The Sun

The spots in this picture would move if we watched the Sun over successive days. Following sunspots across the disk gave astronomers a way to detect the Sun’s rotation. The movement also reveals that different parts turn at different rates. The equatorial region goes around faster than the poles. That is possible because the Sun is made of gas and plasma, with no rigid surface holding it to one pace.

Light also carries evidence of what a star contains. In her 1925 doctoral thesis, Cecilia Payne used stellar spectra and the physics of ionization to study the abundance of elements. Her results pointed to enormous quantities of hydrogen and helium. That challenged the prevailing idea that stars had a composition much like Earth’s. The astronomer Henry Norris Russell urged caution, and Payne qualified the conclusion in her thesis. Four years later, Russell’s own work supported it. The bright disk above is the visible layer of a star made mostly of those two elements.

The source of the light lies much deeper than the layer we can see. In the core, at about 15 million degrees Celsius, nuclear reactions fuse hydrogen into helium and release energy. Energy moves outward through the radiative zone and then the convection zone before reaching the photosphere. The visible surface shown here is about 5,500 degrees Celsius. It is the place from which much of the visible light escapes, far above the core that powers it.

The faint corona brought another surprise. Its spectrum contained bright lines that did not match known materials, so astronomers proposed an element called coronium. The explanation turned out to be extreme heat. At temperatures above a million degrees Celsius, atoms lose many of their electrons. Heavier elements, including iron, can still retain a few, and those highly ionized atoms produce the unfamiliar lines. The delicate glow around the disk comes from gas hotter than the visible surface beneath it.

For a long time, the corona was studied from a distance. On April 28, 2021, Parker Solar Probe passed through a boundary where measurements showed that it had entered the magnetically dominated solar atmosphere. It sampled particles and magnetic fields directly. The phrase ‘touching the Sun’ describes that passage through the corona. There was no solid ground to land on. Farther out, escaping coronal material becomes the solar wind, carrying the Sun’s influence through the region we have been exploring.