NZ
science teacher
sound
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What are the properties of sound waves?
What are the sounds of the stars?
Amplitude and wavelength (or frequency) are two important properties of the wave. For sound waves in air, amplitude relates to ‘loudness’, frequency corresponds to ‘pitch’, whilst the speed of sound in a medium is dependent on the properties of the medium and is largely independent of the amplitude and frequency of the wave. Properties of the medium that are important are elasticity, density, and temperature. The sound speed is highest for solids, lower for liquids, and lowest for gases. In gases the important properties of the medium are temperature, molecular structure of the gas and molecular weight. The sound speed is faster at higher temperature and for lower molecular weights. Table 1 shows the speed of sound for various gases at 0°C. The composition of the matter in the Universe is approximately 75% hydrogen and 25% helium. The properties of these two gases are therefore extremely important for common astronomical objects such as stars.
The Sun, or a star, can easily have sound waves moving through the ionised gas (plasma) in a somewhat similar way to seismic waves propagating through and around the Earth. In the case of the Earth, seismologists are able to use the initiation and propagation of earthquakes to deduce the detailed interior structure of our planet. In the science of Helioseismology (for the Sun) or Asteroseismology (for the stars), we apply the science of seismology to the stars, decoding their internal structure from studying the multiple tiny vibrations or ‘starquakes’ on the star’s surface (Figure 3). Many stars, including our own Sun, ‘ring’ like bells and show patterns on their surfaces like those on a drum that has been struck by a drumstick. In stars this ‘ringing’ can be observed by measuring the Doppler shift of electromagnetic radiation from the stellar surfaces (see Figure 4). Stars do not resonate with just one frequency or tone, but can have many modes excited simultaneously. The result is that each star has a unique musical ‘voice’ wholly dependent on the internal properties of that particular star. Using asteroseismology, we can learn about the stellar interior by doing a detailed comparison between observations and complex theoretical models.
Table 1. The speed of sound for various gases at 0° C. Gas Air Carbon Dioxide Oxygen Helium Hydrogen
Speed (m/s) 331 259 316 965 1290
So, are there any sounds in space? Yes and no. Sound waves can easily travel through astronomical objects, like the stars, the Sun, or even the Earth. Think of earthquakes or seismic waves propagating through the Earth, or radar/sonar waves travelling through water, or the perhaps more familiar sound waves travelling through the air. Sound waves can propagate through the gases that make up most astronomical objects in the Universe. We can see, or otherwise detect, the effects of sound waves propagating through astronomical objects like the Sun, the stars, or through gas in space. We do this by measuring the motions of the gas particles due to the compressions or shock waves propagating through the gases. However, we do not listen to sound waves travelling through space.
Sound in space What do astronomers mean when they talk about sounds in space? Can you listen to the Sun, a star, a pulsar, a black hole, a galaxy or a universe? Many astronomical objects (such as the Sun, stars, pulsars or black holes) vibrate or pulsate in various ways. Some of these vibrations are random events (such as a supernova explosion), whilst other oscillations are extremely periodic (such as the rotational frequency of a pulsar, or the pulsations of a Cepheid star). We can convert, or scale, the frequencies we detect in astronomical objects to frequencies that are readily detectable by the human ear. By scaling these frequencies to the audible range of humans, we can then listen to these ‘sounds of space’ even though the vibrations may not be sound waves that we are familiar with. The relationship between the frequencies (the ‘pitch’ or the ‘notes’) is preserved, so in a way, it is like scaling a piece of music up or down an octave (or 57 octaves in the case of a black hole!).
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Is it (scientifically) useful to listen to the sounds of the stars? Yes! Actually it is scientifically useful because the interiors of stars are among the most difficult regions of the universe to observe. Stars are opaque to visible light and there are few other scientific techniques available to explore inside stars. (And we certainly don’t want to send Ripley and her mining ship out for this information!) Asteroseismological studies allow us to determine whether the composition of an object is changing (since the slow fusing of hydrogen to helium in its core will change the sound speed, thus changing the ‘pitch’ or ‘tone’ of the star’s voice). We are also able to detect the effect of binary companions on the stars through their gravitational influence (similar to the Moon’s effect of raising oceanic tides on the Earth). The ultimate goal of asteroseismology is to improve the evolutionary models of stars, so that we understand the details of how a star is born, lives and dies (and how its structure changes throughout its life). Stars are the very constituents of star clusters and galaxies, as well as the crucibles for all the elements in the Universe (besides hydrogen and helium), so improving our understanding of their internal structure and the evolutionary processes improves our understanding of the Universe as a whole.
What is ‘The Music of the Stars’? Recent technological advances have meant that astronomers now have instruments with the precision necessary to detect the tiny surface waves of stars, thus allowing us to probe deep within the opaque stellar interiors. One of these precise astronomical instruments is the HERCULES spectrograph, which is used on the 1-metre telescope at the University of Canterbury’s Mt John University Observatory in Tekapo. Using this instrument, we have just started a research project to study the sound waves in stars that are similar to the Sun. This research, supported by the Marsden Fund, allows New Zealand astronomers not just to listen to, but to fundamentally understand and to mathematically describe, these stellar sound waves – the ‘Music of the Stars’. For further information contact: karen.pollard@canterbury.ac.nz