# How hot is the Sun? > How hot the Sun's surface, core and corona are, why the corona is hotter than the surface, and how much of the Sun's energy reaches Earth. HTML: https://www.theuniversecounts.com/solar-system/the-sun/how-hot-is-the-sun Numbers as of 2026-10-31 03:45 UTC. Live ones keep changing on the HTML page. The Sun's visible surface glows at 5,499 °C. Its core, where its energy is made, is about 15.7 million degrees Celsius. Strangest of all, its thin outer atmosphere, the corona, is far hotter than the surface below it: between 1 million and 3 million degrees. Nobody yet knows for certain why. All that heat leaves the Sun as light. The small share that Earth catches carries about 9,300 times the power humanity uses. In under an hour, sunlight brings Earth more energy than all of us use in a year. - The Sun's X-ray brightness at Earth (GOES, 0.1–0.8 nm): B1.9 - Solar wind speed measured upstream of Earth: 352 km/s - Time since the last major solar flare (class M or X): 12 days - Sunspot number, monthly mean (latest month): 60.3 ## Where the heat begins What is the Sun? The Sun is a star: a huge ball of hot gas, mostly hydrogen and helium, held together by its own gravity. Its heat starts at the very centre. There, gravity squeezes the gas so hard that hydrogen fuses into helium. Each fusion turns a tiny amount of mass into energy. The core is about 15.7 million degrees Celsius. Nobody can measure it directly. The number comes from careful models of the Sun's interior, which agree closely with what we can measure from outside. From the core, the energy works its way slowly outward. First it travels as light, bouncing from particle to particle. Farther out, great bubbles of hot gas carry it upward, the way water rolls in a boiling pot. At last it reaches the surface and escapes into space. ## The surface we see The photosphere The Sun has no solid surface. What we see is the photosphere, the layer where its light finally escapes into space. Its temperature is 5,499 °C. NASA notes that this is hot enough to make carbon, the stuff of diamonds and graphite, not just melt but boil. That number is the Sun's effective temperature: the temperature that matches the light it gives off. The layer itself is hotter at the bottom than at the top, so it is the honest single number for the surface. Sunspots are patches where strong magnetic fields push up through the surface. They are slightly cooler than the gas around them, so they look dark beside it. ## A mystery above the surface The corona Above the surface lies the corona, the Sun's thin outer atmosphere. We see it with our own eyes only during a total eclipse, as a white crown around the black Moon. It is far hotter than the surface beneath it: between 1 million and 3 million degrees, depending on where and when we measure. That should not happen. The Sun's energy comes from its core, and things usually get cooler the farther they are from the fire. It is as if the air grew hotter as you walked away from a campfire. Scientists call it the coronal heating problem, and it has puzzled them for generations. The leading ideas both involve the Sun's tangled magnetic field. Waves running up the field may carry energy into the corona. Or countless small explosions, called nanoflares, may heat it as field lines snap and reconnect. Many scientists think both play a part. NASA's Parker Solar Probe now flies through the corona itself. With ESA's Solar Orbiter, it has found strong signs that turbulence in the field does much of the work. The question is closing, but it is not closed. ## Flares, sudden bursts of energy Solar flares Sometimes, above a group of sunspots, the twisted magnetic field snaps into a calmer shape. It releases its energy in minutes, as light and fast-moving particles. That burst is a solar flare. Flares shine brightest in X-rays, and NOAA's GOES satellites measure the Sun's X-rays every minute. Flares are graded by letter, A, B, C, M and X, and each letter means ten times the X-ray brightness of the one before. Right now the X-ray level is B1.9. The last flare of class M or X began on 19 October 2026. A flare's light travels at the speed of light, so it reaches us 8.3 minutes after it happens. A strong one can black out shortwave radio on the daylit side of Earth. Big flares are most common near the peak of the solar cycle. ## The heat that reaches us Back to Earth Almost all the Sun's energy leaves as light. Right now it crosses to us in 8.3 minutes. Earth catches only the sliver that falls on the disc our planet turns towards the Sun. Even so, that sunlight carries about 9,300 times as much power as humanity uses: every fuel we burn and all the electricity we make, together. Put another way, sunlight brings Earth a whole year of humanity's energy in under an hour. Since you opened this page, it has brought Earth [counts up live on the page] joules. Part of that light bounces straight back to space. The rest warms the oceans and the air, drives the winds and the rain, and feeds every green leaf. Most life on Earth runs on it. ## FAQ ### How hot is the Sun? Its visible surface is 5,499 °C. The core is about 15.7 million degrees Celsius, and the outer atmosphere, the corona, is between 1 million and 3 million degrees. ### How hot is the Sun's core? About 15.7 million degrees Celsius. That figure comes from models of the Sun's interior, because nothing can see the core directly. It is hot enough, and dense enough, for hydrogen to fuse into helium. ### Why is the Sun's corona hotter than its surface? Nobody knows for certain yet; it is called the coronal heating problem. Most scientists think the Sun's magnetic field carries the energy up, through waves, small explosions called nanoflares, or turbulence. Parker Solar Probe and Solar Orbiter are testing which. ### What is the hottest part of the Sun? The core, at about 15.7 million degrees Celsius. Next comes the corona, the outer atmosphere, at between 1 million and 3 million degrees. The visible surface, at 5,499 °C, is cool by comparison. ### How much of the Sun's energy reaches Earth? Only a tiny share of the Sun's light falls on Earth. Even so, it carries about 9,300 times the power humanity uses. In under an hour, it brings more energy than we use in a year. ### Is the Sun on fire? No. Fire is a chemical reaction that needs oxygen. The Sun shines because of nuclear fusion in its core, where hydrogen turns into helium and a little mass becomes energy. ## Related - [What is the Sun doing right now?](https://www.theuniversecounts.com/solar-system/the-sun/what-is-the-sun-doing-now.md) - [How big is the Sun?](https://www.theuniversecounts.com/solar-system/the-sun/how-big-is-the-sun.md) - [Geomagnetic storms](https://www.theuniversecounts.com/solar-system/space-weather/geomagnetic-storms.md) - [The solar cycle](https://www.theuniversecounts.com/solar-system/the-sun/solar-cycle.md) The warmth on your face on a sunny day left the Sun only minutes ago, and began as fusion in its core. What does it change to know that? ## Sources - Diameter of a regulation football (size 5) (Measured, with error): [The IFAB: Laws of the Game, Law 2 – The Ball](https://www.theifab.com/laws/latest/the-ball/) - Earth's circumference at the equator (Measured): [NASA NSSDCA: Earth Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html) - Earth's diameter at the equator (Measured): [NASA NSSDCA: Earth Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html) - Great-circle distance between London and Sydney (Measured, with error): [NASA NSSDCA: Earth Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html) - Humanity's average rate of energy use (total energy supply, 2024) (Measured, with error): [Energy Institute: Statistical Review of World Energy 2025, Key highlights](https://www.energyinst.org/__data/assets/pdf_file/0007/1658077/Statistical-Review-of-World-Energy.pdf) - Planetary K-index (Kp), latest 3-hour value (Measured, with error): [NOAA SWPC: Planetary K-index](https://www.swpc.noaa.gov/products/planetary-k-index); [NOAA SWPC: NOAA Space Weather Scales](https://www.swpc.noaa.gov/noaa-scales-explanation) - Solar wind speed measured upstream of Earth (Measured): [NOAA SWPC: Solar Wind Data and Display Changes (30 June 2026)](https://www.spaceweather.gov/news/solar-wind-data-and-display-changes); [NOAA SWPC: SWPC home-page summary bar script (shows this file as "Solar Wind Speed ... km/sec")](https://www.swpc.noaa.gov/modules/custom/swpc_summary/swpc_summary.js); [The Astrophysical Journal: Extremely Slow (Vsw < 300 km/s) Solar Winds (ESSWs) at 1 au: Causes of Extreme Geomagnetic Quiet at Earth (Tsurutani & Hajra 2022, ApJ 936, 155)](https://doi.org/10.3847/1538-4357/ac7444); [Journal of Geophysical Research: Extremely high speed solar wind: 29-30 October 2003 (Skoug et al. 2004, JGR 109, A09102)](https://doi.org/10.1029/2004JA010494); [Space Weather (AGU): On the Little-Known Consequences of the 4 August 1972 Ultra-Fast Coronal Mass Ejecta (Knipp et al. 2018, Space Weather 16)](https://doi.org/10.1029/2018SW002024) - Speed of light in a vacuum (Measured): [BIPM: SI defining constants](https://www.bipm.org/en/measurement-units/si-defining-constants) - Sunlight power that Earth intercepts (Measured, with error): [The Astronomical Journal (IAU): Nominal values for selected solar and planetary quantities: IAU 2015 Resolution B3 (Prša et al., AJ 152, 41, 2016; text also at arXiv:1510.07674)](https://doi.org/10.3847/0004-6256/152/2/41); [NASA NSSDCA: Earth Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html) - Sunspot number, monthly mean (latest month) (Measured, with error): [NOAA SWPC: Observed solar cycle indices](https://services.swpc.noaa.gov/json/solar-cycle/observed-solar-cycle-indices.json); [SILSO, Royal Observatory of Belgium: SILSO data files](https://www.sidc.be/SILSO/datafiles) - Temperature at the centre of the Sun (Estimated): [NASA NSSDCA: Sun Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html); [NASA Science: Sun: Facts](https://science.nasa.gov/sun/facts/) - Temperature of the Sun's corona (outer atmosphere) (Estimated): [ESA / NASA: 10 Years of SOHO (ESA Bulletin 126, May 2006)](https://soho.nascom.nasa.gov/publications/ESA_Bull126.pdf); [NASA Science: Sun: Facts](https://science.nasa.gov/sun/facts/); [NASA: Sun Shines in High-Energy X-rays](https://www.nasa.gov/image-article/sun-shines-high-energy-x-rays/) - Temperature of the Sun's visible surface (effective temperature) (Measured, with error): [The Astronomical Journal (IAU): Nominal values for selected solar and planetary quantities: IAU 2015 Resolution B3 (Prša et al., AJ 152, 41, 2016; text also at arXiv:1510.07674)](https://doi.org/10.3847/0004-6256/152/2/41); [NASA NSSDCA: Sun Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html) - The International Space Station's orbital speed (Measured): [NASA: Station Facts](https://www.nasa.gov/international-space-station/space-station-facts-and-figures/) - The Moon's average distance from Earth (Measured): [NASA: Moon Facts](https://science.nasa.gov/moon/facts/) - The Sun's distance from Earth (Measured): [NASA NSSDCA: Sun Fact Sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html) - The Sun's X-ray brightness at Earth (GOES, 0.1–0.8 nm) (Measured): [NOAA SWPC: Solar Flares (Radio Blackouts)](https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts) - Time since the last major solar flare (class M or X) (Measured, with error): [NOAA SWPC: GOES X-ray Flux](https://www.swpc.noaa.gov/products/goes-x-ray-flux)