R I V U X G This ultraviolet image of the Sun, taken in 2010 by the Solar Dynamics Observatory, shows a prominence (upper left) in which magnetic fields carry gases above the Sun’s surface in a loop that can extend upward hundreds of thousands of kilometers.

(NASA/GSFC/AIA)

The Sun is by far the brightest object in the sky. By earthly standards, the temperature of its glowing surface is remarkably high, about 5800 K, but there are regions of the Sun that reach far higher temperatures of tens of thousands or even millions of kelvins. Gases at such temperatures emit ultraviolet light or X-rays, which makes the Sun’s outer regions prominent in the accompanying image from an ultraviolet telescope in space. Far from being smooth and placid, the Sun is a dynamic object with many features. Some of the hottest and most energetic regions on the Sun spawn immense disturbances and can propel solar material across space to reach Earth and other planets.

In recent decades, we have learned that the Sun shines because at its core hundreds of millions of tons of hydrogen are converted to helium every second. We have confirmed this picture by detecting the by-products of this transmutation—strange, ethereal particles called neutrinos—streaming outward from the Sun into space. We have discovered that the Sun has a surprisingly violent atmosphere, with a host of features such as sunspots whose numbers rise and fall on a predictable 11-year cycle. By studying the Sun’s vibrations, we have begun to probe beneath its surface into hitherto unexplored realms. And we have just begun to investigate how changes in the Sun’s activity can affect Earth’s environment as well as our technological society.