This will be discussed further below. The small dark regions are known as sunspots and were among the first astronomical discoveries made by Galileo, the first scientist to use a telescope to study the heavens. The fundamental cause of sunspots is the existence of localized, high-intensity magnetic fields in the solar photosphere, which otherwise has a very weak magnetic field of at most tens of Gauss. This magnetic pressure is in addition to the ordinary kinetic gas pressure, which is related to the temperature and density of the gas. As the sunspots are known to be stable for periods of weeks, there must be pressure equilibrium between the gas in the spot and that surrounding it.
More detailed discussions are available in the literature.
While the supposition of strong magnetic fields easily explains the darkness of the sunspots, more direct evidence of the magnetic fields would be welcome. We would expect, for example, that the Zeeman effect which you studied in a previous experiment should be visible in at least some of the solar absorption lines. The presence of the two O 2 lines nearby is, once again, a convenience. The difficulty of the observation requires us to check whether the observed splitting or broadening is physical, or merely an artifact of the observation process. The O 2 lines, arising as they do in the Earth's atmosphere, will not be subject to the Zeeman broadening we expect to see in the sunspots.