. After some observation, noticing that the object that he was observing was not moving across the sky, Messier concluded that the object was not a comet. Messier then realised the usefulness of compiling

3784-557: The corona contained substantial density variations. Very rarely, Saturn transits the Crab Nebula. Its transit on 4 January 2003 ( UTC ) was the first since 31 December 1295 ( O.S. ); another will not occur until 5 August 2267. Researchers used the Chandra X-ray Observatory to observe Saturn's moon Titan as it crossed the nebula, and found that Titan's X-ray 'shadow' was larger than its solid surface, due to absorption of X-rays in its atmosphere. These observations showed that

3872-399: The currently cited value. Tracing back its expansion (assuming a constant decrease of expansion speed due to the nebula's mass) yielded a date for the creation of the nebula several decades after 1054, implying that its outward velocity has decelerated less than assumed since the supernova explosion. This reduced deceleration is believed to be caused by energy from the pulsar that feeds into

3960-438: The daytime had been recorded in the same part of the sky by Chinese astronomers on 4 July 1054, and probably also by Japanese observers. In 1913, when Vesto Slipher registered his spectroscopy study of the sky, the Crab Nebula was again one of the first objects to be studied. Changes in the cloud, suggesting its small extent, were discovered by Carl Lampland in 1921. That same year, John Charles Duncan demonstrated that

4048-526: The diameter of a grain of rice. Radio waves with frequencies above about 1 GHz and wavelengths shorter than 30 centimeters are called microwaves . Like all electromagnetic waves, radio waves in vacuum travel at the speed of light , and in the Earth's atmosphere at a slightly lower speed. Radio waves are generated by charged particles undergoing acceleration , such as time-varying electric currents . Naturally occurring radio waves are emitted by lightning and astronomical objects , and are part of

4136-399: The diffuse blue region is predominantly produced by synchrotron radiation , which is radiation given off by the curving motion of electrons in a magnetic field. The radiation corresponded to electrons moving at speeds up to half the speed of light . Three years later, the hypothesis was confirmed by observations. In the 1960s it was found that the source of the curved paths of the electrons

4224-421: The electrons in the antenna emit the energy in discrete packets called radio photons, while in a receiving antenna the electrons absorb the energy as radio photons. An antenna is a coherent emitter of photons, like a laser , so the radio photons are all in phase . However, from Planck's relation E = h ν {\displaystyle E=h\nu } , the energy of individual radio photons

4312-438: The energy is deposited. For example, the 2.45 GHz radio waves (microwaves) in a microwave oven penetrate most foods approximately 2.5 to 3.8 cm . Looking into a source of radio waves at close range, such as the waveguide of a working radio transmitter, can cause damage to the lens of the eye by heating. A strong enough beam of radio waves can penetrate the eye and heat the lens enough to cause cataracts . Since

4400-400: The first pulsars to be discovered. Pulsars are sources of powerful electromagnetic radiation , emitted in short and extremely regular pulses many times a second. They were a great mystery when discovered in 1967, and the team who identified the first one considered the possibility that it could be a signal from an advanced civilization. However, the discovery of a pulsating radio source in

4488-399: The first observation of an electron-capture supernova The 1054 supernova explosion that created the Crab Nebula had been thought to be the best candidate for an electron-capture supernova, and the 2021 paper makes it more likely that this was correct. A significant problem in studies of the Crab Nebula is that the combined mass of the nebula and the pulsar add up to considerably less than

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4576-518: The heating effect is in principle no different from other sources of heat, most research into possible health hazards of exposure to radio waves has focused on "nonthermal" effects; whether radio waves have any effect on tissues besides that caused by heating. Radiofrequency electromagnetic fields have been classified by the International Agency for Research on Cancer (IARC) as having "limited evidence" for its effects on humans and animals. There

4664-403: The inner part of the nebula is the point where the pulsar's equatorial wind slams into the bulk of the nebula, forming a shock front . The shape and position of this feature shifts rapidly, with the equatorial wind appearing as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar to well out into the main body of the nebula. The star that exploded as

4752-508: The known number of such sources. As of 2014, more than 90 sources of teraelectronvolt gamma rays were discovered by H.E.S.S. In 2016, the HESS collaboration reported deep gamma ray observations which show the presence of petaelectronvolt-protons originating from the supermassive black hole at the centre of the Milky Way, and therefore should be considered as a viable alternative to supernova remnants as

4840-464: The levels of electric and magnetic field strength at a measurement location. Another commonly used unit for characterizing an RF electromagnetic field is power density . Power density is most accurately used when the point of measurement is far enough away from the RF emitter to be located in what is referred to as the far field zone of the radiation pattern. In closer proximity to the transmitter, i.e., in

4928-434: The metal antenna elements. For example, a dipole antenna consists of two collinear metal rods. If the rods are horizontal, it radiates horizontally polarized radio waves, while if the rods are vertical, it radiates vertically polarized waves. An antenna receiving the radio waves must have the same polarization as the transmitting antenna, or it will suffer a severe loss of reception. Many natural sources of radio waves, such as

5016-607: The nebula is altered by the transiting body. Lunar transits have been used to map X-ray emissions from the nebula. Before the launch of X-ray-observing satellites, such as the Chandra X-ray Observatory , X-ray observations generally had quite low angular resolution , but when the Moon passes in front of the nebula, its position is very accurately known, and so the variations in the nebula's brightness can be used to create maps of X-ray emission. When X-rays were first observed from

5104-437: The nebula's magnetic field, which expands and forces the nebula's filaments outward. Estimates of the total mass of the nebula are important for estimating the mass of the supernova's progenitor star. The amount of matter contained in the Crab Nebula's filaments (ejecta mass of ionized and neutral gas; mostly helium ) is estimated to be 4.6 ± 1.8  M ☉ . One of the many nebular components (or anomalies) of

5192-445: The night sky except the Moon ) by July. The supernova was visible to the naked eye for about two years after its first observation. In the 1960s, because of the prediction and discovery of pulsars , the Crab Nebula again became a major center of interest. It was then that Franco Pacini predicted the existence of the Crab Pulsar for the first time, which would explain the brightness of

5280-627: The original connection to Chinese observations, in 1934 connections were made to a 13th-century Japanese reference to a " guest star " in Meigetsuki a few weeks before the Chinese reference. The event was long considered unrecorded in Islamic astronomy, but in 1978 a reference was found in a 13th-century copy made by Ibn Abi Usaibia of a work by Ibn Butlan , a Nestorian Christian physician active in Baghdad at

5368-444: The predicted mass of the progenitor star, and the question of where the 'missing mass' is, remains unresolved. Estimates of the mass of the nebula are made by measuring the total amount of light emitted, and calculating the mass required, given the measured temperature and density of the nebula. Estimates range from about 1–5  M ☉ , with 2–3  M ☉ being the generally accepted value. The neutron star mass

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5456-419: The progenitor star is a massive star whose core runs out of fuel to power its nuclear fusion reactions and collapses in on itself, releasing gravitational potential energy in a form that blows away the star's outer layers. Type Ia supernovae do not produce pulsars, so the pulsar in the Crab Nebula shows it must have formed in a core-collapse supernova. Theoretical models of supernova explosions suggest that

5544-403: The pulsar is sufficient to power the Crab Nebula. The discovery of the Crab Pulsar and the knowledge of its exact age (almost to the day) allows for the verification of basic physical properties of these objects, such as characteristic age and spin-down luminosity, the orders of magnitude involved (notably the strength of the magnetic field ), along with various aspects related to the dynamics of

5632-406: The range of practical radio communication systems decreases with increasing frequency. Below about 20 GHz atmospheric attenuation is mainly due to water vapor. Above 20 GHz, in the millimeter wave band, other atmospheric gases begin to absorb the waves, limiting practical transmission distances to a kilometer or less. Radio waves were first predicted by the theory of electromagnetism that

5720-421: The reality of Maxwell's electromagnetic waves by experimentally generating electromagnetic waves lower in frequency than light, radio waves, in his laboratory, showing that they exhibited the same wave properties as light: standing waves , refraction , diffraction , and polarization . Italian inventor Guglielmo Marconi developed the first practical radio transmitters and receivers around 1894–1895. He received

5808-421: The reality of Maxwell's electromagnetic waves by experimentally generating electromagnetic waves lower in frequency than light, radio waves, in his laboratory, showing that they exhibited the same wave properties as light: standing waves , refraction , diffraction , and polarization . Italian inventor Guglielmo Marconi developed the first practical radio transmitters and receivers around 1894–1895. He received

5896-411: The receiver consists of one or more tuned circuits which act like a resonator , similarly to a tuning fork . The tuned circuit has a natural resonant frequency at which it oscillates. The resonant frequency is set equal to the frequency of the desired radio station. The oscillating radio signal from the desired station causes the tuned circuit to oscillate in sympathy, and it passes the signal on to

5984-668: The remnant was expanding, while Knut Lundmark noted its proximity to the guest star of 1054. In 1928, Edwin Hubble proposed associating the cloud with the star of 1054, an idea that remained controversial until the nature of supernovae was understood, and it was Nicholas Mayall who indicated that the star of 1054 was undoubtedly the supernova whose explosion produced the Crab Nebula. The search for historical supernovae started at that moment: seven other historical sightings have been found by comparing modern observations of supernova remnants with astronomical documents of past centuries. After

6072-416: The remnant. The role of this supernova to the scientific understanding of supernova remnants was crucial, as no other historical supernova created a pulsar whose precise age is known for certain. The only possible exception to this rule would be SN 1181 , whose supposed remnant 3C   58 is home to a pulsar, but its identification using Chinese observations from 1181 is contested. The inner part of

6160-400: The rest of the receiver. Radio signals at other frequencies are blocked by the tuned circuit and not passed on. Radio waves are non-ionizing radiation , which means they do not have enough energy to separate electrons from atoms or molecules , ionizing them, or break chemical bonds , causing chemical reactions or DNA damage . The main effect of absorption of radio waves by materials

6248-456: The star responsible for the existence of the nebula. It was identified as such in 1942, when Rudolf Minkowski found that its optical spectrum was extremely unusual. The region around the star was found to be a strong source of radio waves in 1949 and X-rays in 1963, and was identified as one of the brightest objects in the sky in gamma rays in 1967. Then, in 1968, the star was found to be emitting its radiation in rapid pulses, becoming one of

6336-439: The star that exploded to produce the Crab Nebula must have had a mass of between 9 and 11  M ☉ . Stars with masses lower than 8  M ☉ are thought to be too small to produce supernova explosions, and end their lives by producing a planetary nebula instead, while a star heavier than 12  M ☉ would have produced a nebula with a different chemical composition from that observed in

6424-416: The sun, stars and blackbody radiation from warm objects, emit unpolarized waves, consisting of incoherent short wave trains in an equal mixture of polarization states. The polarization of radio waves is determined by a quantum mechanical property of the photons called their spin . A photon can have one of two possible values of spin; it can spin in a right-hand sense about its direction of motion, or in

6512-412: The surface of objects and cause surface heating, radio waves are able to penetrate the surface and deposit their energy inside materials and biological tissues. The depth to which radio waves penetrate decreases with their frequency, and also depends on the material's resistivity and permittivity ; it is given by a parameter called the skin depth of the material, which is the depth within which 63% of

6600-588: The surface of the Earth, is vitally important in the design of practical radio systems. Radio waves passing through different environments experience reflection , refraction , polarization , diffraction , and absorption . Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others. Practical radio systems mainly use three different techniques of radio propagation to communicate: At microwave frequencies, atmospheric gases begin absorbing radio waves, so

6688-437: The theory of electromagnetism that was proposed in 1867 by Scottish mathematical physicist James Clerk Maxwell . His mathematical theory, now called Maxwell's equations , predicted that a coupled electric and magnetic field could travel through space as an " electromagnetic wave ". Maxwell proposed that light consisted of electromagnetic waves of very short wavelength. In 1887, German physicist Heinrich Hertz demonstrated

6776-541: The thickness of Titan's atmosphere is 880 km (550 mi). The transit of Saturn itself could not be observed, because Chandra was passing through the Van Allen belts at the time. Radio wave Radio waves are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum , typically with frequencies below 300 gigahertz (GHz) and wavelengths greater than 1 millimeter ( 3 ⁄ 64 inch), about

6864-570: The time of the supernova. Given its great distance, the daytime "guest star" observed by the Chinese could only have been a supernova —a massive, exploding star, having exhausted its supply of energy from nuclear fusion and collapsed in on itself. Recent analysis of historical records have found that the supernova that created the Crab Nebula probably appeared in April or early May, rising to its maximum brightness of between apparent magnitude −7 and −4.5 (brighter even than Venus' −4.2 and everything in

6952-459: The universe. Gamma-ray producing sources include supernova remnants , active galactic nuclei and pulsar wind nebulae . It also actively tests unproven theories in physics such as looking for the predicted gamma-ray annihilation signal from WIMP dark matter particles and testing Lorentz invariance predictions of loop quantum gravity . H.E.S.S. is located in the Khomas highlands of Namibia near

7040-420: The wave's electric field to the next, and is inversely proportional to the frequency f {\displaystyle f} of the wave. The relation of frequency and wavelength in a radio wave traveling in vacuum or air is where Equivalently, c {\displaystyle c} , the distance that a radio wave travels in vacuum in one second, is 299,792,458 meters (983,571,056 ft), which

7128-442: The waves. Since radio frequency radiation has both an electric and a magnetic component, it is often convenient to express intensity of radiation field in terms of units specific to each component. The unit volt per meter (V/m) is used for the electric component, and the unit ampere per meter (A/m) is used for the magnetic component. One can speak of an electromagnetic field , and these units are used to provide information about

7216-600: Was carried out in 1989 by the Whipple Observatory 10m Gamma-Ray telescope, which opened the VHE gamma-ray window and led to the detection of numerous VHE sources since then. In 2019 the Crab Nebula was observed to emit gamma rays in excess of 100  TeV , making it the first identified source beyond 100 TeV. In visible light , the Crab Nebula consists of a broadly oval -shaped mass of filaments, about 6  arcminutes long and 4 arcminutes wide (by comparison,

7304-417: Was composed of a group of stars. William Parsons, 3rd Earl of Rosse observed the nebula at Birr Castle in the early 1840s using a 36-inch (0.9 m) telescope, and made a drawing of it that showed it with arms like those of a crab. He observed it again later, in 1848, using a 72-inch (1.8 m) telescope but could not confirm the supposed resemblance, but the name stuck nevertheless. The Crab Nebula

7392-488: Was discovered by English astronomer John Bevis in 1731. It corresponds with a bright supernova recorded by Chinese astronomers in 1054 as a guest star . The nebula was the first astronomical object identified that corresponds with a historically-observed supernova explosion. At an apparent magnitude of 8.4, comparable to that of Saturn's moon Titan , it is not visible to the naked eye but can be made out using binoculars under favourable conditions. The nebula lies in

7480-588: Was observing a bright comet . Messier catalogued it as the first entry in his catalogue of comet-like objects; in 1757, Alexis Clairaut reexamined the calculations of Edmund Halley and predicted the return of Halley's Comet in late 1758. The exact time of the comet's return required the consideration of perturbations to its orbit caused by planets in the Solar System such as Jupiter, which Clairaut and his two colleagues Jérôme Lalande and Nicole-Reine Lepaute carried out more precisely than Halley, finding that

7568-399: Was proposed in 1867 by Scottish mathematical physicist James Clerk Maxwell . His mathematical theory, now called Maxwell's equations , predicted that a coupled electric and magnetic field could travel through space as an " electromagnetic wave ". Maxwell proposed that light consisted of electromagnetic waves of very short wavelength. In 1887, German physicist Heinrich Hertz demonstrated

7656-417: Was the first astronomical object recognized as being connected to a supernova explosion. In the early twentieth century, the analysis of early photographs of the nebula taken several years apart revealed that it was expanding. Tracing the expansion back revealed that the nebula must have become visible on Earth about 900 years before. Historical records revealed that a new star bright enough to be seen in

7744-552: Was the strong magnetic field produced by a neutron star at the centre of the nebula. Even though the Crab Nebula is the focus of much attention among astronomers, its distance remains an open question, owing to uncertainties in every method used to estimate its distance. In 2008, the consensus was that its distance from Earth is 2.0 ± 0.5 kpc (6,500 ± 1,600 ly). Along its longest visible dimension, it thus measures about 4.1 ± 1 pc (13 ± 3 ly) across. The Crab Nebula currently

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