Strike a guitar string and it sounds. Along its length, certain points barely move; these are called nodes. The Sun has similar nodes: in the layer known as the chromosphere, waves can get trapped between two hot boundaries and oscillate in place, like a string.

Astronomers have long suspected that such standing waves should exist in dark spots on the Sun called pores. But direct observation proved elusive. Now, researchers used a large telescope and a special instrument that splits light into its constituent colors. They tracked temperature and velocity oscillations in the chromosphere of a pore at different heights.

They found that at a certain height, the temperature abruptly flips the phase of its oscillations, as if the wave had hit an invisible wall and reflected. At that same point, the oscillation power drops to nearly zero. That is the node of a standing wave. Its height varies: higher at the pore's center, lower toward the edges. Outside the pore, such nodes are often absent, because there the waves freely escape upward.

Now that we know where the node lies, we can measure chromospheric properties: how high the reflecting layer is and how fast sound travels. This is akin to determining a string's tension from its pitch. It turns out the Sun can be listened to as well.