The first recorded observations of sunspots were made by Chinese astronomers over two thousand years ago. They didn't have telescopes, but when looking through
the smoke from forest fires, they noticed that the Sun had small dark spots on it. (Be careful, never look directly at the Sun, you could go blind.) It was not until the invention of the telescope that it was
possible to study sunspots in detail. There is some controversy about who first observed sunspots with a telescope.
Galileo observed the sunspots carefully and saw that they appeared, disappeared and changed their shapes. He noticed that they moved around the Sun, in fact following
the solar rotation. He concluded that they could not possibly be planets, but were actually on the Sun.
In 1611, Galileo wrote: "Spots are on the surface of the solar body where they are produced and also dissolved, some in shorter and others in longer
periods. They are carried around the Sun; an important occurrence in itself." One of Galileo's drawings is shown above. Click the picture to view an animation constructed from a sequence of Galileo's
sunspot drawings. Galileo’s ideas got him into trouble with church leaders, because all heavenly bodies were supposed to be perfect, that is ‘without blemish’.
Galileo also spoke out in support of Copernicus’s theory that the planets (including the Earth) orbit around the Sun. He was kept prisoner in his house
near Florence until he died, because of his scientific work on the Sun and the solar system - a harsh punishment indeed. We now know that his scientific theories were correct.
Sunspots are still a bit of a mystery, even almost 400 years later! We can now observe the Sun at different wavelengths in the electromagnetic spectrum, besides
the visible range. Above right is an image of the Sun in ultraviolet emission obtained with an instrument on the Solar and Heliospheric Observatory (SOHO), a joint NASA and ESA satellite. Click the picture to view a
video clip from SOHO.
The regions on the Sun where the dark sunspots are seen in visible light, are very bright in X-ray and UV emission. They are called solar active regions. They
are regions on the solar surface where the magnetic field is very strong and breaks through the surface from beneath. Magnetic fields dominate the solar atmosphere.
The temperature is so high that the atoms are ionised to form a ‘plasma’, positively charged ions and electrons. This plasma is guided by the
Sun’s magnetic field. The interaction of the motion of the Sun, the magnetic field and the ionised gas is complex. Magnetic fields dominate the solar atmosphere. The temperature is so high that the atoms are
ionised to form a ‘plasma’, positively charged ions and electrons. This plasma is guided by the Sun’s magnetic field. The interaction of the motion of the Sun, the magnetic field and the ionised
gas is complex.
We model the Sun with complex mathematics and computer simulations. These models are compared with the observations of the Sun from Earth and space. Magnetic
energy is released in the active regions causing huge explosions, solar flares, which shoot energetic particles out into space.
These magnetic particles from the Sun sometimes head towards the Earth and cause beautiful aurora and troublesome geomagnetic storms.
Text by Helen Mason.