Teachers should be aware of Galileo’s dilemma about the nature of sunspots, the later discovered magnetic nature of sunspots as well as the differential (non-rigid) rotation of the Sun.
Here we provide a brief overview of the structure of the Sun, Galileo’s historical observations, the nature of sunspots, the non-rigid rotation of the Sun and an introduction to the satellite mission that produced the data used in this activity. At the end of each subsection, we provided links where teachers can read more about these topics.
The composition of the Sun
The Sun is a giant gaseous ball composed of mostly hydrogen and helium. Due to the extremely high temperatures in the Sun, the electrons can detach from their atom’s nuclei and are free to move. This state of matter is called “plasma”. Since the Sun’s matter is charged, it can interact with magnetic fields.
The Sun can be divided into 5 layers: the core, the radiative zone, the convective zone, the photosphere, and the atmosphere. The core is the innermost layer and it is the place where energy is produced by nuclear fusion (~ 15 million °C). The radiative zone extends from the core to about 70% of the solar radius and here the energy is transported mainly through radiation (photons are emitted, absorbed and re-emitted continuously). In the next layer, the convective zone, energy is transported by convection (upward movement of hot matter and downward movement of cold matter, similar to the boiling of a soup). The photosphere, at ~ 6000 °C, sits right above the convective zone. Since it is the layer from which most of the light comes, we call it the solar surface, although we would not be able to stand on it.
Beyond the photosphere we find the solar atmosphere, which is composed of two other layers: the chromosphere and the corona. The chromosphere is a thin reddish gaseous layer immediately above the surface. The corona is the Sun’s very thin plasma atmosphere, extending millions of kilometres into space.
Visit NASA Solar Science website for more information.
Galileo’s sunspot observations
In 1612 Galileo Galilei pointed a telescope at the Sun. He was one of the first to do this, preceded by Thomas Harriott and Johannes Fabricius. Galileo knew that if he looked directly through the telescope, he could burn his eye. Instead, he projected the image on a screen to make careful drawings. In Galileo’s time people believed that the Sun was a still, perfectly immaculate object. To his great surprise, he saw dark spots on the Sun. He was very intrigued by the nature of these spots, and therefore he observed and drew them on a daily basis to study them.
Visit Rice University Galileo webite for more information.
The magnetic nature of sunspots
The nature of sunspots remained an enigma until 1905, when the astronomer George Ellery Hale detected intense magnetic fields within these dark regions. Using a spectroheliograph he found that a certain property of the light (polarisation) emitted by the Sun was altered in a way that is specifically caused by magnetic fields. Today, satellites like Solar Dynamics Observatory (SDO) are equipped with special instruments to detect the location of magnetic fields on the Sun and infer their intensity. Figure 1 shows two images of the same day, obtained by SDO: a visible-light image of the whole solar disc and a map of the orientation and intensity of the magnetic fields present on the solar disc (magnetogram).
Sunspots are seen in the photosphere as dark features in contrast to the rest of the solar surface, because the matter within them is about 2000 °C cooler than their surroundings at ~ 6000 °C. The intense magnetic fields are responsible for this cooling. Since magnetic fields produce pressure, plasma inside sunspots is forced out to maintain pressure equilibrium between the sunspot (gas pressure plus magnetic pressure) and the surrounding plasma (gas pressure). Therefore the plasma inside the sunspot is less dense and a little cooler (if we compare the inside and outside of a sunspot at the same geometrical depth).
Sunspots usually clump together in groups and have lifetimes between several days and weeks. Sunspots are dynamic and evolve together with the magnetic field: they appear, change, disappear. Their number varies periodically with time together with the amount of magnetic field of the Sun, following the so-called 11-year sunspot cycle: every 11 years, the sunspot number and the amount of magnetic field reach a maximum (called “solar maximum”), followed by a minimum with barely no spots on the Sun. The dataset proposed in this activity is chosen close to the solar maximum, in order to display a large number of sunspots.
Sunspots are found in patches like storms on Earth, and are usually located in bands in both the northern and southern hemispheres. The bands that sunspots form in, move from mid latitudes to almost the equator throughout the 11-year sunspot cycle. Note that individual sunspots do not drift much in latitude since they only exist for a few weeks - just the latitudes where new spots form move towards the equator.
Visit Solar Center website for more information on solar magnetograms.
Visit Solar Dynamics Observatory website for more information on visible light images of the Sun.
The rotation of the Sun
Like the Earth, the Sun has a north pole and a south pole, and rotates around its axis. As seen from the Earth the Sun rotates about its axis once every about 27 days. The Sun’s equator is almost in the plane of the Earth’s orbit, and the Sun’s north pole is in the same direction as the Earth’s north pole. Seen from above the solar north pole, the Sun rotates counter-clockwise. Most modern images of the Sun are oriented so that the solar north is up and therefore features on the Sun’s surface appear to move from left to right as the Sun rotates. Note that Galileo’s drawings of sunspots (Fig. 2) are not oriented this way.
The non-rigid rotation of the Sun
Rigid objects do not change shape (i.e. they are non-deformable). Therefore, when rigid objects spin every part rotates at the same rhythm. This means that every part of the object takes the same amount of time to complete a turn. This is called rigid rotation. This is the reason why every spot on Earth takes 24 hours to complete a turn.
In non-rigid objects, i.e. deformable objects, rotation is different in different parts of the object. This is the case of the Sun, since it is made up of a gaseous matter called plasma. Like the Earth, the Sun has a North pole and a South pole, and rotates around its axis. However, the Sun’s plasma near the equator completes a full turn in a little less than 27 days, whereas plasma near the poles can complete a full turn in as much as 35 days. This means that plasma can rotate at different speeds, depending on the latitude they are at: i.e. faster at the equator than at the poles. This is called differential rotation.
If you measure the Earth’s rotation by measuring winds or the motion of clouds, you will find that the rotation of the Earth’s atmosphere also varies with latitude. This is because the Earth’s atmosphere is a gas and not a solid. As seen from space, the atmosphere rotates in less than 24 hours at mid latitudes and in more than 24 hours near the equator. We call this the “Westerlies” and “Trade winds” respectively. Differential rotation is not a unique aspect of the Sun; it is common for rotating bodies such as other stars and gaseous planets to have different rotation rates at different latitudes.
Visit Swinburn University Cosmos website for more information.
The Solar Dynamics Observatory
The Solar Dynamics Observatory (SDO) is a satellite mission from NASA. It was launched into an orbit around the Earth in 2010 and it has been observing the Sun since then. SDO’s main goal is to study the solar atmosphere to understand better the relationship between the solar magnetic fields and energetic, short-term phenomena such as solar flares and coronal mass ejections.
Visit Solar Dynamics Observatory website for more information.
For more information about the solar cycle, see the NASA Solar Science website and YouTube video