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Encyclopedia > White dwarf
Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint dot to the lower left of the much brighter Sirius A.
Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint dot to the lower left of the much brighter Sirius A.

A white dwarf is an astronomical object which is produced when a star of low or medium mass dies. These stars are not massive enough to generate the core temperatures required to continue nuclear fusion reactions. After such a star has become a red giant during its helium-burning phase, it will shed its outer layers to form a planetary nebula, leaving behind an inert core. In most cases the core consists mostly of carbon and oxygen, though helium[1] and rare oxygen-neon white dwarf stars have been recorded. White dwarf, or White Dwarf, has multiple uses: White dwarf, a compact star that is no longer generating energy through nuclear fusion at its core. ... Image File history File linksMetadata Sirius_A_and_B_Hubble_photo. ... Image File history File linksMetadata Sirius_A_and_B_Hubble_photo. ... For information on Sirius satellite radio, see Sirius Satellite Radio. ... The Hubble Space Telescope (HST) is a telescope in orbit around the Earth, named after astronomer Edwin Hubble. ... See also Lists of astronomical objects Category: ... This article is about the astronomical object. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... Cross section of a red giant showing nucleosynthesis and elements formed According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ... For other uses of this term, see Helium (disambiguation). ... NGC 6543, the Cats Eye Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... For other uses of this term, see Helium (disambiguation). ... General Name, Symbol, Number neon, Ne, 10 Chemical series noble gases Group, Period, Block 18, 2, p Appearance colorless Atomic mass 20. ...


This core has no further source of energy, and so will gradually radiate away its energy and cool down. The core, no longer supported against gravitational collapse by fusion reactions, becomes extremely dense, with a typical mass of that of the sun contained in a volume about equal to that of the Earth. The white dwarf is supported only by electron degeneracy pressure. The maximum mass of a white dwarf, beyond which degeneracy pressure can no longer support it, is about 1.44 solar masses. A white dwarf which approaches this limit (known as the Chandrasekhar limit), typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation. Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... In astronomy, the solar mass is a unit of mass used to express the mass of stars and larger objects such as galaxies. ... The Chandrasekhar limit, is the maximum mass possible for a white dwarf (one of the end stages of stars when they cool down) and is approximately 3 × 1030 kg, around 1. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... Carbon detonation is a violent re-ignition of thermonuclear fusion in a dead star, which produces Type Ia supernovae. ...


Eventually, over hundreds of billions of years, white dwarfs will cool to temperatures at which they are no longer visible. However, over the universe's lifetime to the present (about 13.7 billion years) even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins. The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero—the lowest possible temperature where nothing could be colder and no heat energy remains in a substance—is defined as zero kelvin (0 K). ...


As a class, white dwarfs are fairly common; they comprise roughly 6% of all stars in the solar neighborhood.[2]

Contents

Formation & Types

Almost all small and medium-size stars will end up as white dwarfs, after nearly all the hydrogen they contain has been fused into helium. Near the end of its nuclear burning stage, such a star goes through a red giant phase and then expels most of its outer material (creating a planetary nebula) until only the hot (T > 100,000 K) core remains, which then settles down to become a young white dwarf which shines from residual heat. This article is about the chemistry of hydrogen. ... For other uses of this term, see Helium (disambiguation). ... Cross section of a red giant showing nucleosynthesis and elements formed According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ... NGC 6543, the Cats Eye Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ...


A typical white dwarf has around half the mass of the Sun yet is only slightly bigger than the Earth; this makes white dwarfs one of the densest forms of matter (109 kg·m−3), surpassed only by neutron stars, black holes and hypothetical quark stars. The higher the mass of the white dwarf, the smaller the size - however, there is an upper limit to the mass of a white dwarf, the Chandrasekhar limit (about 1.4 times the mass of the Sun). If this limit were exceeded, the pressure exerted by electrons would no longer be able to balance the force of gravity, and the star would begin to collapse. In the absence of available nuclear fuel, the collapse would lead to a neutron star. However, the vast majority of white dwarfs are not massive enough to suffer such a fate, and in a few border cases, accreting processes may delay this collapse for a time.[citation needed] Carbon-oxygen white dwarfs accreting mass from a neighboring star undergo a runaway nuclear fusion reaction (leading to a Type Ia supernova explosion) prior to reaching the limiting mass. The Sun is the star at the center of the Solar System. ... Adjectives: Terrestrial, Terran, Telluric, Tellurian, Earthly Atmosphere Surface pressure: 101. ... A neutron star is one of the few possible endpoints of stellar evolution. ... A black hole is an object predicted by general relativity,[1] with a gravitational field so powerful that even electromagnetic radiation (such as light) cannot escape its pull. ... A strange star or quark star is a hypothetical type of star composed of strange matter, or quark matter. ... The Chandrasekhar limit, is the maximum mass possible for a white dwarf (one of the end stages of stars when they cool down) and is approximately 3 × 1030 kg, around 1. ... e- redirects here. ... Gravity is a force of attraction that acts between bodies that have mass. ... A neutron star is one of the few possible endpoints of stellar evolution. ...


Despite this limit, most stars end their lives as white dwarfs since they tend to eject most of their mass into space before the final collapse (often with spectacular results—see planetary nebula). It is thought that even stars eight times as massive as the Sun will in the end die as white dwarfs, cooling gradually to become black dwarfs. NGC 6543, the Cats Eye Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ... A black dwarf constitutes the remains of a Sun-sized star which has evolved to a white dwarf and subsequently cooled down such that it only emits black body radiation. ...


The chemical composition of the white dwarf depends upon its mass. A star of a few solar masses will ignite carbon fusion to form magnesium, neon, and smaller amounts of other elements, resulting in a white dwarf composed chiefly of oxygen, neon, and magnesium, provided that it can lose enough mass to get below the Chandrasekhar limit, and provided that the ignition of carbon is not so violent as to blow apart the star in a supernova.


However a star of mass on the order of magnitude of the Sun will be unable to ignite carbon fusion, and will produce a white dwarf composed chiefly of carbon and oxygen, and with mass too low to collapse unless further matter is added to it later. A star of less than about half the mass of the Sun will be unable to ignite helium fusion, and will produce a white dwarf composed chiefly of helium.


Characteristics

Many white dwarfs are approximately the size of the Earth, typically 100 times smaller in diameter than the Sun; their average mass is about 0.5-0.6 solar masses, though there is quite a bit of variation.(see link for discussion) Their compactness implies that the same amount of matter is packed in a volume that is typically 106 = 1,000,000 times smaller than the Sun and so the average density of matter in white dwarfs is 1,000,000 times greater than the average density of the Sun. Such matter is called degenerate. Degenerate matter behaves in a seemingly counterintuitive fashion; for instance, white dwarfs grow smaller—and thus their densities increase—with higher mass (see "further reading"). In the 1930s this was explained as a quantum mechanical effect: the weight of the white dwarf is supported by the pressure of electrons (electron degeneracy), which only depends on density and not on temperature. Very useful in understanding this effect is the Fermi gas model. In astronomy, the solar mass is a unit of mass used to express the mass of stars and larger objects such as galaxies. ... Degenerate matter is matter which has sufficiently high density that the dominant contribution to its pressure arises from the Pauli exclusion principle. ... A Fermi gas is a collection of non-interacting fermions. ...


If, for all observed stars, one makes a diagram of (absolute) brightness versus color (Hertzsprung-Russell diagram), not all combinations of brightness and color occur. Few stars are in the low-brightness-hot-color region (the white dwarfs), but most stars follow a strip, called the main sequence. Low mass main sequence stars are small and cool. They look red and are called red dwarfs or (even cooler) brown dwarfs. These form an entirely different class of heavenly bodies than white dwarfs. In red dwarfs, as in all main-sequence stars, the pressure counterbalancing the weight is caused by the thermal motion of the hot gas. The pressure obeys the ideal gas law. Another class of stars is called giants: stars in the high-brightness part of the brightness-color diagram. These are stars blown up by radiation pressure and are very large. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude diagram, or CMD) shows the relationship between absolute magnitude, luminosity, classification, and surface temperature of stars. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... This article is becoming very long. ... This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. ...


Cooling

Most white dwarf stars are extremely hot; hence the bright white light they emit. This heat is a remnant of that generated from the star's collapse, and is not being replenished (unless the white dwarf accretes matter from other nearby stars). However, since white dwarfs have an extremely small surface area from which to radiate this heat, they remain hot for a long period of time. Evidence suggests that their interiors slowly crystallize as they cool and age, ultimately settling into a diamond-like configuration; astronomers know of at least one "diamond white dwarf" already. [1] Crystal (disambiguation) Insulin crystals A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... This article is about the gemstone. ... BPM 37093 is a white dwarf star 50 light-years from Earth, in the constellation Centaurus, for which enough evidence has been gathered to infer that it consists of crystalline carbon, confirming previous theoretical predictions. ...


Eventually, a white dwarf will cool into a black dwarf. Black dwarfs are ambient temperature entities and radiate weakly in the radio spectrum, according to theory. However, the universe has not existed long enough for any white dwarfs to have cooled down this far yet; no black dwarfs are thought to exist, and the coolest white dwarfs found have surface temperatures around 3900 K.[2](see below) and the cooling is slower as it progresses. A white dwarf may cool from 20,000K to 5,000K in the same amount of time it takes to cool from 5,000K to 4,000K. In all, a 0.5 solar mass white dwarf starting at 20,000K would require approximately 25 billion years to cool to ambient. This may be contrasted with the estimated age of the universe, which is 13.7 billion years. A black dwarf is a hypothetical astronomical object: a white dwarf so old that it has cooled down so that it no longer emits significant heat or light. ... Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultra high frequency VHF = Very high frequency HF = High... Universe is a word derived from the Old French univers, which in turn comes from the Latin roots unus (one) and versus (a form of vertere, to turn). Based on observations of the observable universe, physicists attempt to describe the whole of space-time, including all matter and energy and... The age of the universe, according to the Big Bang theory, is defined as the largest possible value of proper time integrated along a time-like curve from the Earth at the present epoch back to the Big Bang. The time that has elapsed on a hypothetical clock which has...


High-energy emissions

Many nearby, young white dwarfs have been detected as sources of soft X-rays (i.e. lower-energy X-rays); soft X-ray and extreme ultraviolet observations enable astronomers to study the composition and structure of the thin atmospheres of these stars. ROSAT image of X-ray fluorescence of, and occultation of the X-ray background by, the Moon. ... UV astronomy is the branch of astronomy and astrophysics which deals with objects visible in ultraviolet (UV) radiation. ...


Evidence exists that the remnants of a white dwarf's planets and comets interact with it and infall to cause X-ray emission. The orbits of the planetary nebula are also chaotic enough to produce frequent collisions and create a zone rich in dust around the white dwarf. [3]


Cataclysmic variables

White dwarfs cannot independently exceed 1.4 solar masses (the Chandrasekhar limit). Most white dwarfs form with a mass close to 0.6 solar masses, but there is a working method to get them close to this limit. White dwarfs in binary systems can steadily accrete material from a companion star. If the accreted material were to push the mass of the white dwarf beyond the 1.4 solar mass limit, degeneracy pressure would no longer support the star, and collapse would ensue. This mechanism was once thought to be the trigger for Type Ia supernovae, the brightest of all supernovae types. Since the 1960s, however, the prevailing view has been that increasing density in the star's interior triggers carbon fusion at a mass slightly below the Chandrasekhar limit, leading to a runaway nuclear fusion reaction in which some of the oxygen is also consumed. The fusion reaction is unregulated because the white dwarf is supported against gravity by quantum degeneracy pressure, not by thermal pressure. Initiation of fusion thus increases the temperature of the star's interior without increasing the pressure, so the white dwarf does not expand and cool in response. However, the increased temperature increases the rate of the fusion reaction, in a process that feeds on itself. This leads to an explosion that obliterates the white dwarf. Artists impression of a binary star system consisting of a black hole, with an accretion disc around it, and a main sequence star. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ...


When accretion does not push the white dwarf close to the Chandrasekhar limit, hydrogen-rich accretion material on the surface may still light up in a thermonuclear explosion. Since the white dwarf's core remains intact, these surface explosions can be repeated as long as accretion continues. This weaker kind of repetitive cataclysmic phenomenon is called a nova. In general, binary systems with a white dwarf accreting matter from a companion are called cataclysmic variables. Artists conception of a white dwarf star accreting hydrogen from a larger companion A nova (pl. ... Artists conception of a cataclysmic variable system Cataclysmic variables are a class of binary stars containing a white dwarf and a companion star. ...


Mass and radius relationship

To find a relationship between the mass of a white dwarf and its radius, one can start from the hydrostatic equilibrium condition: Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ...

frac{dP}{dr} = - frac{GM(r)}{r^2} rho(r) ,
where
frac{dP}{dr} is the rate of change in pressure as a function of radius
G is the gravitational constant
M is the mass inside a specific radius, r
ρ is the density as a function of radius

This derivation will show that higher-mass white dwarfs will have a smaller radius. First, one makes the very rough estimation of an average constant density, given by the mass of the white dwarf divided by its volume: According to the law of universal gravitation, the attractive force between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. ...

rho = frac{M}{frac{4}{3} pi R^3} ,

Putting that into the hydrostatic equilibrium equation and then integrating, one obtains an equation for pressure inside the center of the star to be:

P approx frac{GM^2}{R^4} ,

Now, for a degenerate gas (which is what makes up a white dwarf), pressure is also proportional to density by: Degenerate matter is matter which has sufficiently high density that the dominant contribution to its pressure arises from the Pauli exclusion principle. ...

P sim rho^{5/3} ,

So setting these two equations of pressure proportional:

frac{GM^2}{R^4} sim left(frac{M}{frac{4}{3} pi R^3} right)^{5/3} ,

Now this is a relationship between mass of a white dwarf to its radius. So, drop all the constants to see it more clearly:

R sim frac{1}{M^{1/3}}

After this rough derivation, what has been shown is that as mass of a white dwarf increases, its radius decreases.


History of discoveries

During the nineteenth century, positional measurements of some stars became sufficiently precise that small changes in their location could be measured. Friedrich Bessel used just such precise measurements to determine that the star Sirius (Alpha Canis Majoris) was changing its position (for reasons unrelated to the parallax effect). In 1844 he predicted that Sirius had an unseen companion, and gave an estimate of the orbital period.[4] Friedrich Wilhelm Bessel (July 22, 1784 – March 17, 1846) was a German mathematician, astronomer, and systematizer of the Bessel functions (which, despite their name, were discovered by Daniel Bernoulli). ... For information on Sirius satellite radio, see Sirius Satellite Radio. ... This does not cite its references or sources. ...


It was not until 1862 that Alvan Graham Clark discovered this dark companion of Sirius. The companion, called Sirius B or the Pup, had a surface temperature of about 25,000 K, so it was classified as a hot star. However, Sirius B was about 10,000 times fainter than the primary, Sirius A. Since it was very bright per unit of surface area, the Pup had to be much smaller than Sirius A, with roughly the diameter of the Earth. Alvan Graham Clark (July 10, 1832 – June 9, 1897), born in Fall River, Massachusetts, was an American astronomer and telescope-maker. ...


Analysis of the orbit of the Sirius star system showed that the mass of the Pup was almost the same as that of our own Sun. This implied that Sirius B was thousands of times more dense than lead. As more white dwarfs were found, astronomers began to discover that white dwarfs are common in our galaxy. In 1917 Adriaan Van Maanen discovered Van Maanen's Star, the second known white dwarf. For PB or pb as an abbreviation, see PB. General Name, Symbol, Number lead, Pb, 82 Chemical series poor metals Group, Period, Block 14, 6, p Appearance bluish gray Atomic mass 207. ... Year 1917 (MCMXVII) was a common year starting on Monday of the Gregorian calendar (see link for calendar) or a common year starting on Tuesday of the 13-day slower Julian calendar (see: 1917 Julian calendar). ... Adriaan van Maanen (March 31, 1884, Sneek – January 26, 1946, Pasadena) was a Dutch-American astronomer. ... Van Maanens Star is a white dwarf, the second such star discovered and the third closest one to the Sun after Sirius B and Procyon B. It is located 14. ...


After the discovery of quantum mechanics in the 1920's, an explanation for the density of white dwarfs was found in 1926. R.H. Fowler explained the high densities in an article "Dense matter" (Monthly Notices R. Astron. Soc. 87, 114-122) using the electron degenerate pressure a few months after the formulation of the Fermi-Dirac statistics for an electron, on which the electron pressure is based. Fig. ... Year 1926 (MCMXXVI) was a common year starting on Friday (link will display the full calendar). ... Ralph Howard Fowler (January 17, 1889 – July 28, 1944) was a British physicist and astronomer. ... Degenerate matter is matter which has sufficiently high density that the dominant contribution to its pressure arises from the Pauli exclusion principle. ... Fermi-Dirac distribution as a function of ε/μ plotted for 4 different temperatures. ...


S. Chandrasekhar discovered in 1930 (Astroph. J. 1931, vol. 74, p. 81-82 [3]) in an article called "The maximum mass of ideal white dwarfs" that no white dwarf can be more massive than about 1.4 solar masses. This is now called the Chandrasekhar limit. Chandrasekhar received the Nobel prize (along with Fowler) in 1983. Chandrasekhar redirects here. ... The Chandrasekhar limit, is the maximum mass possible for a white dwarf (one of the end stages of stars when they cool down) and is approximately 3 × 1030 kg, around 1. ... Hannes Alfvén (1908–1995) accepting the Nobel Prize for his work on magnetohydrodynamics [1]. List of Nobel Prize laureates in Physics from 1901 to the present day. ...


NASA's Spitzer Space Telescope has recently spotted what may be comet dust sprinkled around the white dwarf star G29-38, which died approximately 500 million years ago. The findings suggest the dead star, which most likely consumed its inner planets, is still orbited by a ring of surviving comets and possibly outer planets. This is the first observational evidence that comets can outlive their stars. The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility [SIRTF]) is an infrared space observatory, the fourth and final of NASAs Great Observatories. ... Introductory Material Comet dust refers to cosmic dust that originates from a comet. ... G29-38 (Giclas 29-38, ZZ Psc, WD 2326+049, EG 159, LTT 16907) is a large-amplitude DAV (ZZ Ceti) pulsator White Dwarf with variability first reported by Shulov & Kopatskaya in 1974. ... In the solar system the inner planets are the solid planets nearest the Sun: Mercury, Venus, Earth and Mars. ... Two bodies with a slight difference in mass orbiting around a common barycenter. ... An outer planet of the Solar system is any of the gas giants: Jupiter, Saturn, Uranus or Neptune. ...


In 2004 a team of researchers from Harvard-Smithsonian Center for Astrophysics led by Travis Metcalfe proposed that the carbon interior of white dwarf BPM 37093 may have solidified to form an enormous diamond.[5] The Harvard-Smithsonian Center for Astrophysics (CfA) is located in Cambridge, Massachusetts. ... BPM 37093 is a white dwarf star 50 light-years from Earth, in the constellation Centaurus, for which enough evidence has been gathered to infer that it consists of crystalline carbon, confirming previous theoretical predictions. ...


References

  1. ^ Liebert, J.; Bergeron, P.; Eisenstein, D.; Harris, H. C.; Kleinman, S. J.; Nitta, A.; Krzesinski, J. (2004). "A Helium White Dwarf of Extremely Low Mass". The Astrophysical Journal 606 (2): L147-L149. Retrieved on 2007-03-05. 
  2. ^ Henry, Todd J. (October 1, 2006). The One Hundred nearest Star Systems. RECONS. Retrieved on 2007-03-05.
  3. ^ Comet clash kicks up dusty haze BBC News , 13 February 2007
  4. ^ Fricke, W. (1985). "Friedrich Wilhelm Bessel (1784-1846). In Honor of the 200th Anniversary of Bessel's Birth". Astrophysics and Space Science 110: 11-19. Retrieved on 2007-02-04. 
  5. ^ Dr David Whitehouse (2004). Diamond star thrills astronomers (html). BBC NEWS. Retrieved on 2007-01-06.

2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the Anno Domini (common) era. ... March 5 is the 64th day of the year in the Gregorian Calendar (65th in leap years). ... 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the Anno Domini (common) era. ... March 5 is the 64th day of the year in the Gregorian Calendar (65th in leap years). ... 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the Anno Domini (common) era. ... February 4 is the 35th day of the year in the Gregorian Calendar. ... 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the Anno Domini (common) era. ... January 6 is the 6th day of the year in the Gregorian calendar, with 359 days (360 in leap years) remaining. ...

External links

See also

A Degenerate dwarf is type of star, an alternative name for what is commonly called a White dwarf (see this reference for a more complete article). ... A black dwarf is a hypothetical astronomical object: a white dwarf so old that it has cooled down so that it no longer emits significant heat or light. ... Degenerate matter is matter which has sufficiently high density that the dominant contribution to its pressure arises from the Pauli exclusion principle. ... Degenerate matter is matter which has sufficiently high density that the dominant contribution to its pressure arises from the Pauli exclusion principle. ... This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. ... This article is becoming very long. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. ... Timeline of white dwarfs, neutron stars, and supernovae Note that this list is mainly about the development of knowledge, but also about some supernovae taking place. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ...


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