Delta Cephei (δ Cep, δ Cephei) is a quadruple

star system A star system or stellar system is a small number of stars that orbit each other, bound by gravity, gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally calle ...

located approximately 887

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 King. At this distance, the

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 the star is diminished by 0.23 as a result of

extinction Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce 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 variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmi ...

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-grandfa ...

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 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 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 ...

, with only

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 ...

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 gratin ...

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 bolometric luminosity, luminosity of pulsating variable stars with their pulsation period. The best-known relation is the direct proportionality law holding for Classical Cep ...

, 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 A star cluster is a group of stars held together by self-gravitation. Two main types of star clusters can be distinguished: globular clusters, tight groups of ten thousand to millions of old stars which are gravitationally bound; and open cluster ...

and the availability of precise

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

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 ...

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 Statistical survey, survey. The larger the margin of error, the less confidence one should have that a poll result would reflect the result of ...

: . 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 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 ...

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 AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its mode ...

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

Gaia In Greek mythology, Gaia (; , a poetic form of ('), meaning 'land' or 'earth'),, , . also spelled Gaea (), is the personification of Earth. Gaia is the ancestral mother—sometimes parthenogenic—of all life. She is the mother of Uranus (S ...

measures Delta Cephei's

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 ...

(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 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 ...

B-type star In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

s. With the

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

consumed in their core region, these unstable stars are now passing through later stages of nuclear burning. The star's mass has been determined spectroscopically at 5.3 times the

mass of the Sun The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies a ...

. At this stage of its evolution, the outer layers of the star have expanded to an average of 46 times the girth of the Sun. Standard Candle in the Wind

Delta Cephei is emitting around 1,900 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 stellar atmosphere, 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 spheri ...

, which, in combination with the pulsations and shocks in the star's atmosphere, is ejecting mass at the rate of 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 . 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 a spectral line that is created by a change in the energy state of solitary, electrically neutral hydrogen atoms. It is produced by a spin-flip transition, which means the directio ...

. A

bow shock In astrophysics, bow shocks are shock waves in regions where the conditions of density and pressure change dramatically due to blowing stellar wind. Bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby fl ...

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

interstellar medium The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...

. 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 t ...

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 or angular separation is the measure of the angle between the orientation of two straight lines, rays, or vectors in three-dimensional space, or the central angle subtended by the radii through two points on a sphere. When t ...

of 40

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

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 213306 110991 BD+57 2548 Cephei, 27 TIC objects