Gamma Cephei (γ Cephei, abbreviated Gamma Cep, γ Cep) is a

binary star A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

system approximately 45

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s away in the northern

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

of Cepheus. The primary (designated Gamma Cephei A, officially named Errai , the traditional name of the system) is a stellar class K1 orange

giant In folklore, giants (from Ancient Greek: ''wiktionary:gigas, gigas'', cognate wiktionary:giga-, giga-) are beings of humanoid appearance, but are at times prodigious in size and strength or bear an otherwise notable appearance. The word ''gia ...

subgiant A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...

star; it has a

red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

companion (Gamma Cephei B). An

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

(designated

Gamma Cephei Ab Gamma Cephei Ab (abbreviated γ Cephei Ab, γ Cep Ab), formally named Tadmor , is an exoplanet approximately 45 light-years away in the constellation of Cepheus that orbits Gamma Cephei A. The planet was suspected to exist in 1988, and later c ...

, later named Tadmor) has been confirmed to be orbiting the primary. Gamma Cephei is the naked-eye star that will succeed

Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris (Latinisation of names, Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an ...

as the Earth's northern

pole star A pole star is a visible star that is approximately aligned with the axis of rotation of an astronomical body; that is, a star whose apparent position is close to one of the celestial poles. On Earth, a pole star would lie directly overhead when ...

, due to

axial precession In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In parti ...

. It will be closer to the northern

celestial pole The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at ...

than Polaris around 3160 CE and will make its closest approach around 4250 CE. The 'title' will pass to Iota Cephei some time around 5200 CE.

Description

Gamma Cephei has an

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

of 3.21, nearly all of which is accounted for by the primary component, Gamma Cephei A. It is a binary star system with an

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

of 66.8 years and an

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

(ovalness) of 0.14. The pair are orbiting at a separation of . The primary is about 5.7 billion years old and has evolved off the

main sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...

, having fused all of the hydrogen in its core. It has 1.3 times the mass of the Sun and has expanded to 4.7 times the Sun's radius. The star is radiating nearly 12 times the luminosity of the Sun from its

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

at an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of 4,806 K. The secondary component, Gamma Cephei B, has a mass approximately 0.40 times that of the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

. It is probably a

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

M4, 6.2 magnitudes fainter than the primary. It is assumed to be of similar age to its primary. The

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

of this star has served as one of the stable anchor points by which other stars are classified. It was listed as a standard star for the spectral class K1 IV in 1943, 1953 and 1973. However, in 1989, it was given as a spectral standard for K1 III-IV. Its spectrum is notable for the strength of the

cyano radical The cyano radical (or cyanido radical) is a radical with molecular formula CN, sometimes written •CN. The cyano radical was one of the first detected molecules in the interstellar medium, in 1938. Its detection and analysis was influential in as ...

(CN) bands. Analysis of the spectrum in 2018 gave a best match for a spectral type of K1 III. γ Cephei is catalogued as a suspected

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

with a brightness range between magnitudes 3.18 and 3.24, based on its inclusion in an 1884 list of suspected variable stars.

Nomenclature

''γ Cephei'' ( Latinised to ''Gamma Cephei'') is the system's

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

. Under the rules for naming objects in multiple star systems the two components are designated A and B. Following its discovery the planet was designated Gamma Cephei Ab. The system bore a traditional name variously spelled as ''Errai'', ''Er Rai'' or ''Alrai'', deriving from the

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

الراعي (ar-rā‘ī), meaning 'the shepherd'. (The star Beta Ophiuchi is sometimes also called Alrai, but it is more commonly known as ''Cebalrai'' or ''Kelb Alrai'', meaning 'shepherd's dog'.) In 2016, the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included ''Errai'' for Gamma Cephei A. In July 2014 the

launched

NameExoWorlds NameExoWorlds (also known as IAU NameExoWorlds) were various projects managed by the International Astronomical Union (I.A.U.) to encourage names to be submitted for astronomical objects, notably exoplanets. The accepted names would later be cons ...

, a process for giving proper names to certain exoplanets. The process involved public nomination and voting for the new names. In December 2015, the IAU announced the winning name was Tadmor for this planet. It was submitted by the

Syria Syria, officially the Syrian Arab Republic, is a country in West Asia located in the Eastern Mediterranean and the Levant. It borders the Mediterranean Sea to the west, Turkey to Syria–Turkey border, the north, Iraq to Iraq–Syria border, t ...

n Astronomical Association and is the ancient Semitic name and modern Arabic name for the city of

Palmyra Palmyra ( ; Palmyrene dialect, Palmyrene: (), romanized: ''Tadmor''; ) is an ancient city in central Syria. It is located in the eastern part of the Levant, and archaeological finds date back to the Neolithic period, and documents first menti ...

, a

World Heritage Site World Heritage Sites are landmarks and areas with legal protection under an treaty, international treaty administered by UNESCO for having cultural, historical, or scientific significance. The sites are judged to contain "cultural and natural ...

. In Chinese, the star is named (Shàowèi Zēng Bā, literally, the 8th added star of the '' Xingguan Shaowei'', ''Shaowei'': the Minor Guard) belonging to the Left Wall of the

Purple Forbidden enclosure The Purple Forbidden enclosure ( Zǐ wēi yuán) is one of the San Yuan ( Sān yuán) or Three Enclosures. Stars and constellations of this group lie near the north celestial pole and are visible all year from temperate latitudes in the Northern ...

(, ), which refers to an asterism consisting of Gamma Cephei,

Iota Draconis Iota Draconis (ι Draconis, abbreviated Iota Dra, ι Dra), also named Edasich , is a star in the northern circumpolar constellation of Draco. A visually unremarkable star of apparent magnitude 3.3, in 2002 it was discovered to have a ...

Theta Draconis Theta Draconis, a name Latinized from θ Draconis, is a binary star system in the northern circumpolar constellation of Draco. It is faintly visible to the naked eye at night with an apparent visual magnitude of 4.12. Parallax measu ...

Eta Draconis Eta Draconis (η Draconis, abbreviated Eta Dra, η Dra) is a binary star in the northern circumpolar constellation of Draco. Despite having an apparent visual magnitude of only +2.73, it is the second-brightest star in this generally f ...

Zeta Draconis Zeta Draconis (ζ Draconis, abbreviated Zet Dra, ζ Dra) is a binary star in the northern circumpolar constellation of Draco. With an apparent visual magnitude of +3.17, it is the fifth-brightest member of this generally faint constellat ...

, Upsilon Draconis,

73 Draconis 73 may refer to: * 73 (number) * One of the years 73 BC, AD 73, 1973, 2073 * 73 (magazine), ''73'' (magazine), a United States-based amateur radio magazine * 73 Best regards, a popular Morse code abbreviations, Morse code abbreviation * ''No. 73'', ...

and 23 Cassiopeiae.

Planetary system

A planet orbiting Gamma Cephei A with a 2.7-year period was discovered in 1988. Its existence was also announced in 1989. This was the first-discovered extrasolar planet and its ostensible discovery was based on the same

radial velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

technique later used successfully by others. However, the claim was challenged in 1992 by a paper which favoured K-giant variability with a period equal to the stellar rotation, but in 2002, the existence of a planet with an orbital period of about 2.5 years was confirmed. The secondary star B orbits A at only 9.8 times the

semimajor axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...

of A's planet. The orbit of the planet is roughly perpendicular to the orbit of the binary. Dynamical modelling suggests that the Kozai–Lidov mechanism is at play here.

References

External links

* http://jumk.de/astronomie/exoplanets/errai.shtml
''McDonald Observatory'': Planet Search finds first planet orbiting close-in binary star

{{DEFAULTSORT:Gamma Cephei K-type subgiants M-type main-sequence stars Emission-line stars Binary stars Planetary systems with one confirmed planet Northern pole stars Errai

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BD+76 0928 Cephei, 35 116727 222404