The sun is a star at the center of our solar system, and its gravity holds the solar system together. We cannot dive into the sun to look around, so helioseismology is the only way to see inside it. The inside of the sun has three parts: the core, the radiative zone and the convection zone. Helioseismic imaging helps delineate the boundaries between the convective zone, radiative zone, and the core. The deeper radiative zone, including the core, rotates reasonably uniformly with a period of about twenty-seven days. At the equator the sun's surface spins around once about every 25 Earth days, but at its poles it rotates once every 36 Earth days, and it takes photons around 170,000 years to pass through the radiative zone, whose temperature is about seven million degrees Celsius at the base and two million at the top. The sun is not a solid ball, and you could not stand on its surface. It is about 100 times wider than Earth, and it would take 1.3 million Earths to fill it.

Several instruments watch the sun all the time. NASA's Solar Dynamics Observatory instruments measure the Doppler shifts due to oscillation velocities over the entire visible disk and make high-resolution measurements of the magnetic field over the entire visible disk, while ESA's Solar and Heliospheric Observatory, called SOHO, launched in 1995. SOHO and the Global Oscillations Network Group, called GONG, continuously monitor solar activity, and GONG has provided extensive observations of solar oscillations. People have tracked sunspots in detail since the invention of the telescope four centuries ago, and sunspots are dark, cooler spots on the sun, and they typically stretch 1,000 to 100,000 miles across. The number of sunspots goes up and down in a cycle of about 11 years. Never look directly at the sun without special eye protection made for looking at the sun.

Now let's look at the outside of the sun. Above the photosphere the chromosphere warms from about 6,000 to about 20,000 degrees Celsius, and across the thin transition region the plasma soars to nearly a million degrees. The photosphere, the sun's visible surface, is roughly 4,500 to 6,000 degrees Celsius, and NASA gives about 5,500. The corona, the outer part of the sun's atmosphere, is much hotter than the sun's surface. Scientists are still studying why the corona is hotter than the layers just below it. As of November 2026, Parker Solar Probe launched on Aug. 12, 2018. In 2021 it became the first spacecraft to fly through the corona, in what NASA calls the first time a spacecraft had touched the sun. On Dec. 24, 2024, it flew about 3.8 million miles above the sun's surface at about 430,000 miles per hour, a record for the closest approach to the sun by a spacecraft. Its 4.5-inch-thick carbon-composite shield can withstand temperatures reaching nearly 2,500 degrees Fahrenheit.

Watching matters because the sun's storms reach Earth. Solar eruptions can accelerate charged particles that cross roughly 93 million miles from the sun to Earth in about 30 minutes or less. High speed solar wind brings geomagnetic storms while slow speed wind brings calm space weather, and the effects also depend on the wind's density and magnetic field direction. The fastest coronal mass ejections aimed at Earth can reach it in as little as 15 to 18 hours. A fast coronal mass ejection drives a shock wave that accelerates charged particles ahead of it and can give 15 to 60 minutes of advance warning of shock arrival at Earth. Coronal mass ejections that reach Earth can induce electrical currents in power grids and temporarily heat Earth's upper atmosphere so it swells, increasing drag on some satellites.

Words to know

• helioseismology: study of oscillations
• chromosphere: layer above photosphere
• oscillations: back-and-forth motions
• corona: outer atmosphere