Epsilon Cassiopeiae or ε Cassiopeiae, officially named Segin (), is a single

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

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 ...

Cassiopeia Cassiopeia or Cassiopea may refer to: Greek mythology * Cassiopeia (mother of Andromeda), queen of Aethiopia and mother of Andromeda * Cassiopeia (wife of Phoenix), wife of Phoenix, king of Phoenicia * Cassiopeia, wife of Epaphus, king of Egy ...

. With an

apparent visual magnitude Apparent magnitude () is a measure of the 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 of the object's light ca ...

of 3.4, this is one of the brightest stars in the constellation. The distance to this star has been determined directly using

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

measurements, yielding a value of around from 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 drifting closer with a

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 ...

of −8 km/s.

Nomenclature

''ε Cassiopeiae'', Latinised to ''Epsilon Cassiopeiae'', is the star's Bayer catalog designation. The star bore the traditional name ''Segin'', which probably originates from an erroneous transcription of ''Seginus'', the traditional name for

Gamma Boötis Gamma Boötis is a binary star system in the northern constellation of Boötes the herdsman, forming the left shoulder of this asterism. Its name is a Bayer designation that is Latinised from γ Boötis, and abbreviated Gamma Boo or � ...

, which itself is of uncertain origin. Different sources report varying pronunciations, with SEG-in the most common but the variants SAY-gin and seg-EEN also appearing. In 2016, the IAU 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 approved the name ''Segin'' for this star on 5 September 2017 and it is now so included in the List of IAU-approved Star Names. While some published reports incorrectly claim that this star was designated by NASA as ''Navi'' ('Ivan', backwards), in honor of astronaut Virgil Ivan "Gus" Grissom, one of the three astronauts who died in the

Apollo 1 Apollo 1, initially designated AS-204, was planned to be the first crewed mission of the Apollo program, the American undertaking to land the first man on the Moon. It was planned to launch on February 21, 1967, as the first low Earth orbital ...

accident, the actual star so designated is

Gamma Cassiopeiae Gamma Cassiopeiae, Latinized from γ Cassiopeiae, is a bright star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest st ...

in the center of the constellation. In Chinese, (), meaning '' Flying Corridor'', refers to an asterism consisting of Epsilon Cassiopeiae, Iota Cassiopeiae, Delta Cassiopeiae,

Theta Cassiopeiae Theta Cassiopeiae or θ Cassiopeiae is a solitary star in the northern constellation of Cassiopeia (constellation), Cassiopeia. It shares the traditional name Marfak with μ Cassiopeiae, positioned less than half a degree to the WSW, w ...

, Nu Cassiopeiae and

Omicron Cassiopeiae Omicron Cassiopeiae (ο Cas, ο Cassiopeiae) is a triple star system in the constellation Cassiopeia (constellation), Cassiopeia. It is approximately 700 light-years from Earth, based on its stellar parallax, parallax. It is visible to t ...

. Consequently, the

Chinese name Chinese may refer to: * Something related to China * Chinese people, people identified with China, through nationality, citizenship, and/or ethnicity **Han Chinese, East Asian ethnic group native to China. **'' Zhonghua minzu'', the supra-ethni ...

for Epsilon Cassiopeiae itself is (, .)

Properties

Epsilon Cassiopeiae has a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

of B3 V, indicating that it is a

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 ...

star fusing hydrogen in its core. Cote ''et al.'' (2003) indicate that it displays the spectral properties of a

Be star Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer ...

, even though it is not categorized as such. The presence of

emission lines A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...

in the spectrum indicates the presence of a circumstellar shell of gas that has been thrown off by the star. Epsilon Cassiopeiae has nine times the mass and six times the radius of the Sun. It is radiating 3,000 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 , giving it the blue-white hue of a B-type star. Observation during the

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

mission suggest that the star may undergo weak periodic variability. The amplitude of this variation is 0.0025 in magnitude with a frequency of 11.17797 times per day, or one cycle every 2.15 hours. The

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...

for this measured variation is 4.978. Hipparcos measurements of the space velocity components for this star suggest that it is a member of the Cas-Tau group of co-moving stars, with a 93% likelihood. This group may be kinematically associated with the

alpha Persei Cluster The Alpha Persei Cluster, also known as Melotte 20 or Collinder 39, is an open cluster of stars in the northern constellation of Perseus (constellation), Perseus. To the naked eye, the cluster consists of several blue-hued spectral type B stars. ...

, indicating that the Cas-Tau group, including Epsilon Cassiopeiae, may have been separated from the cluster through tidal interactions.

References

External links

* {{Stars of Cassiopeia B-type main-sequence stars Be stars Suspected variables Cassiopeia (constellation) Cassiopeiae, Epsilon Durchmusterung objects Cassiopeiae, 45 011415 008886 0542 Segin