Many of you will have heard of Pythagoras' Harmony of the Spheres,
the idea that the movement of the planets and stars correspond to musical notes
and together create a symphony.
The stars though make their own music, one which with the billions
of stars around us, combine to form an orchestra like no other... If only you
know what to listen for.
Asteroseismology, the study of the internal structure of the
stars, is based on the fact that they oscillate. Within the stars, certain
frequencies are amplified by constructive interference setting up standing
waves which can penetrate the stellar structure to varying depths. These frequencies, and thus the combined oscillation, changes with the age and size of the star (the higher the oscillation, the smaller the star)
and provides information on the density of its internal regions. If we record
this oscillation, we can hear this cosmic soundtrack. The music of the stars.
Whilst they may not sound amazing individually, imagine what a galaxy of stars would sound like...
So why do stars oscillate?
So why do stars oscillate?
The oscillations are driven by the conversion of the stellar
thermal energy from the radiative zone to kinetic energy in the convection
zone. A process know as the Kappa Mechanism!
What is this Kappa Mechanism? Well, within stars there are layers of partially ionized elements
which are excited by the energy radiated outwards from the core. This
excitation provides the elements with more energy causing the layers to expand.
Once this occurs however, the elements are able to cool down and recombine,
allowing the radiation to pass through. These layers contract as they cool,
thus returning to their original position, and so the cycle begins again!
Simple right?
Not Quite! This oscillation is not singular, instead there are multiple types or "modes" of oscillation each with their own driving force. The three main modes common to sun-like stars are:
1) The pressure or 'p' modes. These are driven by fluctuations
in the internal stellar pressure and are sometimes called acoustic modes as
their fluctuations are determined by the local speed of sound.
2) The gravity or 'g' modes. These are driven by
the buoyancy of the surrounding stellar layers but are usually
confined to the inner region of the star due to the convective region.
3) The surface gravity or 'f ' modes. These modes are similar
to ocean waves, moving along the stellar surface.
Of these three modes, the most prominent at the surface and thus
the easiest to detect are the p-modes.
Different oscillations within Stars
How do we detect these oscillations?
These oscillations are assumed to be quite small, occurring in
isolated stars and appearing spherically symmetric. They lead to slight
variations in the luminosity of these stars and it is these variations which can be detected. Space-based telescopes including SOHO, which was launched to study this
phenomena in the sun (a discipline known as helioseismology), MOST, the first
space telescope dedicated to asteroseismology, and KEPLER are our best tools for this detection.
Indeed KEPLER scientists, held a press conference concerning this
process and the role of the KEPLER telescope in measuring these oscillations
last year:
What have we learnt so far?
Our understanding
of stellar evolution has already been enriched by this research field, with the
direct observation of two stages of the life cycle of red giants. Red giants
which are undergoing hydrogen shell fusion have a g-mode period of 50s whilst those
undergoing helium core fusion have a period between 100 and 300s. This led to
proof of the theoretical calculations suggesting that the core rotates faster
than the surface during these stages.
So what's next?? A more complete understanding of neutron stars? Observations of stellar structure in super giants? Perhaps measurements of the stellar structure just before a super nova?
So what's next?? A more complete understanding of neutron stars? Observations of stellar structure in super giants? Perhaps measurements of the stellar structure just before a super nova?
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