2006 IRISH SCIENTIST YEAR BOOK

Home Page

Table of contents

Index by Author

Index by topics

Search


Armagh Observatory
C Simon Jeffery
Stellar seismology
Everything in the world around us has its natural modes of oscillation. The most obvious can be seen and heard. Consider the beating of the waves on the seashore, the clarion of the school bell, and the melancholy slides of a tenor saxophone.

Others are less obvious, like the vibrations of a soap bubble blown by a child, or of a building shaken by an earthquake. Amazingly, we often only need to hear or feel these vibrations to know what made them! The frequencies of natural modes provide a unique signature describing mass, stiffness and shape.

In the cosmos, stars, galaxies, and even the Universe itself, have natural modes. Generally, we do not see stars vibrate unless they are driven at one or more of their natural frequencies by some internal process.

Even when they do, these vibrations are often very small. Large stars, like red giants, vibrate slowly, with periods of hundreds of days. Tiny neutron stars vibrate with periods of seconds. Ordinary stars like the Sun vibrate with thousands of periods of just a few minutes.

Just as a violin can be distinguished from a 'cello by its sound, measuring oscillation periods enables astronomers to learn about internal stellar structure. In the case of the Sun, its internal density and rotation have been measured with very high precision.


A three-colour light curve of sub-luminous B star KPD2109+4401. Note the beating between closely-spaced frequencies and that the amplitude is larger in blue than in green and red. The black data are for a non-varying comparison star. An amplitude of 0.1 magnitudes represents a variation of 10% in brightness. With an exposure time of Is, the data comprised 75,000 CCD images. Gaps in the data are due to cloud.

Armagh astronomers have been studying a group of rare and small stars � sub-luminous B stars. Half the mass of the Sun, these stars produce energy by converting helium to carbon in their cores. They have a very thin layer of hydrogen on their surfaces and show several simultaneous oscillations with periods between 2 and 10 minutes (Fig. 1). Using a very high-speed colour camera called ULTRACAM on the William Herschel Telescope in La Palma and the Very Large Telescope in Chile, they have measured both the frequencies and the shapes of the oscillations. Disentangling these data will enable us to probe inside the stars, to measure the total mass and the mass of the hydrogen layer, and to deduce the way that heavy elements are separated into layers.

Ultimately, we will learn how these stars were formed and, indeed, why they vibrate in the first place.

Contact: Dr C Simon Jeffery,
Armagh Observatory, College Hill,
Armagh BT61 9DG
E-mail: [email protected]. Web: star.arm.ac.uk/~csj