Delta Cephei (δ Cep, δ Cephei) is the

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. ...

for a quadruple

star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speak ...

located approximately 887

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 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 origins of the e ...

of Cepheus, the King. At this distance, the

visual magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

of the star is diminished by 0.23 as a result of

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

caused by gas and dust along the line of sight. It is the prototype of the

Cepheid variable A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid varia ...

stars that undergo periodic changes in luminosity.

Discovery

Delta Cephei was discovered to be variable by

John Goodricke John Goodricke FRS (17 September 1764 – 20 April 1786) was an English amateur astronomer. He is best known for his observations of the variable star Algol (Beta Persei) in 1782. Life and work John Goodricke, named after his great-grandfath ...

during 1784. He describes his first observation on October 19, 1784, followed by a regular series of observations most nights until December 28. Further observations were made during the first half of 1785, the variability was described in a letter dated June 28, 1785, and formally published on January 1, 1786. This was the second variable star of this type, with

eta Aquilae Eta Aquilae (η Aql, η Aquilae) is the Bayer designation for a multiple star in the equatorial constellation of Aquila, the eagle. It was once part of the former constellation Antinous. On average, this star has an apparent visual magnitu ...

being discovered just a few weeks earlier, on September 10, 1784.

Properties

As well as being the prototype of the Cepheid class of variable stars, Delta Cephei is among the closest stars of this type of variable to 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 ...

, with only

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

being nearer. Its variability is caused by regular pulsations in the outer layers of the star. It varies from

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

3.48 to 4.37, and its

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 grati ...

also varies, from about F5 to G3. The pulsation period is 5.366249 days, with a rise to maximum occurring quicker than the subsequent decline to minimum. Delta cephei ubvri engle 2014

Because the period of this class of variable is dependent on the star's luminosity, Delta Cephei is of particular importance as a calibrator for the

period-luminosity relationship In astronomy, a period-luminosity relation is a relationship linking the luminosity of pulsating variable stars with their pulsation period. The best-known relation is the direct proportionality law holding for Classical Cepheid variables, somet ...

, since its distance is now one of the most precisely established for a Cepheid. This accuracy is thanks in part to its membership of a

star cluster Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clust ...

and the availability of precise

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

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 of celestial obj ...

parallaxes. Hence, in 2002, the

was used to determine the distance to Delta Cephei within a 4%

margin of error The margin of error is a statistic expressing the amount of random sampling error in the results of a survey. The larger the margin of error, the less confidence one should have that a poll result would reflect the result of a census of the e ...

: . However, a re-analysis of

data found a larger parallax than before, leading to a shorter distance of , which is equivalent to 800 light-years. Radial Velocity Measurements of Delta Cephei

Radial Velocity Measurements of Delta Cephei

Radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...

measurements of Delta Cephei have revealed the presence of a small spectroscopic companion star on a 6-year orbit around Delta Cephei A. The mass of this companion is about one tenth of the mass of Delta Cephei and the two come to within 2

astronomical units The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...

at pericenter passage. The presence of this companion will have to be taken into account when

Gaia In Greek mythology, Gaia (; from Ancient Greek , a poetical form of , 'land' or 'earth'),, , . also spelled Gaea , is the personification of the Earth and one of the Greek primordial deities. Gaia is the ancestral mother—sometimes parthenog ...

measures Delta Cephei's

parallax Parallax is a displacement or difference in the apparent position 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. Due to foreshortening, nearby objects ...

(distance). The outer visual companion Delta Cephei C (HD 213307) may also be a spectroscopic and astrometric binary. Stars of this type are believed to form with masses of 3–12 times that of the

, and then have passed through the

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 ...

B-type star A B-type main-sequence star (B V) is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type star ...

s. With the

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

consumed in their core region, these unstable stars are now passing through later stages of nuclear burning. The estimated mass of Delta Cephei, as derived from the

color index In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The lower the color index, the more blue (or hotter) the object is. Conversely, the large ...

, is times the mass of the Sun. By comparison, the mass derived from evolutionary models is times the Sun's mass. At this stage of its evolution, the outer layers of the star have expanded to an average of 44.5 times the girth of the Sun. Standard Candle in the Wind

Delta Cephei is emitting around 2,000 times the Sun's luminosity from the outer atmosphere. This is producing a strong

stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. D ...

, which, in combination with the pulsations and shocks in the star's atmosphere, is ejecting mass at the rate of masses per year, or the equivalent to the mass of the Sun roughly every million years. This matter is flowing outward at a velocity of about 35 km s⁻¹. The result of this expelled gas is the formation of a nebula about 1 parsec across, centered on Delta Cephei, and containing 0.07–0.21 solar masses of

neutral hydrogen The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...

. A

bow shock In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of ...

is being formed where the stellar wind is colliding with the surrounding

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

. The

peculiar velocity Peculiar motion or peculiar velocity refers to the velocity of an object relative to a ''rest frame'' — usually a frame in which the average velocity of some objects is zero. Galactic astronomy In galactic astronomy, peculiar motion refers to ...

of Delta Cephei is relative to its neighbors. It is a suspected member of the Cep OB6 star cluster, and hence may be around the same age as the cluster: around 79 million years. At an

angular separation Angular distance \theta (also known as angular separation, apparent distance, or apparent separation) is the angle between the two sightlines, or between two point objects as viewed from an observer. Angular distance appears in mathematics (in pa ...

of 40

arc second A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...

s from Delta Cephei is a 7.5 magnitude companion star with the identifier HD 213307, called component C in multiple star catalogues, which is visible in small telescopes. HD 213307 itself is a binary star system with a combined stellar classification of B7–8 III–IV. It is heating the matter being ejected by the stellar wind of Delta Cephei, causing the surrounding circumstellar material to emit infrared radiation.

References

External links

* * {{DEFAULTSORT:Delta Cephei Cephei, Delta Classical Cepheid variables Cepheus (constellation) G-type supergiants F-type supergiants 4 8571 Cephei, 27 213306 110991 BD+57 2548 Cephei, 27 TIC objects