Phi Persei (Phi Per, φ Persei, φ Per) is a Class B2Vpe fourth-magnitude

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

in the

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 origins of the e ...

Perseus In Greek mythology, Perseus (Help:IPA/English, /ˈpɜːrsiəs, -sjuːs/; Greek language, Greek: Περσεύς, Romanization of Greek, translit. Perseús) is the legendary founder of Mycenae and of the Perseid dynasty. He was, alongside Cadmus ...

, location about 720

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s from

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

System

Phi Persei is

spectroscopic binary A binary star is a system of two star, stars that are gravity, 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 using a telescope as separa ...

consisting of a blue

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

primary of class B2 and a

hot subdwarf A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral ...

secondary. The two stars have an orbit of 217 days and are separated by about . Phi Persei is a

runaway star In astronomy, stellar kinematics is the Observational astronomy, observational study or measurement of the kinematics or motions of stars through space. Stellar kinematics encompasses the measurement of stellar Velocity, velocities in the Milky W ...

and extrapolating its space velocity backwards by the modelled age of the system (57 million years) places it within 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. To the naked eye, the cluster consists of several blue-hued spectral type B stars. The most luminous member ...

. The primary star rotates rapidly with a projected equatorial velocity of . Due to its rapid rotation, the primary star has a polar radius about and an equatorial radius of about . With 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 nearly , it has a

bolometric luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a s ...

nearly 15,000 times higher than the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. The rapidly-spinning star is surrounded by a

circumstellar disk A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the rese ...

. The binary orbit, the spin of the primary star, and the disk are all seen nearly edge-on. There are no eclipses, but models of the system show that the disk significantly obscures the primary star. The primary formed as a star and has accreted material from its companion. The secondary star is also a class O

subdwarf A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral ...

, hotter than the primary but smaller and less massive. It is proposed that it is the core of a star, with the outer layers stripped as it expanded away from the

. Its luminosity is higher than expected for a normal

helium star A helium star is a class O or B star (blue), which has extraordinarily strong helium lines and weaker than normal hydrogen lines, indicating strong stellar winds and a mass loss of the outer envelope. '' Extreme helium stars'' (EHe) entirely lack ...

, which suggests it may have evolved to helium shell burning. Although the subdwarf has a bolometric luminosity about half that of the primary, it emots most of its radiation as

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

, being only about 3% as bright as the primary at visible wavelengths.

Variability

Phi Persei is a

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

with both rapid and longterm variations in its brightness and spectrum. Variations occur on a daily timescale which may be related to the rotation of the primary star. Any variations originating with the secondary are difficult to detect due to its comparative faintness. Slower variations, including deep fades, are also seen. Some of the variations may correspond to the orbital period, possibly eclipses of gas streams or hot spots, but the occasional deep fades do not match any particular orbital phase. The variations are classified as γ Cassiopeiae-type,

shell star A shell star is a star having a spectrum that shows extremely broad absorption lines, plus some very narrow absorption lines. They typically also show some emission lines, usually from the Balmer series but occasionally of other lines. The broad ...

s with eruptions and irregular fading.

Naming

Flamsteed John Flamsteed (19 August 1646 – 31 December 1719) was an English astronomer and the first Astronomer Royal. His main achievements were the preparation of a 3,000-star catalogue, ''Catalogus Britannicus'', and a star atlas called ''Atlas Coe ...

followed

Ptolemy Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of importanc ...

in treating φ Persei as being in Andromeda and gave it the designation 54 Andromedae. It is isolated from the main stars of Perseus, but lies within its formal borders. In

Chinese Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of ...

, (), meaning '' Heaven's Great General'', refers to an asterism consisting of φ Persei, γ Andromedae,

51 Andromedae 51 Andromedae, abbreviated 51 And and formally named Nembus , is the 5th brightest star in the northern constellation of Andromeda, very slightly dimmer than the Andromeda Galaxy also being of 4th magnitude. It is an orange K-type giant ...

49 Andromedae 49 Andromedae (abbreviated 49 And) is a star in the constellation Andromeda. ''49 Andromedae'' is the Flamsteed designation though it also bears the Bayer designation A Andromedae. It is visible to the naked eye under good viewing co ...

, χ Andromedae, υ Andromedae, τ Andromedae,

56 Andromedae 56 Andromedae, abbreviated 56 And, is a probable binary star system in the northern constellation of Andromeda. ''56 Andromedae'' is the Flamsteed designation. It has a combined apparent visual magnitude of 5.69, which is just bright enou ...

, β Trianguli, γ Trianguli and δ Trianguli. Consequently, the

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are often a ...

for φ Persei itself is (, en, the Second Star of Heaven's Great General.).

References

External links

HST Press release on the φ Per binary system
{{DEFAULTSORT:Phi Persei Persei, Phi Perseus (constellation) B-type main-sequence stars 08068 0496 010516 BD+49 0444 Gamma Cassiopeiae variable stars Binary stars