Alpha Cassiopeiae
Encyclopedia
Alpha Cassiopeiae (α Cas, α Cassiopeiae ) is a second magnitude star in the constellation
Cassiopeia
, with the traditional name Schedar, sometimes spelt Shedir. Though listed as the alpha star by Johann Bayer
, Schedar's visual brightness closely rivals the beta star in the constellation, Caph
. Depending on which passband
is used, α Cas may be marginally brighter or dimmer than β Cas. However, recent calculations from the All-Sky catalog indicate that Schedar is the brightest in Cassiopeia, with a visual magnitude of 2.240. With Schedar located farther away from Earth than Caph, at 228 light years versus 54, Schedar is almost 18 times brighter that its rival insofar as intrinsic brightness
is concerned.
. The star is yet discernible to most observers across the globe reaching as far south as Perth, Australia
, Santiago, Chile
and other settlements north ± 33° South latitude, albeit just above the horizon. Schedar is located in line-of-sight of the Milky Way
galaxy
, so there are other notable celestial objects one can view close to this star—the Pacman Nebula, NGC 436 and NGC 457
being a few.
Alpha Cas reaches its zenith
above cities like Edinburgh
, Copenhagen
and Moscow
. It is circumpolar
throughout Europe, Russia, and as far south as Los Angeles, California on the North American continent as well as other locations around the globe having a latitude greater than ± 33° North. Since Schedar is listed as a second magnitude star, it is easily observable to the naked eye as long as one's stargazing is not hindered by the light pollution
common to most cities.
The best time for your observation is during the late autumn months of the northern hemisphere, when Cassiopeia passes the meridian at midnight, but given its circumpolar nature in many northern localities, it is visible to many of the world's inhabitants throughout the year.
used, Schedar's apparent magnitude is marginally brighter or dimmer than Caph, the beta star in the constellation. The Johnson UBV system
throughout the latter half of the twentieth century indicated a visual magnitude that ranged from about 2.20 to 2.23, thus making it slightly brighter than Caph with its variable magnitude ranging from 2.25 to 2.31. However recent calculations in the optical V band between 500—600 nanometers (nm) by Floor Van Leeuwen in 2007 produced a new measurement for both stars, 2.4107 for Schedar and 2.3579 for Caph, suggesting that Caph ranks as the brightest in the constellation. The confusion arises because of the use of different passband
s.
in the 1990s, Schedar's angular diameter
was measured in 1998 at various wavelength
s ranging from 500—850 nm. The result was a limb darkened
angular measurement of 5.62 ± 0.06 milliarcseconds (mas), a diameter which equates to roughly 0.393 AU
or 42.3R☉, assuming a parallax of 14.29 mas. With the planet
Mercury orbiting the Sun at approximately 0.4 AU
, Schedar's photosphere
extends to roughly half the mercurial
orbit.
whose spectral classification in the SIMBAD
astronomical database is listed as K0IIIa, a stellar class that is notably cooler than the Sun
. However, because the orange giant is nearing the final stages of its evolution
, the photosphere has expanded substantially, yielding a bolometric
luminosity
that is approximately 676L☉
According to Hipparcos
, the New Reduction (van Leeuwen, 2007), the estimated distance to the star is about 70 parsec
s or 228 light years. Like all giant stars, Schedar rotates slowly with an approximate velocity of 21 km/s—a speed which takes the star approximately 102 days to make one complete revolution on its axis.
Schedar has been sometimes classified as a variable star
, but no variability has been detected since the 19th century. Also, three companions to the star have been listed in the Washington Double Star Catalog
, but it seems that all of them are just line-of-sight optical components
.
of the thirteenth century. The term comes from the Arabic word صدر şadr, meaning "breast", a word which is derived from its relative position in the heart of the mythological queen Cassiopeia
. Johannes Hevelius
used the name "Schedir" in his writings, although there are additional spellings of this Arabic transliteration
such as Shedar, Shadar, Sheder, Seder, Shedis, and Shedir. Ulug Beg named the star "Al Dhāt al Kursiyy" (Arabic ألضة ألكرسي, meaning "the lady in the chair"), which Giovanni Battista Riccioli
changed to "Dath Elkarti".
In Chinese, refers to the Chinese asterism
Wang Liang
, a famous charioteer during the Spring and Autumn Period. The stellar pattern consists of α Cassiopeiae, β Cassiopeiae
, κ Cassiopeiae
, η Cassiopeiae
and λ Cassiopeiae
. Consequently, α Cassiopeiae itself is known as
, a Flemish
cartographer, produced a celestial globe portraying the 48 traditional Ptolemaic
constellations in addition to two others, Coma Berenices
and Antinous
. On this globe, he represents Cassiopeia as the Queen of Ethiopia, punished for her boasting by being chained to a chair hanging upside-down. Schedar is found near her left breast, reflecting its Arabic etymological origin.
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Cassiopeia
Cassiopeia (constellation)
Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivalled beauty. Cassiopea was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today...
, with the traditional name Schedar, sometimes spelt Shedir. Though listed as the alpha star by Johann Bayer
Johann Bayer
Johann Bayer was a German lawyer and uranographer . He was born in Rain, Bavaria, in 1572. He began his study of philosophy in Ingolstadt in 1592, and moved later to Augsburg to begin work as a lawyer. He grew interested in astronomy during his time in Augsburg...
, Schedar's visual brightness closely rivals the beta star in the constellation, Caph
Beta Cassiopeiae
Beta Cassiopeiae is a Delta Scuti variable star in the constellation Cassiopeia. It is a subgiant or giant star belonging to the spectral class F2, with a mean apparent magnitude of +2.27 and absolute magnitude of +1.16...
. Depending on which passband
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
is used, α Cas may be marginally brighter or dimmer than β Cas. However, recent calculations from the All-Sky catalog indicate that Schedar is the brightest in Cassiopeia, with a visual magnitude of 2.240. With Schedar located farther away from Earth than Caph, at 228 light years versus 54, Schedar is almost 18 times brighter that its rival insofar as intrinsic brightness
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
is concerned.
Visibility
With a declination of 56° 32' North, Schedar is principally visible in the northern hemisphereNorthern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
. The star is yet discernible to most observers across the globe reaching as far south as Perth, Australia
Perth, Western Australia
Perth is the capital and largest city of the Australian state of Western Australia and the fourth most populous city in Australia. The Perth metropolitan area has an estimated population of almost 1,700,000....
, Santiago, Chile
Santiago, Chile
Santiago , also known as Santiago de Chile, is the capital and largest city of Chile, and the center of its largest conurbation . It is located in the country's central valley, at an elevation of above mean sea level...
and other settlements north ± 33° South latitude, albeit just above the horizon. Schedar is located in line-of-sight of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
, so there are other notable celestial objects one can view close to this star—the Pacman Nebula, NGC 436 and NGC 457
NGC 457
NGC 457 is an open star cluster in the constellation Cassiopeia. It was discovered by Friedrich Wilhelm Herschel in 1787. and lies over 7,900 light years away from the Sun. It has an estimated age of 21 million years...
being a few.
Alpha Cas reaches its zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
above cities like Edinburgh
Edinburgh
Edinburgh is the capital city of Scotland, the second largest city in Scotland, and the eighth most populous in the United Kingdom. The City of Edinburgh Council governs one of Scotland's 32 local government council areas. The council area includes urban Edinburgh and a rural area...
, Copenhagen
Copenhagen
Copenhagen is the capital and largest city of Denmark, with an urban population of 1,199,224 and a metropolitan population of 1,930,260 . With the completion of the transnational Øresund Bridge in 2000, Copenhagen has become the centre of the increasingly integrating Øresund Region...
and Moscow
Moscow
Moscow is the capital, the most populous city, and the most populous federal subject of Russia. The city is a major political, economic, cultural, scientific, religious, financial, educational, and transportation centre of Russia and the continent...
. It is circumpolar
Circumpolar star
A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets , due to its proximity to one of the celestial poles...
throughout Europe, Russia, and as far south as Los Angeles, California on the North American continent as well as other locations around the globe having a latitude greater than ± 33° North. Since Schedar is listed as a second magnitude star, it is easily observable to the naked eye as long as one's stargazing is not hindered by the light pollution
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
common to most cities.
The best time for your observation is during the late autumn months of the northern hemisphere, when Cassiopeia passes the meridian at midnight, but given its circumpolar nature in many northern localities, it is visible to many of the world's inhabitants throughout the year.
Photometric variations
Depending on the photometric systemPhotometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...
used, Schedar's apparent magnitude is marginally brighter or dimmer than Caph, the beta star in the constellation. The Johnson UBV system
UBV photometric system
UBV photometric system, also called the Johnson system , is a wide band photometric system for classifying stars according to their colors. It is the first known standardized photoelectric photometric system. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are...
throughout the latter half of the twentieth century indicated a visual magnitude that ranged from about 2.20 to 2.23, thus making it slightly brighter than Caph with its variable magnitude ranging from 2.25 to 2.31. However recent calculations in the optical V band between 500—600 nanometers (nm) by Floor Van Leeuwen in 2007 produced a new measurement for both stars, 2.4107 for Schedar and 2.3579 for Caph, suggesting that Caph ranks as the brightest in the constellation. The confusion arises because of the use of different passband
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
s.
Angular analysis
With the advances in optical interferometryOptical interferometry
Optical interferometry combines two or more light waves in an opticalinstrument in such a way that interference occurs between them.Early interferometers used white light sources and also monochromatic light from atomic sources...
in the 1990s, Schedar's angular diameter
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
was measured in 1998 at various wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s ranging from 500—850 nm. The result was a limb darkened
Limb darkening
Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or "limb" of the image...
angular measurement of 5.62 ± 0.06 milliarcseconds (mas), a diameter which equates to roughly 0.393 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
or 42.3R☉, assuming a parallax of 14.29 mas. With the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
Mercury orbiting the Sun at approximately 0.4 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
, Schedar's photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
extends to roughly half the mercurial
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
orbit.
Properties
Schedar is an orange giant starGiant star
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...
whose spectral classification in the SIMBAD
SIMBAD
SIMBAD is an astronomical database of objects beyond the Solar System...
astronomical database is listed as K0IIIa, a stellar class that is notably cooler than the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. However, because the orange giant is nearing the final stages of its evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
, the photosphere has expanded substantially, yielding a bolometric
Bolometer
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley...
luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
that is approximately 676L☉
According to Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
, the New Reduction (van Leeuwen, 2007), the estimated distance to the star is about 70 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s or 228 light years. Like all giant stars, Schedar rotates slowly with an approximate velocity of 21 km/s—a speed which takes the star approximately 102 days to make one complete revolution on its axis.
Schedar has been sometimes classified as a variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
, but no variability has been detected since the 19th century. Also, three companions to the star have been listed in the Washington Double Star Catalog
Washington Double Star Catalog
The Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory. The catalog contains positions, magnitudes, proper motions and spectral types and has entries for 102,387 pairs of double stars. The catalog also includes multiple stars...
, but it seems that all of them are just line-of-sight optical components
Double star
In observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e...
.
Ethnological influences
Etymology
The traditional name, Schedar, was first encountered in the Alfonsine tablesAlfonsine tables
The Alfonsine tables provided data for computing the position of the Sun, Moon and planets relative to the fixed stars....
of the thirteenth century. The term comes from the Arabic word صدر şadr, meaning "breast", a word which is derived from its relative position in the heart of the mythological queen Cassiopeia
Cassiopeia (mythology)
Cassiopeia is the name of several figures in Greek mythology.-Wife of Cepheus:The Queen Cassiopeia, wife of king Cepheus of Æthiopia, was beautiful but also arrogant and vain; these latter two characteristics led to her downfall....
. Johannes Hevelius
Johannes Hevelius
Johannes Hevelius Some sources refer to Hevelius as Polish:Some sources refer to Hevelius as German:*Encyplopedia Britannica * of the Royal Society was a councilor and mayor of Danzig , Pomeranian Voivodeship, in the Polish-Lithuanian Commonwealth...
used the name "Schedir" in his writings, although there are additional spellings of this Arabic transliteration
Transliteration
Transliteration is a subset of the science of hermeneutics. It is a form of translation, and is the practice of converting a text from one script into another...
such as Shedar, Shadar, Sheder, Seder, Shedis, and Shedir. Ulug Beg named the star "Al Dhāt al Kursiyy" (Arabic ألضة ألكرسي, meaning "the lady in the chair"), which Giovanni Battista Riccioli
Giovanni Battista Riccioli
Giovanni Battista Riccioli was an Italian astronomer and a Catholic priest in the Jesuit order...
changed to "Dath Elkarti".
In Chinese, refers to the Chinese asterism
Asterism (astronomy)
In astronomy, an asterism is a pattern of stars recognized on Earth's night sky. It may form part of an official constellation, or be composed of stars from more than one. Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction,...
Wang Liang
Legs (Chinese constellation)
The Legs mansion is one of the Twenty-eight mansions of the Chinese constellations. It is one of the western mansions of the White Tiger.-Asterisms:...
, a famous charioteer during the Spring and Autumn Period. The stellar pattern consists of α Cassiopeiae, β Cassiopeiae
Beta Cassiopeiae
Beta Cassiopeiae is a Delta Scuti variable star in the constellation Cassiopeia. It is a subgiant or giant star belonging to the spectral class F2, with a mean apparent magnitude of +2.27 and absolute magnitude of +1.16...
, κ Cassiopeiae
Kappa Cassiopeiae
Kappa Cassiopeiae is a star in the constellation Cassiopeia.κ Cassiopeiae is a blue-white B-type supergiant with a mean apparent magnitude of +4.17. It is approximately 4100 light years from Earth. It is classified as an Alpha Cygni type variable star and its brightness varies from magnitude...
, η Cassiopeiae
Eta Cassiopeiae
Eta Cassiopeiae is a star system 19.4 light years away from Earth, in the constellation Cassiopeia. Sometimes the traditional name Achird is used....
and λ Cassiopeiae
Lambda Cassiopeiae
Lambda Cassiopeiae is a binary star in the constellation Cassiopeia. The system has a combined apparent magnitude of +4.74 and is approximately 355 light years from Earth....
. Consequently, α Cassiopeiae itself is known as
Legacy
In 1551, Gerardus MercatorGerardus Mercator
thumb|right|200px|Gerardus MercatorGerardus Mercator was a cartographer, born in Rupelmonde in the Hapsburg County of Flanders, part of the Holy Roman Empire. He is remembered for the Mercator projection world map, which is named after him...
, a Flemish
Flanders
Flanders is the community of the Flemings but also one of the institutions in Belgium, and a geographical region located in parts of present-day Belgium, France and the Netherlands. "Flanders" can also refer to the northern part of Belgium that contains Brussels, Bruges, Ghent and Antwerp...
cartographer, produced a celestial globe portraying the 48 traditional Ptolemaic
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...
constellations in addition to two others, Coma Berenices
Coma Berenices
Coma Berenices is a traditional asterism that has since been defined as one of the 88 modern constellations. It is located near Leo, to which it formerly belonged, and accommodates the North Galactic Pole...
and Antinous
Antinous (constellation)
Antinous is an obsolete constellation no longer in use by astronomers, having been merged into Aquila, which it bordered to the North.The constellation was created by the emperor Hadrian in 132. According to legend, Hadrian was told by an oracle that only death of his most beloved person would save...
. On this globe, he represents Cassiopeia as the Queen of Ethiopia, punished for her boasting by being chained to a chair hanging upside-down. Schedar is found near her left breast, reflecting its Arabic etymological origin.
See also
- List of stars in Cassiopeia
- Class K Stars
- Angular diameterAngular diameterThe angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
- BetelgeuseBetelgeuseBetelgeuse, also known by its Bayer designation Alpha Orionis , is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel only rarely...
for an in-depth discussion red giants.
External links
- Jim Kaler's Stars, University of Illinois:SHEDAR (Alpha Cassiopiae)
- Harvard Map Collection The Mercator Globes
- The Internet Encyclopedia of Science: Shedar (Alpha Cassiopeiae)