At the base of the convection zone is the tachocline, the boundary between the convective and radiative zones, and helioseismology shows a strong shear layer there where the dynamo operates. The sun generates magnetic fields that extend out into space to form the interplanetary magnetic field. A sunspot forms when intense magnetic activity in the convective zone ruptures a thermal column, and sunspots are areas of intense and complex magnetic fields. The number of sunspots goes up and down in a cycle of about 11 years.

The cycle has a visible peak. As of NASA and NOAA's October 2024 announcement, the sun had reached its solar maximum period, a period of the roughly 11-year cycle when the sun's magnetic poles flip, and the most powerful flare of the current solar cycle was an X9.0 on Oct. 3. Solar flares are classified by peak X-ray emission into classes A, B, C, M and X, and each class is ten times higher than the one before. Sunspots typically stretch 1,000 to 100,000 miles across, and people have tracked them in detail since the invention of the telescope four centuries ago.

Helioseismology is the only way to see inside the sun, since we cannot dive in to see inside, and the sun has been likened to a bell being struck continuously. SOHO and the Global Oscillations Network Group continuously monitor solar activity. NASA's Solar Dynamics Observatory carries an instrument called AIA, which makes images of the chromosphere and inner corona at several temperatures at a rapid cadence, including images of the sun in 10 wavelengths every 10 seconds. Helioseismology can also offer a glimpse of the farside of the sun, improving space weather forecasts.

The hottest puzzle is the atmosphere. 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 corona, the outer part of the sun's atmosphere, is much hotter than the sun's surface, and scientists are still studying why it is hotter than the layers just below it. As of November 2026, NASA's Parker Solar Probe, which launched on Aug. 12, 2018, became in 2021 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.

Coronal mass ejections travel at speeds from slower than 250 kilometers per second to nearly 3000 km/s, and the fastest Earth-directed ones arrive in 15-18 hours while slower ones take several days. 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. Under the right conditions, 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. Never look directly at the sun without special eye protection made for looking at the sun.

Cecilia Payne's 1925 thesis showed hydrogen was a million times more abundant than the other elements; the examiner Russell called this clearly impossible, and in the final thesis she wrote that the enormous abundance was almost certainly not real. Today about three quarters of the sun's mass is hydrogen, almost all of the rest is helium, and 1.69 percent is other gases and metals. Fusion happens only in the core, where protons of hydrogen atoms violently collide and fuse to create helium atoms. As of November 2026, the sun fuses around 620 million metric tons of hydrogen into helium every second. Neutrinos are more difficult to detect and account for only about two percent of the sun's total energy.

The sun is about 4.5 to 4.6 billion years old, and as of November 2026, scientists predict it is a little less than halfway through its lifetime. A Sun-like star ends as a red giant that blows away its outer layers as a planetary nebula, leaving only its core as a white dwarf. For now, one open question is still on the table: Solar Orbiter's science question includes what drives the sun's 11-year cycle of rising and subsiding magnetic activity.

Words to know

• tachocline: convection zone base
• dynamo: magnetic field maker
• helioseismology: study of oscillations
• corona: outer atmosphere