Cepheid variable

Cepheid variable

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Cepheids redirects here. For the fictional species, see "Blind Alley".


A Cepheid (icon or ˈ) is a member of a class of very luminous variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

s. The strong direct relationship between a Cepheid variable's luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

 and pulsation period
Periodic function
In mathematics, a periodic function is a function that repeats its values in regular intervals or periods. The most important examples are the trigonometric functions, which repeat over intervals of length 2π radians. Periodic functions are used throughout science to describe oscillations,...

, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.

Cepheid variables are divided into several subclasses which exhibit markedly different masses, ages, and evolutionary histories: Classical Cepheids, Type II Cepheids, Anomalous Cepheids, and Dwarf Cepheids.

The term cepheid originates from Delta Cephei
Delta Cephei
Delta Cephei is a binary star system approximately 891 light-years away in the constellation of Cepheus . Delta Cephei is the prototype of the Cepheid variable stars, and it is among the closest stars of this type to the Sun...

 in the constellation Cepheus
Cepheus (constellation)
Cepheus is a constellation in the northern sky. It is named after Cepheus, King of Aethiopia in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:...

, the first star of this type identified, by John Goodricke
John Goodricke
John Goodricke FRS was an eminent and profoundly deaf amateur astronomer. He is best known for his observations of the variable star Algol in 1782.- Life and work :...

 in 1784.

Classical Cepheids



Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cephei variables) undergo pulsations with very regular periods on the order of days to months. Classical Cepheids are population I variable stars which are 4–20 times more massive than the Sun, and up to 100,000 times more luminous. Cepheids are yellow supergiants of spectral class F6 – K2 and their radii change by (~25% for the longer-period I Carinae
HD 84810
HD 84810, also known as l Carinae is a star in the constellation Carina.l Carinae is a yellow G-type supergiant with a mean apparent magnitude of +3.69. It is approximately 1510 light years from Earth. It is classified as a Cepheid variable star and its brightness varies from magnitude +3.28 to...

) millions of kilometers during a pulsation cycle.

Classical Cepheids are used to determine distances to galaxies within the Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...

 and beyond, and are a means by which the Hubble constant can be established. Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure.

Type II Cepheids



Type II Cepheids (also termed Population II Cepheids) are population II variable stars which pulsate with periods typically between 1 and 50 days. Type II Cepheids are typically metal-poor, old (~10 Gyr), low mass objects (~half the mass of the Sun). Type II Cepheids are divided into several subgroups by period. Stars with periods between 1 and 4 days are of the BL Her subclass, 10–20 days belong to the W Virginis subclass
W Virginis variable
W Virginis variables are a subclass of Type II Cepheids which exhibit pulsation periods between 10–20 days, and are of spectral class F6 – K2.They were first recognized as being distinct from classical Cepheids by Walter Baade in 1942, in a study of Cepheids in the Andromeda Galaxy that proposed...

, and stars with periods greater than 20 days belong to the RV Tauri subclass
RV Tauri variable
RV Tauri variables are supergiant variable stars. They exhibit changes in luminosity which are tied to radial pulsations of their surfaces. Their changes in brightness are also correlated with changes in their spectral type. While at their brightest, the stars have spectral types F or G. At...

.

Type II Cepheids are used to establish the distance to the Galactic center
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...

, globular clusters, and galaxies.

History


On September 10, 1784, Edward Pigott
Edward Pigott
Edward Pigott was an English astronomer, and the son of astronomer Nathaniel Pigott and Anna Mathurine de Bériot . Probably born in Whitton, Middlesex, his elder brother, Charles Gregory, died in young age. He also had a younger sister, Mathurina...

 detected the variability of Eta Aquilae
Eta Aquilae
Eta Aquilae is a star in the constellation Aquila. It was also part of the former constellation Antinous. It is a Cepheid variable star, varying from apparent magnitude 3.5 to 4.4 with a period of 7.176641 days...

, the first known representative of the class of Classical Cepheid variables. However, the namesake for classical Cepheids is the star Delta Cephei
Delta Cephei
Delta Cephei is a binary star system approximately 891 light-years away in the constellation of Cepheus . Delta Cephei is the prototype of the Cepheid variable stars, and it is among the closest stars of this type to the Sun...

, discovered to be variable by John Goodricke
John Goodricke
John Goodricke FRS was an eminent and profoundly deaf amateur astronomer. He is best known for his observations of the variable star Algol in 1782.- Life and work :...

 a few months later.

A relationship between the period and luminosity for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt
Henrietta Swan Leavitt
Henrietta Swan Leavitt was an American astronomer. A graduate of Radcliffe College, Leavitt went to work in 1893 at the Harvard College Observatory in a menial capacity as a "computer", assigned to count images on photographic plates...

 in an investigation of thousands of variable stars in the Magellanic Clouds
Magellanic Clouds
The two Magellanic Clouds are irregular dwarf galaxies visible in the southern hemisphere, which are members of our Local Group and are orbiting our Milky Way galaxy...

. She published it in 1912 with further evidence.

In 1915, Harlow Shapley
Harlow Shapley
Harlow Shapley was an American astronomer.-Career:He was born on a farm in Nashville, Missouri, and dropped out of school with only the equivalent of a fifth-grade education...

 used Cepheids to place initial constraints on the size and shape of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, and of the placement of our Sun within it.

In 1924, Edwin Hubble
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...

 established the distance to Classical Cepheid variables in the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

, and showed that the variables were not members of the Milky Way. That settled the Island Universe debate which was concerned with whether the Milky Way and the Universe were synonymous, or was the Milky Way merely one in a plethora of galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 that constitutes the Universe.

In 1929, Hubble and Milton L. Humason
Milton L. Humason
Milton Lasell Humason was an American astronomer. He was born in Dodge Center, Minnesota.He dropped out of school and had no formal education past the age of 14. Because he loved the mountains, and Mount Wilson in particular, he became a "mule skinner" taking materials and equipment up the...

 formulated what is now known as Hubble's Law
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...

 by combining Cepheid distances to several galaxies with Vesto Slipher
Vesto Slipher
Vesto Melvin Slipher was an American astronomer. His brother Earl C. Slipher was also an astronomer and a director at the Lowell Observatory....

's measurements of the speed at which those galaxies recede from us. They discovered that the Universe is expanding (see the expansion of the Universe
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...

).

In the mid 20th century, significant problems with the astronomical distance scale were resolved by dividing the Cepheids into different classes with very different properties. In the 1940s, Walter Baade recognized two separate populations of Cepheids (classical and Type II). Classical Cepheids are younger and more massive population I stars, whereas Type II Cepheids are older fainter population II stars. Classical Cepheids and Type II Cepheids follow different period-luminosity relationships. The luminosity of Type II Cepheids is, on average, less than classical Cepheids by about 1.5 magnitudes
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...

 (but still brighter than RR Lyrae
RR Lyrae variable
RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances.This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra....

 stars). Initial studies of Cepheid variable distances were complicated by the inadvertent admixture of classical Cepheids and Type II Cepheids. Walter Baade
Walter Baade
Wilhelm Heinrich Walter Baade was a German astronomer who worked in the USA from 1931 to 1959.-Biography:He took advantage of wartime blackout conditions during World War II, which reduced light pollution at Mount Wilson Observatory, to resolve stars in the center of the Andromeda galaxy for the...

's seminal discovery led to a fourfold increase in the distance to M31
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

, and the extragalactic distance scale. RR Lyrae stars
RR Lyrae variable
RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances.This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra....

 were recognized fairly early (by the 1930s) as being a separate class of variable, due in part to their short periods.

Uncertainties in Cepheid determined distances


Chief among the uncertainties tied to the Classical and Type II Cepheid distance scale are: the nature of the period-luminosity relation in various passband
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....

s, the impact of metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

 on both the zero-point and slope of those relations, and the effects of photometric contamination (blending) and a changing (typically unknown) extinction law on Cepheid distances. All these topics are actively debated in the literature.

These unresolved matters have resulted in cited values for the Hubble constant (established from Classical Cepheids) ranging between 60 km/s/Mpc and 80 km/s/Mpc. Resolving this discrepancy is one of the foremost problems in astronomy since the cosmological parameters of the Universe may be constrained by supplying a precise value of the Hubble constant.

Dynamics of the pulsation


The accepted explanation for the pulsation of Cepheids is called the Eddington valve, or κ-mechanism, where the Greek letter κ (kappa) denotes gas opacity.
Helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 is the gas thought to be most active in the process. Doubly ionized
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...

 helium (helium whose atoms are missing two electrons) is more opaque than singly ionized helium. The more helium is heated, the more ionized it becomes. At the dimmest part of a Cepheid's cycle, the ionized gas in the outer layers of the star is opaque, and so is heated by the star's radiation, and due to the increased temperature, begins to expand. As it expands, it cools, and so becomes less ionized and therefore more transparent, allowing the radiation to escape. Then the expansion stops, and reverses due to the star's gravitational attraction. The process then repeats.

The mechanics of the pulsation as a heat-engine was proposed in 1917 by Arthur Stanley Eddington
Arthur Stanley Eddington
Sir Arthur Stanley Eddington, OM, FRS was a British astrophysicist of the early 20th century. He was also a philosopher of science and a popularizer of science...

 (who wrote at length on the dynamics of Cepheids), but it was not until 1953 that S. A. Zhevakin
Sergei Alexandrovich Zhevakin
Sergei Alexandrovich Zhevakin was a Russian astronomer.Zhevakin is credited for identifying ionized helium as the valve for the heat engine that drives the pulsation of Cepheid variable stars.-External links:*...

 identified ionized helium as a likely valve for the engine.

Examples

  • Classical Cepheids include: Eta Aquilae
    Eta Aquilae
    Eta Aquilae is a star in the constellation Aquila. It was also part of the former constellation Antinous. It is a Cepheid variable star, varying from apparent magnitude 3.5 to 4.4 with a period of 7.176641 days...

    , Zeta Geminorum
    Zeta Geminorum
    Zeta Geminorum is a star in the constellation Gemini. It has the traditional name Mekbuda.The name Mekbuda has roots in ancient Arabic where it and the star Mebsuta were the paws of a lion...

    , Beta Doradus
    Beta Doradus
    Beta Doradus is the second brightest star in the constellation of Dorado. It is a Cepheid variable, with an apparent visual magnitude which varies between 3.46 and 4.08. Its spectral type and luminosity class are likewise variable, from F-type to G-type and from a supergiant to a bright...

    , as well as the namesake Delta Cephei
    Delta Cephei
    Delta Cephei is a binary star system approximately 891 light-years away in the constellation of Cepheus . Delta Cephei is the prototype of the Cepheid variable stars, and it is among the closest stars of this type to the Sun...

    . The North Star (Polaris
    Polaris
    Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) 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....

    ) is the closest classical Cepheid, although the star exhibits many peculiarities and its distance is a topic of active debate
  • Type II Cepheids include: W Virginis and BL Hercules.
  • Dwarf Cepheids include: Delta Scuti
    Delta Scuti
    Delta Scuti is a white, F-type giant star in the constellation Scutum. It is approximately 187 light years from Earth. Delta Scuti is the prototype of the Delta Scuti type variable stars. It is a high-amplitude δ Scuti type pulsator with light variations of about 0.15 minutes...

    , SX Phoenicis.

External links