Text: Helmut Hornung
"Be quiet my son and place your trust in God. I assure you that the spots are nothing more than faults in your lenses." Christoph Scheiner, a Jesuit priest from Ingolstadt, found himself in a state of conflict when he read the lines of the Provincial Superior of his order: Did not God create the Sun as a pure and flawless light? On the other hand, Scheiner knew that the black regions on the daystar were by no means faults in his telescope. Other researchers, such as Galileo Galilei or Johannes Fabricius, had also observed this at the beginning of the 17th century. Even ancient Chinese chronicles had reported sunspots, which occasionally revealed themselves to the naked eye.
What does all this have to do with this phenomenon? The Sun does not have a firm shell. What astronomers refer to as "surface" is the approximately 350 kilometre-thin layer, from which the light that is visible to us originates: the photosphere. The entire glowing ball has colossal dimensions: with a diameter of 1.39 million kilometres, there would be room for 1.3 million earths. And with a weight of 2000 septillion tonnes, the Sun has 330000 times the mass of our planet. However, when compared to other stars, the Sun is a dwarf.
The sunspots appear to swim in the photosphere like black islands. As early as the 17th century, researchers realised that at least the largest among them had a dark core, the umbra (from the Latin: shadow). This umbra encloses a brighter half shadow, which is called the penumbra. Most sunspots are larger than the Earth. Some of the groups of spots have an expanse of more than 300000 kilometres - corresponding to a good two thirds of the distance between the Earth and the Moon.
In the 18th century, even serious scientists believed that the dark spots were holes in the Sun's atmosphere that permitted a glimpse of the underlying surface populated by alien beings. However, researchers then determined the temperature of the photosphere to be 5500 degrees Celsius, and the temperature of the umbras to be 4000 degrees Celsius. This difference makes the spots appear noticeably darker in contrast to the undisturbed photosphere. But why are they cooler?
The key to the spots lies “under the skin” to a certain extent. The Sun is a balloon of gas. At its centre lies a fusion reactor, which continuously converts hydrogen to helium at temperatures of roughly 15 million degrees. In the process, the solar power plant generates energy of 380 sextillion kilowatts every hour. Two mechanisms transport it to the surface: radiation and convection.
In the outer convection zone – which does not even make up a quarter of the Sun's radius – hot plasma bubbles rise into the photosphere at a speed of more than 3000 kilometres per hour, where they cool down and sink again a few minutes later. This permanent bubbling and seething gives the photosphere its granular texture. The individual grains, called granules, have a diameter of up to 1500 kilometres.
Due to the Earth's ever-turbulent atmosphere, the granular "sun sand" appears more or less clearly in ground-based telescopes. Freezing an image is a challenge for any astrophotographer. Balloon or satellite telescopes provide the best results outside the disturbing influence of the Earth's atmosphere. The photographs reveal a network of mostly angular granules, which are separated by narrow dark spaces. Time-lapse films show that the network is continuously renewed.
However, not only is the photosphere in motion, hot matter also circulates in the core of the Sun. This plasma – gas, which partially or completely consists of ions and electrons – is electrically conductive. Like every star, the Sun has a magnetic field that extends approximately 200000 kilometres below its surface. When plasma rises upwards due to convection, it pulls the magnetic field lines – like a teaspoon that has been dipped into honey and is then brought to the mouth. The magnetic field lines prevent the granules from rising, which disturbs the energy supply of the photosphere. Here, where the bundled field lines finally break through the surface, it cools down: and a sunspot forms.
The photograph shows two such spots. Johann Hirzberger from the Max Planck Institute for Solar System Research produced the photo with the Swedish Solar Telescope (SST) at the Roque de los Muchachos Observatory in La Palma. With an opening of 98 centimetres, the SST is the second largest refracting telescope in the world. The telescope tube is evacuated in order to prevent air turbulences, which would impair the resolution. Furthermore, adaptive optics compensate for the ever-present shimmer in the Earth's atmosphere. The instrument analyses the image of the Sun 1000 times per second and the optics adjust accordingly. In this way, the Swedish Solar Telescope provides high-quality sharp images, comparable with balloon or space-based observatories.
In the photograph shown here, bright thread-shaped structures can be recognised at the edges of the sunspots. These should actually appear dark as well, because the magnetic fields would have to be strong enough to prevent the reinforcement and cool the region. However, researchers working with Hirzberger have proven that the local magnetic field is loosened in places. The plasma circulates and causes sustained luminous structures, which appear to rotate on their axes.
Spots frequently emerge in groups. Their number is a measurement for solar activity – and not always the same: On average, every eleven years the star suffers, so to speak, from chickenpox – when many spots cover its gleaming face. The last time that this was the case was in 2001. Currently, the number of spots is increasing significantly once again. However, the Sun only appears to gather momentum again slowly. The current maximum is getting off to a weak start.