Van Maanen 2
Van Maanen 2 is a white dwarf. It is a dense, compact stellar remnant, no longer generating energy, having about 68% of the Sun's mass but only 1% of the Sun's radius. Out of the white dwarfs known, it is, at 13.9 light-years, the third closest to the Sun, after Sirius B and Procyon B, in that order, the closest known solitary white dwarf. Discovered in 1917 by Dutch–American astronomer Adriaan van Maanen, Van Maanen 2 was the third white dwarf identified, after 40 Eridani B and Sirius B, the first, not a member of a multi-star system. A spectrographic plate made in 1917 shows evidence of planetary matter around the star. While searching for a companion to the large-proper-motion star Lalande 1299, in 1917 Dutch–American astronomer Adriaan van Maanen discovered a star with an larger proper motion located a few arcminutes to the northeast, he estimated the annual proper motion of the latter as 3 arcseconds. This star had been recorded on a plate taken November 11, 1896 for the Carte du Ciel Catalog of Toulouse, it showed an apparent magnitude of 12.3.
The initial spectral classification was type F0. In 1918, American astronomer Frederick Seares obtained a refined visual magnitude of 12.34, but the distance to the star remained unknown. Two years van Maanen published a parallax estimate of 0.246″, giving it an absolute magnitude of +14.8. This made it the faintest F-type star known at that time. In 1923, Dutch-American astronomer Willem Luyten published a study of stars with large proper motions in which he identified what he called "van Maanen's star" as one of only three known white dwarfs, a term he coined; these are stars that have an unusually low absolute magnitude for their spectral class, lying well below the main sequence on the Hertzsprung–Russell diagram of stellar temperature vs. luminosity. The high mass density of white dwarfs was demonstrated in 1925 by American astronomer Walter Adams when he measured the gravitational redshift of Sirius B as 21 km/s. In 1926, British astrophysicist Ralph Fowler used the new theory of quantum mechanics to show that these stars are supported by electron gas in a degenerate state.
British astrophysicist Leon Mestel demonstrated in 1952 that the energy emitted by a white dwarf is the surviving heat from a prior period of nuclear fusion. He showed that nuclear burning no longer occurs within a white dwarf, calculated the internal temperature of van Maanen 2 as 6 × 106 K, he gave a preliminary age estimate of 1011/A years, where A is the mean atomic weight of the nuclei in the star. In 2016, it was discovered that a spectrographic plate of van Maanen 2 made in 1917 has evidence – the earliest known – of planetary matter outside the solar system. No actual planet has been detected, but the plate reveals the existence of a circumstellar ring of debris, such rings in other cases have been associated with planets. Van Maanen 2 is located 13.9 light-years from the Sun in the constellation Pisces, about 2° to the south of the star Delta Piscium, with a high proper motion of 2.978″ annually along a position angle of 155.538°. It is too faint to be seen with the naked eye. Like other white dwarfs, it is a dense star: its mass has been estimated to be about 68% of the Sun's, yet it has only 1% of the Sun's radius.
The outer atmosphere has a temperature of 6,220 K, cool for a white dwarf. As all white dwarfs radiate away their heat over time, this temperature can be used to estimate its age, thought to be around 3 billion years; the progenitor of this white dwarf had an estimated 2.6 solar masses and remained on the main sequence for about 9 × 108 years. This gives the star a combined age of about 4.1 billion years. When this star left the main sequence, it expanded into a red giant that reached a maximum radius of 650 times the current radius of the Sun, or about 3 astronomical units. Any planets that were orbiting within this radius would have interacted directly with the star's extended envelope; the stellar classification of Van Maanen 2 is DZ8, where the DZ prefix indicates the presence of elements heavier than helium in its spectrum—what astronomers term metals. Indeed, this star is the prototype for white dwarfs of this class. Based upon physical models of white dwarfs, elements with mass greater than helium should sink below the photosphere of the star, leaving only hydrogen and helium to be visible in the spectrum.
Hence, for heavier elements to appear, there must have been an external source. It is unlikely. Instead, the surface of the star was polluted by circumstellar material, such as by the remains of a rocky, terrestrial planet; the total mass of metals in the atmosphere of Van Maanen 2 is estimated to be around 1021 g—about the same mass as a large moon such as Ariel. These pollutants will sink deeper into the atmosphere on time scales of around three million years, which indicates the material is being replenished at a rate of 107 g/s; these materials could have been accreted in the form of multiple planetesimals smaller than around 84 km colliding with the star. White dwarfs with a spectrum that indicates high levels of metal contamination possess a circumstellar disk. In the case of van Maanen 2, observations of the star at a wavelength of 24 μm do not show the infrared excess that might be generated by a dusty disk. Instead there is a noticeable deficit; the predicted flux at 24 μm is 0.23 mJy.
This deficit may be explained by collision-induced absorption in the atmosphere of the star. However, this is only known to happen with white dwarfs that have temperatures below 4,000 K, as a result of collisions between hydrogen molecules or between hydrogen molecules a
The apparent magnitude of an astronomical object is a number, a measure of its brightness as seen by an observer on Earth. The magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude value, with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry. Apparent magnitudes are used to quantify the brightness of sources at ultraviolet and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or simply as V, as in "mV = 15" or "V = 15" to describe a 15th-magnitude object; the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes.
The brightest stars in the night sky were said to be of first magnitude, whereas the faintest were of sixth magnitude, the limit of human visual perception. Each grade of magnitude was considered twice the brightness of the following grade, although that ratio was subjective as no photodetectors existed; this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is believed to have originated with Hipparchus. In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star, 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today; this implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio; the zero point of Pogson's scale was defined by assigning Polaris a magnitude of 2. Astronomers discovered that Polaris is variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess due to a circumstellar disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures. However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the magnitude scale was extrapolated to all wavelengths on the basis of the black-body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, as a function of wavelength, can be computed. Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.
With the modern magnitude systems, brightness over a wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30; the brightness of Vega is exceeded by four stars in the night sky at visible wavelengths as well as the bright planets Venus and Jupiter, these must be described by negative magnitudes. For example, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system; the most used is the AB magnitude system, in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be equal in the V filter band.
As the amount of light received by a telescope is reduced by transmission through the Earth's atmosphere, any measurement of apparent magnitude is corrected for what it would have been as seen from above the atmosphere. The dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of 100. Therefore, the apparent magnitude m, in the spectral band x, would be given by m x = − 5 log 100 , more expressed in terms of common logarithms as m x
NGC 520 is a pair of colliding spiral galaxies about 78 million light-years away in the constellation Pisces and were discovered by astronomer William Herschel on 13 December 1784. The object has an H II nucleus. Interacting galaxies List of NGC objects Pisces NGC 520, An Interacting pair of Spiral Galaxies NGC 520 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
In astronomy, declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the celestial equator, along the hour circle passing through the point in question; the root of the word declination means "a bending away" or "a bending down". It comes from the same root as the words recline. In some 18th and 19th century astronomical texts, declination is given as North Pole Distance, equivalent to 90 -. For instance an object marked as declination -5 would have a NPD of 95, a declination of -90 would have a NPD of 180. Declination in astronomy is comparable to geographic latitude, projected onto the celestial sphere, hour angle is comparable to longitude. Points north of the celestial equator have positive declinations, while those south have negative declinations. Any units of angular measure can be used for declination, but it is customarily measured in the degrees and seconds of sexagesimal measure, with 90° equivalent to a quarter circle.
Declinations with magnitudes greater than 90° do not occur, because the poles are the northernmost and southernmost points of the celestial sphere. An object at the celestial equator has a declination of 0° north celestial pole has a declination of +90° south celestial pole has a declination of −90°The sign is customarily included whether positive or negative; the Earth's axis rotates westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates are inherently relative to the year of their observation, astronomers specify them with reference to a particular year, known as an epoch. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch; the used standard epoch is J2000.0, January 1, 2000 at 12:00 TT. The prefix "J" indicates. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, B1950.0.
A star's direction remains nearly fixed due to its vast distance, but its right ascension and declination do change due to precession of the equinoxes and proper motion, cyclically due to annual parallax. The declinations of Solar System objects change rapidly compared to those of stars, due to orbital motion and close proximity; as seen from locations in the Earth's Northern Hemisphere, celestial objects with declinations greater than 90° − φ appear to circle daily around the celestial pole without dipping below the horizon, are therefore called circumpolar stars. This occurs in the Southern Hemisphere for objects with declinations less than −90° − φ. An extreme example is the pole star which has a declination near to +90°, so is circumpolar as seen from anywhere in the Northern Hemisphere except close to the equator. Circumpolar stars never dip below the horizon. Conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earth's surface. If a star whose declination is δ is circumpolar for some observer a star whose declination is −δ never rises above the horizon, as seen by the same observer.
If a star is circumpolar for an observer at latitude φ it never rises above the horizon as seen by an observer at latitude −φ. Neglecting atmospheric refraction, declination is always 0 ° at west points of the horizon. At the north point, it is 90° − |φ|, at the south point, −90° + |φ|. From the poles, declination is uniform around the entire horizon 0°. Non-circumpolar stars are visible only during certain seasons of the year; the Sun's declination varies with the seasons. As seen from arctic or antarctic latitudes, the Sun is circumpolar near the local summer solstice, leading to the phenomenon of it being above the horizon at midnight, called midnight sun. Near the local winter solstice, the Sun remains below the horizon all day, called polar night; when an object is directly overhead its declination is always within 0.01 degrees of the observer's latitude. The first complication applies to all celestial objects: the object's declination equals the observer's astronomic latitude, but the term "latitude" ordinarily means geodetic latitude, the latitude on maps and GPS devices.
In the continental United States and surrounding area, the difference is a few arcseconds but can be as great as 41 arcseconds. The second complication is that, assuming no deflection of the vertical, "overhead" means perpendicular to the ellipsoid at observer's location, but the perpendicular line does not pass through the center of the earth. For the moon this discrepancy can reach 0.003 degrees.
Delta Piscium is a solitary, orange-hued star in the zodiac constellation of Pisces. It has an apparent visual magnitude of +4.4, so it is bright enough to be faintly visible to the naked eye. Based upon an annual parallax shift of 10.5 mas, it is around 311 light-years from the Sun. The visual magnitude of the star is diminished by an interstellar absorption factor of 0.08 due to interstellar dust. This is an evolved K-type giant star with a stellar classification of K4 IIIb, it has around 1.65 times the mass of the Sun and, at the age of three billion years, has expanded to 44 times the Sun's radius. The star is radiating 447 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 3,963 K; because Delta Piscium is positioned near the ecliptic, so it is subject to lunar occultations. It has a magnitude 13.99 visual companion at an angular separation of 135.0 arc seconds on a position angle of 12°, as of 2011. In Chinese, 外屏, meaning Outer Fence, refers to an asterism of stars, δ Piscium, ε Piscium, ζ Piscium, μ Piscium, ν Piscium, ξ Piscium and α Piscium.
The Chinese name for δ Piscium itself is 外屏一 Kaler, James B. "DELTA PSC", STARS, University of Illinois, retrieved 2017-08-02
Andromeda is one of the 48 constellations listed by the 2nd-century Greco-Roman astronomer Ptolemy and remains one of the 88 modern constellations. Located north of the celestial equator, it is named for Andromeda, daughter of Cassiopeia, in the Greek myth, chained to a rock to be eaten by the sea monster Cetus. Andromeda is most prominent during autumn evenings in the Northern Hemisphere, along with several other constellations named for characters in the Perseus myth; because of its northern declination, Andromeda is visible only north of 40° south latitude. It is one of the largest constellations, with an area of 722 square degrees; this is over 1,400 times the size of the full moon, 55% of the size of the largest constellation and over 10 times the size of the smallest constellation, Crux. Its brightest star, Alpha Andromedae, is a binary star, counted as a part of Pegasus, while Gamma Andromedae is a colorful binary and a popular target for amateur astronomers. Only marginally dimmer than Alpha, Beta Andromedae is a red giant, its color visible to the naked eye.
The constellation's most obvious deep-sky object is the naked-eye Andromeda Galaxy, the closest spiral galaxy to the Milky Way and one of the brightest Messier objects. Several fainter galaxies, including M31's companions M110 and M32, as well as the more distant NGC 891, lie within Andromeda; the Blue Snowball Nebula, a planetary nebula, is visible in a telescope as a blue circular object. In Chinese astronomy, the stars that make up Andromeda were members of four different constellations that had astrological and mythological significance. Andromeda is the location of the radiant for the Andromedids, a weak meteor shower that occurs in November; the uranography of Andromeda has its roots most in the Greek tradition, though a female figure in Andromeda's location had appeared earlier in Babylonian astronomy. The stars that make up Pisces and the middle portion of modern Andromeda formed a constellation representing a fertility goddess, sometimes named as Anunitum or the Lady of the Heavens.
Andromeda is known as "the Chained Lady" or "the Chained Woman" in English. It was known as Mulier Catenata in al-Mar ` at al Musalsalah in Arabic, it has been called Persea or Cepheis, all names that refer to Andromeda's role in the Greco-Roman myth of Perseus, in which Cassiopeia, the queen of Ethiopia, bragged that her daughter was more beautiful than the Nereids, sea nymphs blessed with incredible beauty. Offended at her remark, the nymphs petitioned Poseidon to punish Cassiopeia for her insolence, which he did by commanding the sea monster Cetus to attack Ethiopia. Andromeda's panicked father, was told by the Oracle of Ammon that the only way to save his kingdom was to sacrifice his daughter to Cetus, she was chained to a rock by the sea but was saved by the hero Perseus, who in one version of the story used the head of Medusa to turn the monster into stone. Perseus and Andromeda married. After Andromeda's death Athena placed her in the sky as a constellation. Several of the neighboring constellations represent characters in the Perseus myth.
It is connected with the constellation Pegasus. Andromeda was one of the original 48 constellations formulated by Ptolemy in his 2nd-century Almagest, in which it was defined as a specific pattern of stars, she is depicted with α Andromedae as her head, ο and λ Andromedae as her chains, δ, π, μ, Β, γ Andromedae representing her body and legs. However, there is no universal depiction of Andromeda and the stars used to represent her body and chains. Arab astronomers were aware of Ptolemy's constellations, but they included a second constellation representing a fish at Andromeda's feet. Several stars from Andromeda and most of the stars in Lacerta were combined in 1787 by German astronomer Johann Bode to form Frederici Honores, it was designed to honor King Frederick II of Prussia, but fell into disuse. Since the time of Ptolemy, Andromeda has remained a constellation and is recognized by the International Astronomical Union, although like all modern constellations, it is now defined as a specific region of the sky that includes both Ptolemy's pattern and the surrounding stars.
In 1922, the IAU defined its recommended three-letter abbreviation, "And". The official boundaries of Andromeda were defined in 1930 by Eugène Delporte as a polygon of 36 segments, its right ascension is between 22h 57.5m and 2h 39.3m and its declination is between 53.19° and 21.68° in the equatorial coordinate system. In traditional Chinese astronomy, nine stars from Andromeda, along with seven stars from Pisces, formed an elliptical constellation called "Legs"; this constellation either represented the foot of a wild boar. Gamma Andromedae and its neighbors were called "Teen Ta Tseang Keun", representing honor in astrology and a great general in mythology. Alpha Andromedae and Gamma Pegasi together made "Wall", representing the eastern wall of the imperial palace and/or the emperor's personal library. For the Chinese, the northern swath of Andromeda formed a stable for changing horses and the fa