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The delta Scuti stars are a class of pulsating variable star named for the class prototype -- the fourth-brightest star in the constellation Scutum. They are related to the Cepheid variables, though the delta Scuti stars lie closer to the main sequence, have lower masses, and higher effective temperatures.

Like most other pulsating stars, the delta Scuti stars pulsate because of what is called the opacity- or κ-mechanism. Radiation gets blocked in a narrow ionization layer found inside all stars cooler than the ionization temperatures of hydrogen and helium. When this occurs, the temperature within the layer increases, causing an increase in pressure. This pressure increase can do mechanical work on the outer layers of the star. Delta Scuti stars pulsate because this layer is a just the right height so that

  • the energy doesn't dissipate into the photosphere, and
  • it isn't trapped deep inside, or used for convection.

The former case occurs in hotter, more massive stars, while the latter occurs in cooler, less massive stars. Specifically, what happens is that any compression of the surface of a delta Scuti star yields positive work (at the expense of the radiative luminosity). Thus, once you start the star pulsating, it will keep going and the amplitude will increase. No one yet understands exactly how these stars start pulsating, but we at least know what keeps them going. Like the Cepheids, the RR Lyrae, and the pulsating white dwarf stars, they lie on the instability strip of the Hertzsprung-Russell diagram.

Delta Scuti stars are interesting because unlike their larger Cepheid cousins, they often pulsate in what are called nonradial modes. If you've ever had a course on spherical harmonics, I just mean that the azimuthal quantum number is greater than 0. If you haven't, imagine a balloon: inflating it and deflating would be a radial mode, since the expansion is spherically symmetric. Squishing it flat in the middle and letting it go again would be a nonradial mode (in this case, an "l=2" mode). The reason this is interesting is because the pulsations of a star can tell you something about the interior conditions that we can't normally see, using the principles of asteroseismology. Thus we study these objects to learn more about the physical properties of stars and stellar evolution.

As a group, the delta Scuti stars have spectral types between roughly A5V on the hot side, and F5III on the cool side, and masses about twice that of the Sun. If a delta Scuti star has a chemical composition more deficient in metals than the Sun, it is known as an SX Phoenicis star, again named for the subclass prototype. The SX Phoenicis stars have lower masses, are more evolved, and tend to pulsate only in radial modes. As such, they have been called "dwarf Cepheids" in the past. Like the Cepheids, these stars also have a period-luminosity relation, which means that if you know the period, you can measure how far away it is by measuring its brightness.

Delta Scuti itself is a very ordinary-looking star (α 18h 42m 16.43s, δ -09° 03' 09.2'', mV = 4.7) of spectral type F2III. Its variability was first discovered spectroscopically around the year 1900, and the periodic nature of its pulsations confirmed in the 1930's. Several hundred of these stars are now known, though many of these are recently discovered SX Phoenicis blue straggler stars found in globular clusters and the galactic center.

Delta Scuti was also the subject of the first professional paper I ever wrote: "A new pulsation spectrum and asteroseismology of delta Scuti," Astronomical Journal vol 114, p1592 (1997).

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