Delta Scuti variable
Encyclopedia
A Delta Scuti variable is a variable star
which exhibits variations in its luminosity
due to both radial and non-radial pulsations of the star's surface. Typical brightness fluctuations are from 0.003 to 0.9 magnitude
s in V
over a period of a few hours, although the amplitude
and period
of the fluctuations can vary greatly. The stars are usually A0 to F5 type
giant
or main sequence
stars. The high-amplitude Delta Scuti variables are also called AI Velorum stars. They are the second most abundant variable source in the Milky Way after white dwarfs.
δ – 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 heated it becomes more ionised, which is more opaque. This is because helium only has two electrons, so when it becomes (singly or doubly) ionised either only (one or no) electrons remain in its shell to emit any light. 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, expand, ionise, 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 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
, which exhibits brightness fluctuations from +4.60 to +4.79 in apparent magnitude
with a period of 4.65 hours. Other well known Delta Scuti variables include Altair, Denebola
(β Leonis) and β Cassiopeiae
. Vega
(α Lyrae) is a suspected Delta Scuti variable, but this remains unconfirmed.
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...
which exhibits variations in its 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...
due to both radial and non-radial pulsations of the star's surface. Typical brightness fluctuations are from 0.003 to 0.9 magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
s in V
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...
over a period of a few hours, although the amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
and period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
of the fluctuations can vary greatly. The stars are usually A0 to F5 type
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
giant
Giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...
or main sequence
Main sequence
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 Hertzsprung and Henry Norris Russell...
stars. The high-amplitude Delta Scuti variables are also called AI Velorum stars. They are the second most abundant variable source in the Milky Way after white dwarfs.
δ – 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 heated it becomes more ionised, which is more opaque. This is because helium only has two electrons, so when it becomes (singly or doubly) ionised either only (one or no) electrons remain in its shell to emit any light. 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, expand, ionise, 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 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
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...
, which exhibits brightness fluctuations from +4.60 to +4.79 in apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
with a period of 4.65 hours. Other well known Delta Scuti variables include Altair, Denebola
Denebola
Denebola is the second brightest star in the constellation Leo. It is an A-class star that is about distant from earth, and has a luminosity about twelve times that of the sun. Its apparent magnitude is 2.14...
(β Leonis) and β Cassiopeiae
Beta Cassiopeiae
Beta Cassiopeiae is a Delta Scuti variable star in the constellation Cassiopeia. It is a subgiant or giant star belonging to the spectral class F2, with a mean apparent magnitude of +2.27 and absolute magnitude of +1.16...
. Vega
Vega
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...
(α Lyrae) is a suspected Delta Scuti variable, but this remains unconfirmed.