Science Feature

The Sun may be smaller
than we thought

If correct, then other properties of the Sun such as its internal temperature and density may be slightly different than previously calculated. Understanding the Sun's interior is important as it might help scientists make predictions about space weather and answer questions about the solar system.

The Sun has no solid surface. Its atmosphere merely gets thinner and more transparent farther from its centre.

Instead the Sun's "surface" is defined to be the depth in the Sun's atmosphere where it becomes opaque to light. Scientists measure this by observing the Sun with telescopes and measuring the distance between the centre of the Sun's disc and its "edge" the place where its brightness suddenly drops off. This gives a radius of 695,990 kilometres, or about 109 times the radius of Earth.

A second, completely different way to measure the Sun's size is by using surface gravity waves called f-modes that ripple across the surface of the Sun like water waves on the ocean.

Multiple choice
Theory implies that these waves should appear only at the Sun's opaque surface, and observations of them can be used to measure the Sun's radius, since their wavelength is tied to their distance from the Sun's centre in a predictable way.

Scientists have been puzzled for years because these methods give two different answers. The wave method gives a radius of around 695,700 kilometres, about 300 kilometres smaller than the result from the light drop-off measurement.

Although the difference amounts to just 0.04%, it is large enough to matter when scientists try to gain insights on the Sun's interior by interpreting observations of sound waves which ripple the Sun's surface in addition to the f-modes using a technique called helioseismology.

Understanding the Sun through helioseismology is important as it can help scientists learn about the origins of the magnetic fields that produce sunspots, which in turn can help predict space weather.

Helioseismology has also helped scientists understand some of the mysteries of the solar system for example, the technique was used to solve part of the solar neutrino problem. The technique ruled out changes in the Sun's interior as a cause of this mysterious disappearance of neutrinos flowing from the Sun to Earth.

Real difference
Now, new calculations of how light propagates in the Sun's atmosphere may have resolved the discrepancy in the Sun's radius in favour of the smaller measurement. The new calculations were carried out by a team led by Margit Haberreiter of the World Radiation Centre in Davos, Switzerland.

(New Scientist.com news service)