The heart of the Sun is spinning nearly four times faster than the surface, scientists have discovered. The surprise finding follows  years of study of our local star by a veteran spacecraft called Soho, the Solar and Heliospheric Observatory, which is run jointly by ESA and NASA.

Astronomers had assumed that the core of the Sun rotated at around the same speed as the surface, which spins once every 24.5 days days at the equator, but every 35 days near the poles.

You may wonder how solar scientists were able to study a region which is deeply hidden from view. They used an experiment that measured sound waves in the Sun’s atmosphere, or corona, and how they were affected by conditions inside the Sun.

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By timing precisely how long it took some waves to penetrate to the heart of the Sun and back again, they could tell how they were pulled about on their journey.

The sound waves – more scientifically known as acoustic waves – interacted with gravity waves, which the discovery team compared to the sloshing around of water within a tanker truck driving on a winding mountain road.

The differing rotation times can happen because the Sun is not a solid body. The discovery that the core is rotating much more rapidly, was made by an international team of astronomers.

They used an instrument on Soho called GOLF (Global Oscillations at Low Frequency) to identify the sloshing motion in data collected over 16 years. The technique was developed by a team led by French astronomer Eric Fossat of the Observatoire de la Côte d’Azur in Nice.

Principal investigator of GOLF, and a co-author of the new study, is Patrick Boumier of France’s Institut d’Astrophysique Spatiale.

The scientists believe the outer region’s spin was slowed by the solar wind, which is the stream of charged particles that the Sun releases into space. Their findings will help astronomers learn more about the history of the Sun, which is 4.6 billion years old and half way through its life.

Professor Roger Ulrich, of the University of California, said in a statement: “The most likely explanation is that this core rotation is left over from the period when the Sun formed.

“It’s a surprise, and exciting to think we might have uncovered a relic of what the Sun was like when it first formed.”

The heart of the Sun is a searing 15.7 million degrees Celsius,compared to “only” 5,800 degrees C at the surface.

Astronomers first measured the rotation of the Sun in the 17th century by observing the motion of sunspots.

Soho was a forerunner of a number of space telescopes that are now monitoring the Sun. Launched in December 1995, it is still operating nearly 22 years later.

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