The turbulent lives of stars

Sep 15, 2011
Schematic stellar structure of the Sun in comparison with that of a Delta Scuti star Credit: Victoria Antoci

The stars are boiling. The reason is the energy generated in the center of the star that wants to escape. If this does not happen quickly enough, the star starts to ‘boil’ in the outer layers causing vibrations that result in light variations, like in the Sun. Such oscillations have now been discovered by Victoria Antoci and collaborators using the NASA spacecraft Kepler, but in a much hotter star. The scientists publish this in the most recent issue of Nature.

Besides the discovery of earth-like planets, astronomy is concerned with research on stellar oscillations, among many other topics. The vibrations cause periodic brightness variations of some . Asteroseismology works just like the seismic exploration of the Earth’s interior: the frequencies of seismic waves depend on mass and composition of a body and therefore allow to tomographically reproduce its interior structure.

What causes stellar oscillations?

Several mechanisms maintain periodic oscillations in stars. In the Sun it is “seething” (convection) in the outer layers, comparable with boiling water and the consequent audible sound of the pot. In stars with masses some 1.5 times solar and more it is the so-called “kappa mechanism” that excites periodic pulsations. “This process works like a heat or Diesel engine” explains Victoria Antoci from the Institute of Astronomy at the University of Vienna.

Stellar structure

Because of decades of research on solar oscillations it is known that the energy in the outer 30 per cent of the solar radius is transported by convection and below that, energy is transported by radiation.

Light variations of the star 'HD 187547', measured by the NASA Kepler satellite. Here we show 12 out of 720 hours (Credit: Victoria Antoci).

In stars twice as massive, only one per cent of the envelope is convective. Also in this case, the energy generated in the core is transported by radiation. Stars with even higher mass should no longer possess a convective envelope. However, where exactly it disappears is unknown because of the extreme physical conditions in this domain.

One possibility to explore this is asteroseismology of so-called Delta Scuti stars. These stars are in the mass range where the convective envelope disappears. Delta Scuti stars show periodic light variations due to pulsations excited by the kappa mechanism. “Since more than ten years, scientists predicted that despite the small depth (one per cent) of the convective envelope of Delta Scuti stars convection should have sufficient energy to excite solar-like pulsations as well. Finally we succeeded to prove this”, Victoria Antoci is pleased.

Kepler confirms the theory

In the framework of her PhD thesis the scientist examined hundreds of stars observed with NASA’s Kepler space telescope for solar-like oscillations and made a detection: the Delta Scuti star called HD 187547 is the first representative of the group showing both types of oscillations. “With HD 187547 we found the ideal object to study different processes and their interaction under extreme physical conditions” says Gerald Handler from Nicolaus Copernicus Center in Warsaw, the advisor of Victoria Antoci’s dissertation.
 
Statements about the actual depth of the outer convective layer are possible for the first time thanks to the work published in Nature, as is a calibration of convection models in this temperature domain. In addition, the presence of two different types of stellar oscillations permits to model the interior structure of HD 187547 with unprecedented precision. The scientists also determined that HD 187547 has unusual abundances of certain chemical elements on its surface, most probably a consequence of slow stellar rotation. Heavy elements dwindle down and become less abundant in the star’s spectrum (only the stellar surface is directly observable). On the other hand, light elements are pushed upwards and appear more abundant. This physical process is known as diffusion and is not fully understood in stars such as HD 187547.

Explore further: Astronomers find 'cousin' planets around twin stars

More information: The excitation of solar-like oscillations in a Sct star by efficient envelope convection, V. Antoci, et al. In: Nature, September 14, 2011. DOI:10.1038/nature10389

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omatumr
1 / 5 (2) Sep 15, 2011
The stars are boiling. The reason is the energy generated in the center of the star that wants to escape. If this does not happen quickly enough, the star starts to boil in the outer layers causing vibrations that result in light variations, like in the Sun.


Yes!

Such events are important in the Sun too, as reported in 2002 [1]

1. "Super-fluidity in the solar interior: Implications for solar eruptions and climate", Journal of Fusion Energy 21, 193-198 (2002):

http://arxiv.org/.../0501441

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo
rawa1
not rated yet Sep 16, 2011
The constellations of planets is crucial for periodicity of stellar cycles. The location of centre of mass of solar system is outside of surface of Sun, so that convective currents of plasma beneath the Sun surface are asymmetric, which leads into their additional stirring due the Coriolis force (in the same way, like the convective currents are changing their orientation at the troposphere).

This explains, why solar cycles are driven with periodicity of Jupiter planet. Under symmetric constellation of planets the centre of mass of solar system appears at the centre of Sun, which prohibits the proper stirring of solar plasma. The Sun is behaving like glass of overheated water standing inside of microwave oven and it has a tendency to the explosive boiling and solar eruptions.
Thecis
not rated yet Sep 16, 2011
@rawa1:

Just a question, why is the centre of mass of the solar system oustide the surface of the sun? The sun itself resembles aroudn 98 - 99% of the whole mass. Most orbits of the planets are spherical, meaning they would not shift the centre of mass of the solar system. On the short run, the centre of mass could wobble a little, but to actually be outside the surface of the sun needs some more explanation.

That Jupiter could also affect the sun is likely, as the moon affects earth greatly (being 1/6 of the mass). But to actually shift the centre of mass, not likely

Please provide further detailed information.
lengould100
not rated yet Sep 16, 2011
Thecis: Good answers here.

http://www.physic...567.html