Delta Cephei (δ Cep / δ Cephei) is a
binary starA binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
system approximately 891
light-yearAs defined by the International Astronomical Union , a light-year is the distance that light travels in a vacuum in one Julian year.The speed of light is 299,792,458 metres per second...
s away in the
constellationIn modern astronomy, a constellation is an area of the celestial sphere, defined by exact boundaries.The term "constellation" can also be used loosely to refer to just the more prominent visible stars that seem to form a pattern in that area.-Definitions:...
of
CepheusCepheus is a constellation in the northern sky. It is named after Cepheus, King of Aethiopia in Greek mythology, and is considered to represent a king. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:γ Cephei...
(the
KingKing may be a title for a head of state.King may also refer to:-Places:* King, Ontario, Canada* King, Indiana, United States* King, North Carolina, United States* King, Lincoln County, Wisconsin, United States...
). The names
Alrediph,
Al Radif or variants are derived from the Arabic "الرادف" (al-rādif), or "the Follower". Delta Cephei is the prototype of the
Cepheid variableA Cepheid is a member of a class of pulsating variable stars. The relationship between a Cepheid variable's luminosity and pulsation period is quite precise, securing Cepheids as viable standard candles and the foundation of the Extragalactic Distance Scale.Typical classical Cepheids pulsate with...
stars, and it is among the closest stars of this type to the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
(with
PolarisPolaris is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole , making it the current northern pole star.Polaris is about 430 light-years from Earth...
being closer). Its variability was discovered by
John GoodrickeJohn Goodricke FRS was an eminent and profoundly deaf amateur astronomer. He is best known for his observations of the variable star Algol in 1782...
in 1784, the second Cepheid variable discovered following
Eta AquilaeEta Aquilae is a star in the constellation Aquila. It was also part of the former constellation Antinous. In addition, this star carries the seldom used Hebrew name Bezek, from בֶזֶק bazaq "lightning"....
earlier the same year.
Unlike the eclipsing binary
AlgolAlgol , known colloquially as the Demon Star, is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, the first such star to be discovered, and also one of the first variable stars to be discovered...
, Delta Cephei's variability is due to pulsation of the star.
Delta Cephei (δ Cep / δ Cephei) is a
binary starA binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
system approximately 891
light-yearAs defined by the International Astronomical Union , a light-year is the distance that light travels in a vacuum in one Julian year.The speed of light is 299,792,458 metres per second...
s away in the
constellationIn modern astronomy, a constellation is an area of the celestial sphere, defined by exact boundaries.The term "constellation" can also be used loosely to refer to just the more prominent visible stars that seem to form a pattern in that area.-Definitions:...
of
CepheusCepheus is a constellation in the northern sky. It is named after Cepheus, King of Aethiopia in Greek mythology, and is considered to represent a king. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:γ Cephei...
(the
KingKing may be a title for a head of state.King may also refer to:-Places:* King, Ontario, Canada* King, Indiana, United States* King, North Carolina, United States* King, Lincoln County, Wisconsin, United States...
). The names
Alrediph,
Al Radif or variants are derived from the Arabic "الرادف" (al-rādif), or "the Follower". Delta Cephei is the prototype of the
Cepheid variableA Cepheid is a member of a class of pulsating variable stars. The relationship between a Cepheid variable's luminosity and pulsation period is quite precise, securing Cepheids as viable standard candles and the foundation of the Extragalactic Distance Scale.Typical classical Cepheids pulsate with...
stars, and it is among the closest stars of this type to the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
(with
PolarisPolaris is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole , making it the current northern pole star.Polaris is about 430 light-years from Earth...
being closer). Its variability was discovered by
John GoodrickeJohn Goodricke FRS was an eminent and profoundly deaf amateur astronomer. He is best known for his observations of the variable star Algol in 1782...
in 1784, the second Cepheid variable discovered following
Eta AquilaeEta Aquilae is a star in the constellation Aquila. It was also part of the former constellation Antinous. In addition, this star carries the seldom used Hebrew name Bezek, from בֶזֶק bazaq "lightning"....
earlier the same year.
Stellar system
Unlike the eclipsing binary
AlgolAlgol , known colloquially as the Demon Star, is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, the first such star to be discovered, and also one of the first variable stars to be discovered...
, Delta Cephei's variability is due to pulsation of the star. It varies from
magnitudeThe apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere...
3.6 to 4.3, and its
spectral typeIn astronomy, stellar classification is a classification of stars based on its spectral characteristics. The spectral class of a star, is a designation of a class to a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective...
also varies, from about F5 to G3. The period is 5.36634 days; rise to maximum is quicker than the subsequent decline to minimum. It was later discovered that there are two types of Cepheid variables, and Delta Cephei is now known as a type I (Classical) Cepheid.
Stars of this type are believed to form with masses of 3–30 times that of our
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, and then have passed through the
main sequenceThe main sequence is a continuous and distinctive band of stars that appear on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung-Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
as B-class stars. With the
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
burnt up in their core, these unstable stars are now passing through later stages of nuclear burning.
Knowing the distance of Delta Cephei and others in its class is fundamental to calibrating their period-luminosity relationship; unfortunately, these efforts were until recently marred by only fair
parallaxParallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines...
accuracy. In 2002 however, the
Hubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital...
was used to determine the distance to Delta Cephei (and
RR LyraeRR Lyrae is a variable star in the Lyra constellation. It is the prototype of the RR Lyrae variables star class. It has a period of about 13 hours, and oscillates between apparent magnitudes 7 and 8. Its variable nature was discovered by the Scottish astronomer Williamina Fleming at Harvard...
, another standard candle) within ~4%: 273 parsecs, or 890 light-years. There is a 7.5 magnitude companion star, separated from Delta Cephei by 41 arc seconds and visible in small telescopes.