How hot is the Sun and why is the corona hotter than the surface?

The Sun's photosphere (the visible surface) is about 5,500 °C. The corona — the wispy outer atmosphere that becomes visible during a total solar eclipse — reaches over 1 million °C. This counterintuitive result (the atmosphere hotter than the surface) has puzzled scientists for decades. The leading explanation involves magnetic waves (Alfvén waves) and magnetic reconnection events that deposit energy from the Sun's magnetic field into the corona. NASA's Solar Dynamics Observatory studies this continuously.

What is the solar cycle?

The Sun's magnetic field flips polarity approximately every 11 years, marking a solar cycle. During solar maximum, the magnetic field is tangled and active: sunspots, solar flares, and coronal mass ejections are more frequent. During solar minimum, activity quietens. We are currently in Solar Cycle 25 (began December 2019), which reached solar maximum in 2025. Activity during Cycle 25 exceeded predictions. The solar cycle affects space weather, aurora visibility, and long-distance radio communications. More on the effects in our solar wind and magnetosphere guide.

Is it safe to look at the Sun through a telescope?

Never look at the Sun through a telescope or binoculars without a proper solar filter — even a fraction of a second can cause permanent blindness. For visual observation, use only certified solar filter glasses (ISO 12312-2) for naked-eye observation, or a proper front-mounted solar filter for telescopes. White-light solar filters show sunspots and solar granulation. Hydrogen-alpha (Hα) filters — available for amateur telescopes — reveal prominences and flares in spectacular detail. During a total solar eclipse, it is safe to look with naked eyes only during the brief totality phase. See NASA's solar observing safety guide.

What will happen to the Sun when it dies?

In about 5 billion years the Sun will exhaust its core hydrogen. It will then expand into a red giant — swelling to perhaps 200 times its current radius, engulfing Mercury and Venus and possibly Earth. After the red giant phase, the Sun will shed its outer layers as a planetary nebula and leave behind a white dwarf — an Earth-sized, incredibly dense, slowly cooling remnant about half the Sun's current mass. This endpoint is typical for stars below about 8 solar masses. See the full sequence in our stellar life cycle guide.

The Sun — Our Star Explained

Structure of the Sun

The Sun is not a uniform ball of gas — it has distinct layers, each with different properties and roles. At the centre, the core spans about 25% of the radius but contains 99% of the energy-producing nuclear reactions. Temperatures here reach 15 million °C and pressures are 340 billion times Earth's atmospheric pressure — enough to force hydrogen nuclei together to fuse.

Core (0–25% R☉) — nuclear fusion, 15 million °C

Radiative zone (25–70% R☉) — photons bounce for ~100,000 years

Convective zone (70–100% R☉) — hot plasma rises, cools, sinks

Photosphere — visible surface, 5,500 °C, sunspots form here

Chromosphere — 10,000–20,000 °C, pink during eclipses

Corona — over 1 million °C, extends millions of km

Observing the Sun safely

The Sun is the only star close enough to show surface detail from Earth. Sunspots — temporary dark patches caused by magnetic flux tubes inhibiting convection — can be tracked day to day with a solar-filtered telescope. Near solar maximum (2025 in Cycle 25), large active regions may be visible to the naked eye through a proper solar filter.

The most dramatic solar observing opportunity is a total solar eclipse, when the Moon exactly covers the Sun's disc, revealing the corona as a white halo stretching several solar diameters. The 2027 total solar eclipse crosses Spain, North Africa, and the Middle East on 2 August 2027 — the most favourable solar eclipse in years. For more on the solar wind and space weather that the Sun drives, see our solar wind guide.

The Sun

Structure of the Sun

Observing the Sun safely

Frequently asked questions