A Delta Scuti variable (sometimes termed dwarf cepheid when the V-band amplitude is larger than 0.3 mag.) is a class of pulsating star, comprising several sub-classes of object with A- or F-type spectra. The variables follow a period-luminosity relation in certain passbands like other

standard candles The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible ...

such as

Cepheids 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 ...

. and, together with

classical cepheids Classical Cepheids are a type of Cepheid variable star. They are young, population I variable stars that exhibit regular radial Stellar pulsation, pulsations with periods of a few days to a few weeks and visual amplitudes ranging from a few tenth ...

, are important

. They have been used to establish the distance to the

Large Magellanic Cloud The Large Magellanic Cloud (LMC) is a dwarf galaxy and satellite galaxy of the Milky Way. At a distance of around , the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal Galaxy, Sagittarius Dwarf ...

globular clusters A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting ...

, open clusters, and the

Galactic Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...

. The OGLE and MACHO surveys have detected nearly 3,000 Delta Scuti variables in the

. Typical brightness fluctuations of Delta Scuti variables are from 0.003 to 0.9 magnitudes in V over a period of a few hours, although the

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...

and period of the fluctuations can vary greatly. They are usually A0 to F5

type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * ...

giant In folklore, giants (from Ancient Greek: ''wiktionary:gigas, gigas'', cognate wiktionary:giga-, giga-) are beings of humanoid appearance, but are at times prodigious in size and strength or bear an otherwise notable appearance. The word ''gia ...

subgiant A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...

, or

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 ...

stars. The high-amplitude Delta Scuti variables are also called AI Velorum stars, after the prototype AI Velorum. SX Phoenicis variables are generally considered to be a subclass of Delta Scuti variables that contain Population II stars, often blue stragglers, and can be found in globular clusters. SX Phe variables also follow a period-luminosity relation. One last sub-class are the pre-main sequence (PMS) Delta Scuti variables, stars that are more luminous than main sequence stars of the same temperature, still contracting towards the main sequence. Delta Scuti stars exhibit both radial and non-radial luminosity pulsations. Non-radial pulsations are when some parts of the surface move inwards and some outward at the same time. Radial pulsations are a special case, where the star expands and contracts around its equilibrium state by altering the radius to maintain its spherical shape. The variations are due to the swelling and shrinking of the star through the Eddington Valve or Kappa-mechanism. The stars have a helium rich atmosphere. As helium is compressed it becomes more ionised, which is more opaque. So at the dimmest part in the cycle the star has highly ionised opaque helium in its atmosphere blocking part of the light from escaping. The energy from this “blocked light” causes the helium to heat up then expand, become more transparent and therefore allow more light through. As more light is let through the star appears brighter and, with the expansion, the helium begins to cool down. Hence the helium contracts under gravity and heats up again and the cyclical process continues. Throughout their lifetime Delta Scuti stars exhibit pulsation when they are situated on the classical Cepheid instability strip. They then move across from the main sequence into the giant branch. The prototype of these sorts of variable stars is Delta Scuti (δ Sct), which exhibits brightness fluctuations from +4.60 to +4.79 in

apparent magnitude Apparent magnitude () is a measure of the Irradiance, 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 (astronomy), ...

with a period of 4.65 hours. Other well known Delta Scuti variables include

Altair Altair is the brightest star in the constellation of Aquila (constellation), Aquila and the list of brightest stars, twelfth-brightest star in the night sky. It has the Bayer designation Alpha Aquilae, which is Latinisation of name ...

and Denebola (β Leonis).

Vega Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only from the Sun, and ...

(α Lyrae) is a suspected Delta Scuti variable, but this remains unconfirmed.

Examples

Other examples include -

σ Octantis Sigma Octantis is a solitary star in the Octans constellation that forms the pole star of the southern celestial hemisphere, Southern Hemisphere. Its name is also written as σ Octantis, abbreviated as Sigma Oct or σ Oct, and it is offic ...

and β Cassiopeiae

Examples

References

Further reading