Seeing Inside the Sun

In the sky this week…

  • Jupiter is still clear and lies low in the south-western sky after sunset, setting about 8 p.m
  • Saturn rises around 10 p.m. and Venus around 5 a.m.
  • The Moon will reach Last Quarter on February 24.

Ken Tapping, February 23, 2011

Until very recently there was only one way to investigate the inside of the Sun, by doing physics. We know what the Sun looks like from the photosphere and outwards, we know its mass and composition, and we know that it is holding itself together by its own gravity. It is easy to measure the total amount of energy being radiated into space, and we know it is being produced in the Sun’s core. That is enough for us to calculate the density and temperature from the core to the photosphere. The calculation shows that the temperature and density in the core of the Sun is high enough for nuclear fusion to occur, where hydrogen turns to helium and releases energy. In addition, we know the Earth is roughly 4.5 billion years old, so the Sun must be of comparable age, which leaves us no other viable options. The Sun has to be a fusion reactor. These calculations seem to work for the Sun and many other stars but is that really proof? Luckily a solution was found and it came from somewhere unexpected - the study of earthquakes here on Earth.

The energy released in a large earthquake is enough to send waves throughout the body of the Earth. By analysing the waves reaching distant locations around the world, we can map the inside of the Earth. This science is known as seismology.

The Sun is being wracked with explosions all the time, and there is the continual rumbling of fluid motions. A couple of decades ago, it was discovered that we could image the waves flowing around on the solar photosphere. In theory we would be able to calculate the Sun’s activity by finding out what the conditions inside would have to be like to support the waves. The problem was that this would require continuous observations of the Sun, not just half of each day. The solution was the Global Oscillation Network Group, or GONG. A number of identical special telescopes were deployed worldwide, so that we could monitor the Sun, 24/7. A new science, called helioseismology was born. Helios is the Greek Sun God. What we found was that our calculations were right. As the measurements have piled up over decades, and our analysis techniques have improved, we have become able to detect even small changes in the Sun. The Sun has been changing its behaviour in recent years and helioseismology is proving a powerful tool for gaining a better understanding of how the Sun is reacting and what it might mean for us.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mail: ken.tapping@nrc-cnrc.gc.ca

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