How hot is the Sun?

In words, the Sun's surface is roughly ten thousand degrees Fahrenheit, nearly 50 times hotter than boiling water on the Fahrenheit scale. But "how hot is the Sun" has more than one right answer, because the Sun is not one temperature. It's a layered ball of plasma that runs from a 27-million-degree core to a surface cool enough (by star standards) to have dark spots, wrapped in an atmosphere that mysteriously heats back up to millions of degrees.

How hot is the Sun in Fahrenheit, Celsius and Kelvin?

Here are the Sun's key temperatures in all three units. In the United States most people search in degrees Fahrenheit; scientists use kelvin (K), which starts at absolute zero, so it has no negative numbers.

The precise surface figure, 5,772 K, is the Sun's official "effective temperature" adopted by the International Astronomical Union. Converted, that's 5,499°C or 9,930°F, which is why you'll usually see it rounded to 5,500°C or 10,000°F. When numbers reach the millions, Celsius and kelvin are practically the same, since they differ by only 273 degrees.

How hot is each layer of the Sun?

From the center out, the Sun has three inner zones (core, radiative zone, convection zone), a visible surface (the photosphere), and a three-part atmosphere (chromosphere, transition region, corona). Temperature falls steadily from the core to the surface, then does something unexpected on the way out. You can tap each layer in the cutaway at the top of the page.

The core: the hottest steady place in the solar system About 27 million°F · 15 million°C · 15.7 million K

The Sun's core, the inner quarter of its radius, is where its energy is made. Here gravity squeezes the gas to about 150 times the density of water (roughly 13 times denser than lead) at pressures hundreds of billions of times Earth's air pressure. At about 15 million°C, hydrogen nuclei slam together hard enough to fuse into helium. At the very center, solar models put the temperature near 15.7 million K, just over 28 million°F. Even though it's that hot, the core isn't a single "flame": temperature drops as you move outward, reaching about 7 million K at its edge.

The radiative zone About 7 million K falling to 2 million K

Above the core, energy travels outward as light, but the plasma is so dense that each photon is absorbed and re-emitted in a random direction almost immediately. Temperatures fall from roughly 7 million K (12.6 million°F) at the bottom to about 2 million K (3.6 million°F) at the top, 70% of the way to the surface.

The convection zone About 2 million K falling to 5,800 K

In the outer 30% of the Sun, roughly 125,000 miles (200,000 km) deep, the gas is cool and opaque enough that heat moves by boiling. Giant columns of hot plasma rise, cool at the top, and sink again, like a pot of simmering soup. The tops of these currents are visible as "granules" on the surface, each about the size of Texas, lasting 8 to 20 minutes. People sometimes ask how hot the Sun's "mantle" or "outer core" is; the Sun doesn't have those Earth-style layers, but the convection zone is the closest match to a mantle.

The photosphere: the Sun's surface About 10,000°F · 5,500°C · 5,772 K

The photosphere is the visible surface of the Sun, the first layer you see, and the answer most people want when they ask how hot the Sun's surface is. It's not solid. It is the depth where the Sun's plasma becomes transparent enough for light to escape straight into space, a layer only a few hundred miles thick. Its bottom is around 6,400 K and its top around 4,400 K, averaging to the 5,772 K figure. In Fahrenheit, the Sun's surface temperature is about 10,000°F; in Celsius, about 5,500°C.

Sunspots are cooler patches, about 3,800 K (6,400°F), where tangled magnetic fields block the heat rising from below. They look black only because the surrounding surface is so much brighter. On their own, they would glow a brilliant orange.

The chromosphere About 4,100 K rising to 20,000 K

Just above the surface, temperature reaches its lowest point, about 4,100 K (6,900°F), then begins to climb. The chromosphere ("color sphere") rises about 1,200 miles (2,000 km) and heats to roughly 20,000 K (35,500°F). It glows pink-red from hydrogen and is visible for a few seconds at the edges of a total solar eclipse. Solar prominences, huge loops of plasma anchored here, are relatively cool and dense at about 7,500 to 10,000 K (13,000 to 17,500°F) even while they float in the million-degree corona.

The transition region 20,000 K jumping to about 1 million K

In a thin, ragged boundary layer, the temperature leaps by a factor of about 50 over a short distance. This is where the Sun's upper atmosphere goes from warm to extraordinarily hot.

The corona: the Sun's outer atmosphere 1–3 million°C · 1.8–5.4 million°F

The corona is the pearly white halo seen during a total solar eclipse, like the one millions of Americans watched on April 8, 2024. Its plasma is typically 1 to 3 million K, hotter over active, sunspot-covered regions. The outer corona gradually flows away as the solar wind, which is still around 100,000 K (about 180,000°F) when it passes Earth. Solar flares, the Sun's biggest explosions, can heat plasma to 10 to 20 million K and sometimes more, briefly making them the hottest spots on the Sun, hotter even than the core.

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

If you walk away from a campfire, you get colder. The Sun seems to break that rule: its atmosphere gets hotter the farther you go from the surface, jumping from about 6,000 K to over a million. This is called the coronal heating problem, and it's one of the biggest open questions in astronomy.

Scientists have two main suspects, and both involve magnetism. In the nanoflare idea, magnetic field lines rooted in the churning surface get twisted and braided until they snap and reconnect, releasing energy in millions of tiny explosions. In the wave-heating idea, magnetic waves (called Alfvén waves) carry energy up from below and dump it into the corona. The real answer may be both. NASA's Parker Solar Probe and the European Space Agency's Solar Orbiter are now flying close enough to measure the corona directly; Parker found sudden kinks in the magnetic field, nicknamed "switchbacks," and Solar Orbiter spotted countless miniature flares nicknamed "campfires."

Why doesn't the corona blind us or scorch the planets? Because it's incredibly thin, billions of times less dense than the air you breathe. High temperature measures how fast particles are moving, not how much heat they can deliver. We explain this, with a distance calculator, on the page how hot is the Sun in space.

How hot is the Sun compared to lava, lightning, fire and Earth's core?

Temperatures on the Sun span such a huge range that the chart below uses a logarithmic scale: each gridline is ten times hotter than the last.

• Sun vs. boiling water: water boils at 212°F (100°C). The Sun's surface is about 47 times hotter in Fahrenheit, and about 15 times hotter in kelvin, the scale that measures true thermal energy.
• Sun vs. fire: a wood campfire reaches roughly 1,100 to 2,000°F (600 to 1,100°C). The Sun's surface is about 5 to 9 times hotter in Fahrenheit, and its core is more than ten thousand times hotter. A candle flame's hottest zone is about 2,500°F (1,400°C).
• Sun vs. lava and volcanoes: erupting lava runs 1,300 to 2,200°F (700 to 1,200°C). The Sun's surface is roughly 4 to 6 times hotter in kelvin.
• Sun vs. Earth's core: Earth's inner core is estimated at about 9,400°F (5,200°C), almost as hot as the Sun's surface. The Sun's core, though, is about 3,000 times hotter.
• Sun vs. lightning: a lightning bolt heats the air around it to about 50,000°F (27,700°C), roughly five times hotter than the Sun's surface, according to NOAA. It lasts only microseconds, while the Sun's core has been at 15 million°C for billions of years.
• Sun vs. welding: an oxy-acetylene torch burns at about 6,000°F (3,300°C), and the core of an electric welding arc can reach temperatures comparable to or above the Sun's surface, which is why welders need dark shields.
• Sun vs. Earth: Earth's average surface temperature is about 59°F (15°C). Its record high is 134°F, set in Death Valley, California, in 1913.

Why is the Sun so hot?

The short answer: gravity and nuclear fusion. The Sun contains 99.8% of all the mass in the solar system. Its own weight crushes its center to extreme density and temperature, and at about 15 million°C hydrogen nuclei (protons) move fast enough to fuse. Strictly by classical physics even that isn't hot enough for protons to overcome their electric repulsion; fusion happens anyway thanks to a quantum effect called tunneling.

That energy output is staggering. The Sun radiates about 3.8 × 10²⁶ watts, which means 3.8 × 10²⁶ joules every second. (Joules and watts measure energy and power, not temperature, but they explain why the Sun stays so hot.) Earth intercepts less than one two-billionth of that, and it's still enough to power nearly all life on the planet. The Sun is a star, not a planet, and a fairly ordinary one: a G-type main-sequence star about 4.6 billion years old.

How long does it take the Sun's heat to reach us?

Energy made in the core takes a surprisingly long time to escape. In the radiative zone, a photon travels only a tiny distance before being absorbed and re-emitted in a random direction. NASA estimates the resulting zig-zag trip takes anywhere from about 10,000 to 170,000 years. Convection then carries the energy the last stretch to the surface in weeks to months. Once it leaves the surface, light crosses the 93 million miles to Earth in just 8 minutes and 20 seconds.

How do scientists know how hot the Sun is?

No thermometer has touched the Sun, yet its temperatures are known with surprising precision. Four independent methods agree.

1. Its color and spectrum. Hot objects glow with a characteristic spread of colors. Sunlight peaks at a wavelength near 500 nanometers, and by Wien's law that corresponds to about 5,800 K.
2. Its total brightness and size. Measure how much energy reaches Earth (1,361 watts per square meter above the atmosphere), scale it up to the whole Sun, divide by its surface area, and the Stefan–Boltzmann law returns 5,772 K.
3. Spectral lines. Atoms stripped of many electrons, such as iron missing 13 of its 26, can only exist at millions of degrees. Spotting their light in the corona proved its temperature in the 1940s.
4. Sound waves and neutrinos for the core. The Sun rings like a bell with sound waves that satellites measure (helioseismology), revealing temperature deep inside. Neutrinos from fusion stream straight out of the core and reach Earth in about eight minutes; how many arrive is extremely sensitive to the core's temperature. In 2020 the Borexino detector in Italy even caught neutrinos from the Sun's rarer CNO fusion cycle.

What color is the Sun, and how hot is it compared to other stars?

The Sun is white. Its light looks yellow or orange from the ground because air scatters away some blue light, especially at sunrise and sunset, but astronauts in space see a brilliant white Sun. A star's color reveals its temperature: cooler stars glow red-orange, hotter ones blue-white.

• Blue stars ("blue suns") are the hottest common stars, with surfaces of 10,000 K to over 40,000 K (18,000 to 72,000°F). Rigel, in Orion, is about 12,000 K.
• Red stars are the coolest, about 2,500 to 3,900 K. Red dwarfs like Proxima Centauri, the nearest star to the Sun, are around 3,000 K.
• The biggest stars aren't the hottest. UY Scuti, one of the largest stars known, is roughly 900 to 1,700 times wider than the Sun (estimates vary) but only about 3,400 K at its surface, cooler than our Sun.
• The hottest stars in the universe include Wolf–Rayet stars such as WR 102, at around 210,000 K. There are no green or purple stars: a star that peaks in green light still looks white to our eyes.

How hot was the Sun in the past, and how hot will it get?

• When it formed 4.6 billion years ago, the Sun "switched on" once its core reached about 10 million K and fusion began. Before that, it was a collapsing cloud heated only by gravity.
• 4 to 4.5 billion years ago the young Sun was about 30% dimmer than today, with a slightly cooler surface and core. This is the "faint young Sun" puzzle, since early Earth still had liquid water.
• Is the Sun getting hotter each year? Yes, but imperceptibly. It brightens by roughly 1% every 100 million years as helium builds up in the core. Year to year, that change is far too small to measure, and it's not a factor in recent climate change.
• In about 1 billion years the Sun will be about 10% brighter, likely enough to boil away Earth's oceans.
• In about 5 billion years the Sun will run out of hydrogen in its core and swell into a red giant. Its surface will cool to around 3,000 to 4,000 K and glow orange-red, while its core heats to about 100 million K, hot enough to fuse helium into carbon.
• As a dying Sun, it will shed its outer layers and leave behind a white dwarf about the size of Earth, starting out at over 100,000 K and then cooling for trillions of years.

Is the Sun hotter today, in summer, or in Florida?

The Sun's temperature doesn't change from day to day. Right now, today, and tomorrow, its surface is about 10,000°F (5,500°C), in the morning, at noon, and at night, when your side of Earth simply faces away from it. What changes is how much of its energy reaches you:

• Sun angle. In summer, and in places like Florida, Hawaii, Jamaica, Dubai or the Philippines, the Sun sits higher in the sky, so its light hits the ground more directly and passes through less atmosphere. That's why summer, and the tropics, feel hotter.
• Distance barely matters. Earth is actually closest to the Sun in early January (about 91.4 million miles) and farthest in early July (about 94.5 million miles). Seasons come from Earth's 23.4° tilt, not distance.
• The solar cycle. Sunspot activity rises and falls about every 11 years. NASA and NOAA announced in October 2024 that the Sun had reached solar maximum in the current cycle, Solar Cycle 25, yet the Sun's total energy output varies by only about 0.1% across a cycle.
• UV index isn't temperature. The daily UV index measures ultraviolet light reaching the ground, which causes sunburn and tanning. It depends on sun angle, clouds and ozone, not on how hot the Sun is.

Sunlight itself doesn't have a temperature until it's absorbed; its colors match a 5,772 K glow. Mirrors and lenses can concentrate it (France's giant solar furnace at Odeillo reaches about 3,500°C), but the laws of thermodynamics say no "solar death ray" can ever heat something hotter than the Sun's own surface.