How Hot Is the Sun?

How hot is each layer of the Sun?

The Sun is a huge ball of hydrogen and helium held together by its own gravity.^[1] It has no solid surface, because it is made of plasma, a super-hot, electrically charged gas.^[1] NASA divides it into inner regions (the core, the radiative zone and the convection zone) and outer layers (the photosphere, the chromosphere, the transition zone and the corona).^[1] The table below uses the rounded figures from NASA's Sun facts page.^[1]

The photosphere is much cooler than the core, but NASA notes that it is still hot enough to make carbon not just melt but boil.^[1]

What are the Sun's temperatures in kelvin?

NASA's Sun Fact Sheet gives a model temperature at the center of the Sun of 1.571 × 10⁷ K, which works out to about 15.7 million kelvin.^[3] It lists the Sun's effective temperature as 5,772 K, with about 6,600 K at the bottom of the photosphere and 4,400 K at the top.^[3] A NASA page on the Sun's layers, which credits the National Solar Observatory, gives slightly different values: about 6,500 K at the bottom of the photosphere and 4,000 K at the top.^[2]

Above the photosphere, the temperature climbs again.^[2] In the chromosphere it rises from about 4,000 K at the bottom to about 8,000 K at the top.^[2] In the transition region, a layer only about 100 km (60 miles) thick, it jumps from about 8,000 K to about 500,000 K.^[2] The same page puts the corona at 500,000 K or more, up to a few million K.^[2]

NASA pages describe the corona's heat differently: the Sun facts page says it reaches up to 2 million °C (3.5 million °F), while NASA's Parker Solar Probe pages say coronal temperatures spike upward of 2 million °F, which works out to about 1.1 million °C.^[1][6]

Why is the corona hotter than the Sun's surface?

NASA calls this one of the Sun's biggest mysteries: the corona is much hotter than the layers just below it, which is like walking away from a bonfire and getting warmer.^[1] Scientists call it the coronal heating problem, and a 2018 NASA article described it as one of the greatest unanswered questions in astrophysics.^[4]

The puzzle began with a green spectral line observed during a total solar eclipse in 1869.^[4] It matched no element known on Earth, so scientists thought they might have found a new one and called it coronium.^[4] About 70 years later, a Swedish physicist found that the line came from iron that had been ionized 13 times, leaving it with half the usual number of electrons.^[4] Such heavy ionization required a corona of around 2 million °F, nearly 200 times hotter than the surface.^[4]

The Sun's outer layers constantly churn: huge cells of plasma move like bubbles in a pot of boiling water, creating tangled magnetic fields that reach far up into the corona.^[4] One idea is that the motion launches magnetic waves, called Alfvén waves, into the corona, where they set charged particles spinning and heat the atmosphere, a bit like ocean waves pushing surfers.^[4] Another is that bomb-like explosions called nanoflares release heat when twisted magnetic field lines snap.^[4] The two ideas are not mutually exclusive, and many scientists think both may be involved.^[4] NASA's Sun facts page still describes the source of coronal heating as a major unsolved puzzle.^[1]

What is Parker Solar Probe finding out?

NASA launched Parker Solar Probe on Aug. 12, 2018.^[6] Its goals are to trace the flow of energy, study the heating of the corona and explore what accelerates the solar wind.^[5] In 2021 it became the first spacecraft to fly through the corona, and on Dec. 14, 2021, NASA announced that it had sampled particles and magnetic fields there.^[5] On Dec. 24, 2024, the probe passed just 3.8 million miles (around 6.1 million kilometers) from the Sun's surface, closer than any human-made object had come to a star.^[7]

Getting close matters because the slow solar wind takes around four days to reach Earth, enough time for it to mix with other particles and lose its defining features.^[4] Two of the probe's instrument suites measure electric and magnetic fields and the hot plasma itself, so scientists can look for the distinct signature that each heating mechanism should leave.^[4] The Johns Hopkins Applied Physics Laboratory, which built and operates the spacecraft, says zig-zag magnetic structures called switchbacks, seen in abundance close to the Sun, hold key insight into how the plasma is heated and accelerated.^[7]

Where does the Sun's heat come from?

The Sun's heat and light are powered by nuclear reactions in the core, where hydrogen is fused into helium.^[1] The core is hot enough to sustain this fusion, which creates outward pressure that keeps the Sun's huge mass from collapsing.^[1] NASA explains that fusion happens when atoms are heated and squeezed so hard that their nuclei merge into a new element, a process that often creates a photon, a particle of light.^[8]

How long does the Sun's energy take to escape?

In the radiative zone, the energy is carried by radiation that is absorbed and given off again by atoms over and over.^[1][8] NASA's figures for this trip differ: its Sun facts page says the radiation takes about 170,000 years to get from the core to the top of the convection zone, while a NASA visualization first published in 2012 says a photon spends about 40,000 years being absorbed and re-emitted before it reaches the surface.^[1][8]

In the convection zone, large bubbles of hot plasma carry the energy up toward the photosphere.^[1] Once light leaves the surface, it crosses the 93 million miles (150 million kilometers) to Earth in about eight minutes.^[1][8]