Seeing inside the sun

  • Feb. 22, 2011 11:00 a.m.


Many science and astronomy books contain illustrations showing what the inside of the Sun is like. They show the nuclear fusion zone in the core, where the energy is produced, how the energy gets transferred to what looks like the surface, and then radiated into space. How can we do this with any confidence? How do we know there is nuclear fusion going on in the core? We cannot visit the interior of the Sun or send instruments there to find out. We are stuck with observing and measuring the surface, more correctly called the photosphere, and what lies above it, and what the Sun radiates.

Until very recently there was only one way to investigate the inside of the Sun, and that was to do physics. We know what the Sun looks like from the photosphere and outwards, we know the Sun’s mass and what it is made of. We know also 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 about 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 analyzing 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 found that we could image the waves flowing around on the solar photosphere. In theory we would be able to calculate what the Sun is like by finding out what it had to be like inside to support the waves we are seeing. 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.  Moreover, as the measurements have piled up over decades, and our analysis techniques have improved, we have become able to detect small even small changes in the Sun. The Sun has been changing its behaviour in recent years, and helioseismology is proving a powerful tool for getting some idea as to what is going on and what it might mean for us. 

Jupiter is low in the southwestern sky after sunset, setting about 8 p.m., but still conspicuous. Saturn rises around 10 p.m., and Venus around 5am.The Moon will reach Last Quarter on the 24th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

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