Pulsation is the astronomer's word for vibration or oscillation. Every physical object has natural patterns or modes of vibration, each with a corresponding period – the time required for one vibration. We commonly observe this in musical instruments. A bugler can make the air column in a bugle vibrate in different modes or harmonics, with different periods which we perceive as different pitches. A violin string can vibrate in several pitches at once, producing a complex sound which may be pleasant or not, depending on the quality of the violin and the skill of the musician. It is tempting to call pulsation ‘the music of the spheres’, especially as most stars are spherical; the sun is spherical to 1 part in 100 000. This concept comes from an excellent recent review article on stellar pulsation, part of a conference in July 2005, and published in Communications in Asteroseismology, volume 147:

http://www.univie.ac.at/tops/CoAst/archive/cia147.pdf

Pulsation modes

Stars are, to a first approximation, spherical. The simplest form of pulsation is radial pulsation – a simple, spherically symmetric in-and-out expansion and contraction. A star has an infinite number of modes of radial pulsation. The simplest is called the fundamental mode. In this mode, all parts of the star expand together and contract together, in unison. The next-simplest mode is the first overtone. In this mode, there is a nodal sphere in the star, where the material remains at rest. When the part of the star outside this sphere is expanding, the part inside is contracting, and vice versa. In the second overtone mode, there are two nodal spheres, where the material remains at rest.