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71-590: A nova ( pl. novae or novas ) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", Latin for "new") that slowly fades over weeks or months. All observed novae involve white dwarfs in close binary systems , but causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. The main sub-classes of novae are classical novae, recurrent novae (RNe), and dwarf novae . They are all considered to be cataclysmic variable stars . Classical nova eruptions are

142-415: A helium flash ) is a proposed category of nova event that lacks hydrogen lines in its spectrum . The absence of hydrogen lines may be caused by the explosion of a helium shell on a white dwarf. The theory was first proposed in 1989, and the first candidate helium nova to be observed was V445 Puppis , in 2000. Since then, four other novae have been proposed as helium novae. Astronomers have estimated that

213-675: A classical nova may brighten by more than 12 magnitudes. Although it is estimated that as many as a quarter of nova systems experience multiple eruptions, only ten recurrent novae (listed below) have been observed in the Milky Way. Several extragalactic recurrent novae have been observed in the Andromeda Galaxy (M31) and the Large Magellanic Cloud . One of these extragalactic novae, M31N 2008-12a , erupts as frequently as once every 12 months. On 20 April 2016,

284-516: A combination of the Pauli exclusion principle and quantum confinement . The Pauli principle allows only one fermion in each quantum state and the confinement ensures that energy of these states increases as they are filled. The lowest states fill up and fermions are forced to occupy high energy states even at low temperature. While the Pauli principle and Fermi-Dirac distribution applies to all matter,

355-409: 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 also can emit gamma rays (>100 MeV). Potentially, a white dwarf can generate multiple novae over time as additional hydrogen continues to accrete onto its surface from its companion star. Where this repeated flaring

426-407: A few times per century. The last bright nova was V1369 Centauri , which reached 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 ( Latin for "concerning the new star"), giving rise to the adoption of the name nova . In this work he argued that

497-467: A finite volume may take only a discrete set of energies, called quantum states . The Pauli exclusion principle prevents identical fermions from occupying the same quantum state. At lowest total energy (when the thermal energy of the particles is negligible), all the lowest energy quantum states are filled. This state is referred to as full degeneracy. This degeneracy pressure remains non-zero even at absolute zero temperature. Adding particles or reducing

568-441: A gas of particles that became degenerate at low temperature; he also pointed out that ordinary atoms are broadly similar in regards to the filling of energy levels by fermions. Milne proposed that degenerate matter is found in most of the nuclei of stars, not only in compact stars . Degenerate matter exhibits quantum mechanical properties when a fermion system temperature approaches absolute zero . These properties result from

639-452: A lesser one at −7.5. Novae also have roughly the same absolute magnitude 15 days after their peak (−5.5). Nova-based distance estimates to various nearby galaxies and galaxy clusters have been shown to be of comparable accuracy to those measured with Cepheid variable stars . A recurrent nova ( RN ) is an object that has been seen to experience repeated nova eruptions. The recurrent nova typically brightens by about 9 magnitudes, whereas

710-456: A manner similar to Cooper pairing in electrical superconductors . The equations of state for the various proposed forms of quark-degenerate matter vary widely, and are usually also poorly defined, due to the difficulty of modelling strong force interactions. Quark-degenerate matter may occur in the cores of neutron stars, depending on the equations of state of neutron-degenerate matter. It may also occur in hypothetical quark stars , formed by

781-435: A nearby object should be seen to move relative to the fixed stars, and thus the nova had to be very far away. Although SN 1572 was later found to be a supernova and not a nova, the terms were considered interchangeable until the 1930s. After this, novae were called classical novae to distinguish them from supernovae, as their causes and energies were thought to be different, based solely on the observational evidence. Although

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852-477: A nova is less dependent on the accretion rate of the white dwarf than on its mass; with their powerful gravity, massive white dwarfs require less accretion to fuel an eruption than lower-mass ones. Consequently, the interval is shorter for high-mass white dwarfs. V Sagittae is unusual in that the time of its next eruption can be predicted fairly accurately; it is expected to recur in approximately 2083, plus or minus about 11 years. Novae are classified according to

923-433: A result, the white dwarf steadily captures matter from the companion's outer atmosphere in an accretion disk, and in turn, the accreted matter falls into the atmosphere. As the white dwarf consists of degenerate matter , the accreted hydrogen is unable to expand even though its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 million K , initiating nuclear burning via

994-451: A semi-classical model for electrons in a metal. The model treated the electrons as a gas. Later in 1927, Arnold Sommerfeld applied the Pauli principle via Fermi-Dirac statistics to this electron gas model, computing the specific heat of metals; the result became Fermi gas model for metals. Sommerfeld called the low temperature region with quantum effects a "wholly degenerate gas". Also in 1927 Ralph H. Fowler applied Fermi's model to

1065-414: A solid. In degenerate gases the kinetic energies of electrons are quite high and the rate of collision between electrons and other particles is quite low, therefore degenerate electrons can travel great distances at velocities that approach the speed of light. Instead of temperature, the pressure in a degenerate gas depends only on the speed of the degenerate particles; however, adding heat does not increase

1136-408: A sufficiently drastic increase in temperature (such as during a red giant star's helium flash ), matter can become non-degenerate without reducing its density. Degeneracy pressure contributes to the pressure of conventional solids, but these are not usually considered to be degenerate matter because a significant contribution to their pressure is provided by electrical repulsion of atomic nuclei and

1207-437: A well known high energy electromagnetic transient. The proposed ULTRASAT satellite will observe a field of more than 200 square degrees continuously in an ultraviolet wavelength that is particularly important for detecting supernovae within minutes of their occurrence. Degenerate matter Degenerate matter occurs when the Pauli exclusion principle significantly alters a state of matter at low temperature. The term

1278-400: Is an extremely compact star composed of "nuclear matter", which is predominantly a degenerate neutron gas with a small admixture of degenerate proton and electron gases. Neutrons in a degenerate neutron gas are spaced much more closely than electrons in an electron-degenerate gas because the more massive neutron has a much shorter wavelength at a given energy. This phenomenon is compounded by

1349-477: Is around 1.38 solar masses. The limit may also change with the chemical composition of the object, as it affects the ratio of mass to number of electrons present. The object's rotation, which counteracts the gravitational force, also changes the limit for any particular object. Celestial objects below this limit are white dwarf stars, formed by the gradual shrinking of the cores of stars that run out of fuel. During this shrinking, an electron-degenerate gas forms in

1420-447: Is heated by the hot white dwarf and eventually reaches a critical temperature, causing ignition of rapid runaway fusion . The sudden increase in energy expels the atmosphere into interstellar space, creating the envelope seen as visible light during the nova event. In past centuries such an event was thought to be a new star. A few novae produce short-lived nova remnants , lasting for perhaps several centuries. A recurrent nova involves

1491-461: Is in contrast to the timescale of the millions or billions of years during which the galaxies and their component stars in our universe have evolved. Singularly, the term is used for violent deep-sky events, such as supernovae , novae , dwarf nova outbursts, gamma-ray bursts , and tidal disruption events , as well as gravitational microlensing . Time-domain astronomy also involves long-term studies of variable stars and their changes on

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1562-449: Is increased, electrons progressively fill the lower energy states and additional electrons are forced to occupy states of higher energy even at low temperatures. Degenerate gases strongly resist further compression because the electrons cannot move to already filled lower energy levels due to the Pauli exclusion principle. Since electrons cannot give up energy by moving to lower energy states, no thermal energy can be extracted. The momentum of

1633-482: Is observed, the object is called a recurrent nova. An example is RS Ophiuchi , which is known to have flared seven times (in 1898, 1933, 1958, 1967, 1985, 2006, and 2021). Eventually, the white dwarf can explode as a Type Ia supernova if it approaches the Chandrasekhar limit . Occasionally, novae are bright enough and close enough to Earth to be conspicuous to the unaided eye. The brightest recent example

1704-471: Is pressure, k B is the Boltzmann constant , N is the number of particles (typically atoms or molecules), T is temperature, and V is the volume, the pressure exerted by degenerate matter depends only weakly on its temperature. In particular, the pressure remains nonzero even at absolute zero temperature. At relatively low densities, the pressure of a fully degenerate gas can be derived by treating

1775-464: Is used in astrophysics to refer to dense stellar objects such as white dwarfs and neutron stars , where thermal pressure alone is not enough to prevent gravitational collapse . The term also applies to metals in the Fermi gas approximation. Degenerate matter is usually modelled as an ideal Fermi gas , an ensemble of non-interacting fermions. In a quantum mechanical description, particles limited to

1846-521: The Sky & Telescope website reported a sustained brightening of T Coronae Borealis from magnitude 10.5 to about 9.2 starting in February 2015. A similar event had been reported in 1938, followed by another outburst in 1946. By June 2018, the star had dimmed slightly but still remained at an unusually high level of activity. In March or April 2023, it dimmed to magnitude 12.3. A similar dimming occurred in

1917-477: The CNO cycle . If the accretion rate is just right, hydrogen fusion may occur in a stable manner on the surface of the white dwarf, giving rise to a supersoft X-ray source , but for most binary system parameters, the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other, heavier chemical elements in a runaway reaction, liberating an enormous amount of energy. This blows

1988-937: The Gravitational-wave Optical Transient Observer (GOTO) began looking for collisions between neutron stars. The ability of modern instruments to observe in wavelengths invisible to the human eye ( radio waves , infrared , ultraviolet , X-ray ) increases the amount of information that may be obtained when a transient is studied. In radio astronomy the LOFAR is looking for radio transients. Radio time domain studies have long included pulsars and scintillation. Projects to look for transients in X-ray and gamma rays include Cherenkov Telescope Array , eROSITA , AGILE , Fermi , HAWC , INTEGRAL , MAXI , Swift Gamma-Ray Burst Mission and Space Variable Objects Monitor . Gamma ray bursts are

2059-466: The Heisenberg uncertainty principle . However, because protons are much more massive than electrons, the same momentum represents a much smaller velocity for protons than for electrons. As a result, in matter with approximately equal numbers of protons and electrons, proton degeneracy pressure is much smaller than electron degeneracy pressure, and proton degeneracy is usually modelled as a correction to

2130-644: The MACHO Project . These efforts, beside the discovery of the microlensing events itself, resulted in the orders of magnitude more variable stars known to mankind. Subsequent, dedicated sky surveys such as the Palomar Transient Factory , the spacecraft Gaia and the LSST , focused on expanding the coverage of the sky monitoring to fainter objects, more optical filters and better positional and proper motions measurement capabilities. In 2022,

2201-720: The Milky Way experiences roughly 25 to 75 novae per year. The number of novae actually observed in the Milky Way each year is much lower, about 10, probably because distant novae are obscured by gas and dust absorption. As of 2019, 407 probable novae had been recorded in the Milky Way. In the Andromeda Galaxy , roughly 25 novae brighter than about 20th magnitude are discovered each year, and smaller numbers are seen in other nearby galaxies. Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium. Classical nova explosions are galactic producers of

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2272-494: The equations of state of electron-degenerate matter. At densities greater than those supported by neutron degeneracy, quark matter is expected to occur. Several variations of this hypothesis have been proposed that represent quark-degenerate states. Strange matter is a degenerate gas of quarks that is often assumed to contain strange quarks in addition to the usual up and down quarks. Color superconductor materials are degenerate gases of quarks in which quarks pair up in

2343-410: The light curve decay speed, referred to as either type A, B, C and R, or using the prefix "N": Some novae leave behind visible nebulosity , material expelled in the nova explosion or in multiple explosions. Novae have some promise for use as standard candle measurements of distances. For instance, the distribution of their absolute magnitude is bimodal , with a main peak at magnitude −8.8, and

2414-547: The Pauli principle, exert pressure preventing further compression. The allocation or distribution of fermions into quantum states ranked by energy is called the Fermi-Dirac distribution . Degenerate matter exhibits the results of Fermi-Dirac distribution. Unlike a classical ideal gas , whose pressure is proportional to its temperature P = k B N T V , {\displaystyle P=k_{\rm {B}}{\frac {NT}{V}},} where P

2485-402: The amount of material ejected in a nova 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

2556-408: The chances of looking in the right place at the right time were low. Schmidt cameras and other astrographs with wide field were invented in the 20th century, but mostly used to survey the unchanging heavens. Historically time domain astronomy has come to include appearance of comets and variable brightness of Cepheid-type variable stars . Old astronomical plates exposed from the 1880s through

2627-492: The collapse of objects above the Tolman–Oppenheimer–Volkoff mass limit for neutron-degenerate objects. Whether quark-degenerate matter forms at all in these situations depends on the equations of state of both neutron-degenerate matter and quark-degenerate matter, both of which are poorly known. Quark stars are considered to be an intermediate category between neutron stars and black holes. Quantum mechanics uses

2698-444: The core, providing sufficient degeneracy pressure as it is compressed to resist further collapse. Above this mass limit, a neutron star (primarily supported by neutron degeneracy pressure) or a black hole may be formed instead. Neutron degeneracy is analogous to electron degeneracy and exists in neutron stars , which are partially supported by the pressure from a degenerate neutron gas. Neutron stars are formed either directly from

2769-507: The degenerate particles are neutrons. A fermion gas in which all quantum states below a given energy level are filled is called a fully degenerate fermion gas. The difference between this energy level and the lowest energy level is known as the Fermi energy. In an ordinary fermion gas in which thermal effects dominate, most of the available electron energy levels are unfilled and the electrons are free to move to these states. As particle density

2840-620: The early 1990s held by the Harvard College Observatory are being digitized by the DASCH project. The interest in transients has intensified when large CCD detectors started to be available to the astronomical community. As telescopes with larger fields of view and larger detectors come into use in the 1990s, first massive and regular survey observations were initiated - pioneered by the gravitational microlensing surveys such as Optical Gravitational Lensing Experiment and

2911-509: The element lithium . The contribution of novae to the interstellar medium is not great; novae supply only 1 ⁄ 50 as much material to the galaxy as do supernovae, and only 1 ⁄ 200 as much as red giant and supergiant stars. Observed recurrent novae such as RS Ophiuchi (those with periods on the order of decades) are rare. Astronomers theorize, however, that most, if not all, novae recur, albeit on time scales ranging from 1,000 to 100,000 years. The recurrence interval for

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2982-560: The event is usually classified as a Type Ia supernova . Novae most often occur in the sky along the path of the Milky Way , especially near the observed Galactic Center in Sagittarius; however, they can appear anywhere in the sky. They occur far more frequently than galactic supernovae , averaging about ten per year in the Milky Way. Most are found telescopically, perhaps only one every 12–18 months reaching naked-eye visibility. Novae reaching first or second magnitude occur only

3053-414: The fact that the pressures within neutron stars are much higher than those in white dwarfs. The pressure increase is caused by the fact that the compactness of a neutron star causes gravitational forces to be much higher than in a less compact body with similar mass. The result is a star with a diameter on the order of a thousandth that of a white dwarf. The properties of neutron matter set an upper limit to

3124-430: The fermions in the fermion gas nevertheless generates pressure, termed "degeneracy pressure". Under high densities, matter becomes a degenerate gas when all electrons are stripped from their parent atoms. The core of a star, once hydrogen burning nuclear fusion reactions stops, becomes a collection of positively charged ions , largely helium and carbon nuclei, floating in a sea of electrons, which have been stripped from

3195-399: The gas. All matter experiences both normal thermal pressure and degeneracy pressure, but in commonly encountered gases, thermal pressure dominates so much that degeneracy pressure can be ignored. Likewise, degenerate matter still has normal thermal pressure; the degeneracy pressure dominates to the point that temperature has a negligible effect on the total pressure. The adjacent figure shows

3266-402: The gas. At very high densities, where most of the particles are forced into quantum states with relativistic energies , the pressure is given by P = K ( N V ) 4 / 3 , {\displaystyle P=K\left({\frac {N}{V}}\right)^{4/3},} where K is another proportionality constant depending on the properties of the particles making up

3337-490: The growth of a new field of astrophysics research, time-domain astronomy , which studies the variability of brightness and other parameters of objects in the universe in different time scales." Also the 2017 Dan David Prize was awarded to the three leading researchers in the field of time-domain astronomy: Neil Gehrels ( Swift Gamma-Ray Burst Mission ), Shrinivas Kulkarni ( Palomar Transient Factory ), Andrzej Udalski ( Optical Gravitational Lensing Experiment ). Before

3408-415: The interesting cases for degenerate matter involve systems of many fermions. These cases can be understood with the help of the Fermi gas model. Examples include electrons in metals and in white dwarf stars and neutrons in neutron stars. The electrons are confined by Coulomb attraction to positive ion cores; the neutrons are confined by gravitation attraction. The fermions, forced in to higher levels by

3479-449: The interior of white dwarfs are two examples. Following the Pauli exclusion principle, there can be only one fermion occupying each quantum state. In a degenerate gas, all quantum states are filled up to the Fermi energy. Most stars are supported against their own gravitation by normal thermal gas pressure, while in white dwarf stars the supporting force comes from the degeneracy pressure of the electron gas in their interior. In neutron stars,

3550-653: The invention of telescopes , transient events that were visible to the naked eye , from within or near the Milky Way Galaxy, were very rare, and sometimes hundreds of years apart. However, such events were recorded in antiquity, such as the supernova in 1054 observed by Chinese, Japanese and Arab astronomers, and the event in 1572 known as " Tycho's Supernova " after Tycho Brahe , who studied it until it faded after two years. Even though telescopes made it possible to see more distant events, their small fields of view – typically less than 1 square degree – meant that

3621-437: The mass is increased, the particles become spaced closer together due to gravity (and the pressure is increased), so the object becomes smaller. Degenerate gas can be compressed to very high densities, typical values being in the range of 10,000 kilograms per cubic centimeter. There is an upper limit to the mass of an electron-degenerate object, the Chandrasekhar limit , beyond which electron degeneracy pressure cannot support

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3692-414: The mass of a neutron star , the Tolman–Oppenheimer–Volkoff limit , which is analogous to the Chandrasekhar limit for white dwarf stars. Sufficiently dense matter containing protons experiences proton degeneracy pressure, in a manner similar to the electron degeneracy pressure in electron-degenerate matter: protons confined to a sufficiently small volume have a large uncertainty in their momentum due to

3763-419: The most common type. This type is usually created in a close binary star system consisting of a white dwarf and either a main sequence , subgiant , or red giant star . If the orbital period of the system is a few days or less, the white dwarf is close enough to its companion star to draw accreted matter onto its surface, creating a dense but shallow atmosphere . This atmosphere, mostly consisting of hydrogen,

3834-443: The normalization of pairs of images. Due to large fields of view required, the time-domain work involves storing and transferring a huge amount of data. This includes data mining techniques, classification, and the handling of heterogeneous data. The importance of time-domain astronomy was recognized in 2018 by German Astronomical Society by awarding a Karl Schwarzschild Medal to Andrzej Udalski for "pioneering contribution to

3905-532: The nuclei. Degenerate gas is an almost perfect conductor of heat and does not obey ordinary gas laws. White dwarfs are luminous not because they are generating energy but rather because they have trapped a large amount of heat which is gradually radiated away. Normal gas exerts higher pressure when it is heated and expands, but the pressure in a degenerate gas does not depend on the temperature. When gas becomes super-compressed, particles position right up against each other to produce degenerate gas that behaves more like

3976-411: The object against collapse. The limit is approximately 1.44 solar masses for objects with typical compositions expected for white dwarf stars (carbon and oxygen with two baryons per electron). This mass cut-off is appropriate only for a star supported by ideal electron degeneracy pressure under Newtonian gravity; in general relativity and with realistic Coulomb corrections, the corresponding mass limit

4047-570: The reduction of the specific heat of gases at very low temperature as "degeneration"; he attributed this to quantum effects. In subsequent work in various papers on quantum thermodynamics by Albert Einstein , by Max Planck , and by Erwin Schrödinger , the effect at low temperatures came to be called "gas degeneracy". A fully degenerate gas has no volume dependence on pressure when temperature approaches absolute zero . Early in 1927 Enrico Fermi and separately Llewellyn Thomas developed

4118-531: The remaining gases away from the surface of the white dwarf and produces an extremely bright outburst of light. The rise to peak brightness may be very rapid, or gradual; after the peak, the brightness declines steadily. The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used for grouping novae into speed classes. Fast novae typically will take less than 25 days to decay by 2 magnitudes, while slow novae will take more than 80 days. Despite its violence, usually

4189-412: The same processes as a classical nova, except that the nova event repeats in cycles of a few decades or less as the companion star again feeds the dense atmosphere of the white dwarf after each ignition, as in the star T Coronae Borealis . Under certain conditions, mass accretion can eventually trigger runaway fusion that destroys the white dwarf rather than merely expelling its atmosphere. In this case,

4260-768: The screening of nuclei from each other by electrons. The free electron model of metals derives their physical properties by considering the conduction electrons alone as a degenerate gas, while the majority of the electrons are regarded as occupying bound quantum states. This solid state contrasts with degenerate matter that forms the body of a white dwarf, where most of the electrons would be treated as occupying free particle momentum states. Exotic examples of degenerate matter include neutron degenerate matter, strange matter , metallic hydrogen and white dwarf matter. Degenerate gases are gases composed of fermions such as electrons, protons, and neutrons rather than molecules of ordinary matter. The electron gas in ordinary metals and in

4331-514: The speed of light (particle kinetic energy larger than its rest mass energy ) is called relativistic degenerate matter . The concept of degenerate stars , stellar objects composed of degenerate matter, was originally developed in a joint effort between Arthur Eddington , Ralph Fowler and Arthur Milne . Eddington had suggested that the atoms in Sirius B were almost completely ionised and closely packed. Fowler described white dwarfs as composed of

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4402-400: The speed of most of the electrons, because they are stuck in fully occupied quantum states. Pressure is increased only by the mass of the particles, which increases the gravitational force pulling the particles closer together. Therefore, the phenomenon is the opposite of that normally found in matter where if the mass of the matter is increased, the object becomes bigger. In degenerate gas, when

4473-573: The supernova of stars with masses between 10 and 25 M ☉ ( solar masses ), or by white dwarfs acquiring a mass in excess of the Chandrasekhar limit of 1.44  M ☉ , usually either as a result of a merger or by feeding off of a close binary partner. Above the Chandrasekhar limit, the gravitational pressure at the core exceeds the electron degeneracy pressure, and electrons begin to combine with protons to produce neutrons (via inverse beta decay , also termed electron capture ). The result

4544-415: The system as an ideal Fermi gas, in this way P = ( 3 π 2 ) 2 / 3 ℏ 2 5 m ( N V ) 5 / 3 , {\displaystyle P={\frac {(3\pi ^{2})^{2/3}\hbar ^{2}}{5m}}\left({\frac {N}{V}}\right)^{5/3},} where m is the mass of the individual particles making up

4615-508: The term "stella nova" means "new star", novae most often take place on white dwarfs , which are remnants of extremely 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

4686-468: The thermal pressure (red line) and total pressure (blue line) in a Fermi gas, with the difference between the two being the degeneracy pressure. As the temperature falls, the density and the degeneracy pressure increase, until the degeneracy pressure contributes most of the total pressure. While degeneracy pressure usually dominates at extremely high densities, it is the ratio between degenerate pressure and thermal pressure which determines degeneracy. Given

4757-720: The timescale of minutes to decades. Variability studied can be intrinsic , including periodic or semi-regular pulsating stars , young stellar objects , stars with outbursts , asteroseismology studies; or extrinsic , which results from eclipses (in binary stars , planetary transits ), stellar rotation (in pulsars , spotted stars), or gravitational microlensing events . Modern time-domain astronomy surveys often uses robotic telescopes , automatic classification of transient events, and rapid notification of interested people. Blink comparators have long been used to detect differences between two photographic plates, and image subtraction became more used when digital photography eased

4828-453: The volume forces the particles into higher-energy quantum states. In this situation, a compression force is required, and is made manifest as a resisting pressure. The key feature is that this degeneracy pressure does not depend on the temperature but only on the density of the fermions. Degeneracy pressure keeps dense stars in equilibrium, independent of the thermal structure of the star. A degenerate mass whose fermions have velocities close to

4899-406: The word 'degenerate' in two ways: degenerate energy levels and as the low temperature ground state limit for states of matter. The electron degeneracy pressure occurs in the ground state systems which are non-degenerate in energy levels. The term "degeneracy" derives from work on the specific heat of gases that pre-dates the use of the term in quantum mechanics. In 1914 Walther Nernst described

4970-1053: The year before the 1945 outburst, indicating that it would likely erupt between March and September 2024. As of 5 October 2024, this predicted outburst has not yet occurred. Novae are relatively common in the Andromeda Galaxy (M31); several dozen novae (brighter than apparent magnitude +20) are discovered in M31 each year. The Central Bureau for Astronomical Telegrams (CBAT) has tracked novae in M31, M33 , and M81 . Transient astronomical event Time-domain astronomy studies transient astronomical events (" transients "), which include various types of variable stars, including periodic , quasi-periodic , high proper motion stars, and lifecycle events ( supernovae , kilonovae ) or other changes in behavior or type. Non-stellar transients include asteroids , planetary transits and comets . Transients characterize astronomical objects or phenomena whose duration of presentation may be from milliseconds to days, weeks, or even several years. This

5041-481: Was Nova Cygni 1975 . This nova appeared on 29 August 1975, in the constellation Cygnus about 5 degrees north of Deneb , and reached magnitude  2.0 (nearly as bright as Deneb). The most recent were V1280 Scorpii , which reached magnitude 3.7 on 17 February 2007, and Nova Delphini 2013 . Nova Centauri 2013 was discovered 2 December 2013 and so far is the brightest nova of this millennium, reaching magnitude 3.3. A helium nova (undergoing

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