Sounding the Stars: OCTAVE, the Birmingham Seismology Programme

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Stars are the visible building-blocks of the Universe and the nearest star to us, the Sun, is vital to life on Earth. Our research aims to 'look' deep inside the Sun and stars to help us understand how they work and evolve. Although you can't see it without special equipment, the Sun is shaking, and many stars are too. Bubbling motion just under the visible surface of the star constantly feeds in energy, and the star responds by vibrating like a (very) large musical instrument. The 'notes' of the stars are very interesting to astronomers. They are produced by sound waves which have travelled deep inside and their frequencies (the 'pitches' of the notes) depend on the conditions they meet on the way, such as the density and temperature of stellar material. By studying them astronomers essentially get an 'ultrasound scan' of a star - we can understand better everything from what makes the vibrations in the first place, all the way to the nuclear reactor at the star's core. By studying the Sun we examine one star in exquisite detail, whilst gaining information on a range of stars allows us to test our models of stars with different masses, compositions and ages. We can then study how stars like the Sun evolve - we can see the future of the Sun Turning these ideas into scientific reality is very difficult. The shaking is very small - the surface moves to and fro at a couple of metres per second (in the solar case) and cycles in about 5 minutes. Just detecting the small movements takes specially designed instruments (called spectrometers, because they examine a star's spectrum). At the same time the star's brightness changes by about 1 part in 10,000, just detectable from Earth for the Sun, but too subtle to be measured in other stars through the Earth's atmosphere. Some interesting effects on stars can last for several years. For example, we know that stars have cycles of magnetic activity which we don't fully understand yet - the Sun's has periods of 'spottiness' every 11 years or so. For this and other reasons, you would really like to observe the stars' vibrations continuously for many years. One way is to put a number of robotic spectrometers in different places around the world, so that when a star sets on one instrument it has already risen on another. We have been doing this for the Sun for about 30 years, by operating the Birmingham Solar Oscillations Network (BiSON), and now aim to use this expertise in the development of a network for studying stars. Colleagues in Denmark are designing the prototype instrument for SONG (the Stellar Oscillations Network Group). We aim to contribute to instrument design and control to help SONG develop, as well as continuing our solar observations. Another way to study the oscillations of stars is to observe their small brightness fluctuations from space, and a number of current satellites can do this. Techniques we have developed for studying the Sun are transferrable to other stars and the satellite data. We have been working in recent years with international colleagues to prepare for data that the NASA satellite, Kepler, will produce (from the middle of 2009) and we aim to work on hundreds of stars in the coming years. We will also use data from the SMEI satellite to study the rather different pulsations of O and B stars, which are much hotter than the Sun. Here the pulsations are so dramatic that they may play a part in the stars losing mass from their outer layers - our work will help to examine this mass loss. So our new proposal is to extend our observational and analytical experience from our solar work to other stars and to continue to operate the BiSON network, collecting high quality solar data. We will use stellar and solar data to provide new information on the deep interiors of stars (especially to study stellar evolution) and to improve our understanding of stellar activity cycles and of the outer convective layers of stars where the vibrations are generated.