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How hot is the sun?

How hot is the sun? The temperature of the sun varies considerably between each layer.
How hot is the sun? The temperature of the sun varies considerably between each layer. (Image credit: Xurzon via Getty Images)

Life on Earth would not exist without our huge hot glowing ball of gas. But just how hot is the sun? Well, that depends… 

The temperature of the sun varies from around 27 million degrees Fahrenheit (15 million degrees Celsius) at the core to only about 10,000 degrees F (5,500 degrees C) at the surface, according to NASA (opens in new tab)

Every 1.5 millionths of a second, the sun releases more energy than all humans consume in an entire year according to NASA Space Place. (opens in new tab) Here, we explore how hot each layer of the sun is and why the temperatures vary so much. 

Related: When will the sun die? 

Where does the sun's heat come from?

The sun is made of gas and plasma. Most of the gas — 92% — is hydrogen. If the sun were smaller, it would just be a huge ball of hydrogen akin to Jupiter. According to NASA Space Place, the hydrogen in the sun's core is held together by a lot of gravity resulting in high pressure. The pressure is so high that when hydrogen atoms collide with enough force they create a new element — helium — in a process called nuclear fusion. 

The continual nuclear fusion, causes energy to build up and the sun's core reaches temperatures of about 27 million degrees F (15 million degrees C). The energy then radiates outward to the sun's surface, atmosphere and beyond.

Radiative zone temperatures

Outside the sun's core lies the radiative zone where temperatures range from 12 million degrees F (7 million degrees C) nearest the core to around 4 million degrees F (2 million degrees C) in the outer radiative zone, according to the educational website Study.com (opens in new tab). No thermal convection occurs in this layer, according to the science news website Phys.org (opens in new tab). Instead, heat is transferred via thermal radiation whereby hydrogen and helium emit photons that travel a short distance prior to getting reabsorbed by other ions. It can take light particles (photons) thousands of years to meander their way through this layer before reaching the surface of the sun. 

Convection zone temperatures

Beyond the radiative zone is the sun's convective zone which extends for 120,000 miles (200,000 kilometers) according to Study.com. Temperatures in the convection zone are approximately 4 million degrees F (2 million degrees C). Plasma in this layer moves in a convective motion — like boiling water — bubbles of hot plasma transport heat to the surface of the sun. 

The sun's atmosphere: Photosphere, chromosphere and corona temperatures

The temperature of the sun varies between each layer of the atmosphere.  (Image credit: NASA/SDO)
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Temperatures in the sun's atmosphere also vary considerably between the layers. In the photosphere, temperatures reach about 10,000 degrees F (5,500 degrees C) according to the educational website (opens in new tab) The Sun Today. It is here that the sun's radiation is detected as visible light. Sunspots on the photosphere appear dark because they are cooler than the other parts of the sun's surface. The temperature of sunspots can be as low as 5,400 to 8,100 degrees F (3,000 to 4,500 degrees C) according to the University Corporation of Atmospheric Research (UCAR) (opens in new tab).

The chromosphere lies above the photosphere and temperatures range from approximately 11,000 degrees F (6,000 degrees C) nearest the photosphere to about 7,200 degrees F (4,000 degrees C) a couple of hundred miles higher up. 

Now here is where things get a little bit strange. Above the chromosphere lies the corona — the outermost layer of the sun's atmosphere. The sun's corona extends thousands of miles above the visible "surface" (photosphere) of the sun. Now you might think that temperatures here must be the lowest here since we are the farthest away from the heat-generating core… but that isn't the case. At all. 

The sun's corona can reach temperatures of around 1.8 million degrees F to 3.6 million degrees F (1 to 2 million degrees C), that's up to 500 times hotter than the photosphere. But how is the sun's upper atmosphere hotter than the surface? It's a great question, and one that has scientists rather stumped. There are some ideas about where the energy comes from that heats the corona, but a definitive conclusion is yet to be made. If you'd like to read more about this solar mystery check out this article on "Why is the sun's atmosphere hotter than its surface?"

Touching the sun: The Parker Solar Probe

One of the key missions of the Parker Solar Probe, launched in August 2018 and currently orbiting our star, will be to investigate why the corona defies stellar dynamic models by having a temperature greater that the photosphere.

The craft will fly through the sun's atmosphere, withstanding violent temperatures, often coming as close to its surface as 3.8 million miles (6.1 million kilometers). As it does this it will collect measurements of the corona and important data on solar winds, also taking images of the star. 

In 2021 the probe became the fastest craft ever created by humans, passing the sun at 364,621 mph (692,018 kph). When it is closest to the sun, the Parker Solar Probe travels at 430,000 mph (700,000 kph), according to NASA's Parker Solar Probe page (opens in new tab).

NASA's Parker Solar Probe

NASA's Parker Solar Probe launched on August 12, 2018 on a mission to study the sun.  (Image credit: NASA/Johns Hopkins APL/Steve Gribben)

The hottest and coolest stars

Stars come in a variety of sizes and colors so it should be no surprise that they have different temperatures too. Astronomers can tell a lot about the temperature of a star by its color, or its spectral type. 

There are 7 spectral types designated by the letters O, B, A, F, G, K, and M. The hottest stars are O and B stars which shine mostly blue light with a great deal of their light in the ultraviolet spectrum. M-type stars are the coolest class more prominent in red wavelengths but also emitting a lot of infrared light. 

Blue stars have estimated surface temperatures of 25,000 kelvin (K) (44,540 degrees F/ 24,726 degrees C), while red stars are much cooler at around 3,000 K (4,940 degrees F/ 2,726 degrees C), according to the University of Central Florida (opens in new tab). In between these are white stars with temperatures of around 10,000 K (17,540 degrees F/ 9,726 degrees C), yellow stars, like the sun, at 6,000 K (10,340 degrees F/ 5,726 degrees C), and cooler orange stars with temperatures in the region of 4,000 K (6,740 degrees F/ 3,726 degrees C).

Additional reading

You can explore the sun in more detail with NASA's Solar Dynamics Observatory (opens in new tab) or keep up to date with the latest findings (opens in new tab) from NASA's Parker Solar Probe on its mission to "touch" the sun. If you want to improve your knowledge and understanding of the sun check out this free course courtesy of the Open University (opens in new tab). Read about energy from the sun and how we can make use of it in this informative guide from the National Energy Education Development Project (NEED) (opens in new tab)

Bibliography

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Daisy Dobrijevic
Reference Writer

Daisy Dobrijevic joined Space.com in February 2022 as a reference writer having previously worked for our sister publication All About Space magazine as a staff writer. Before joining us, Daisy completed an editorial internship with the BBC Sky at Night Magazine and worked at the National Space Centre in Leicester, U.K., where she enjoyed communicating space science to the public. In 2021, Daisy completed a PhD in plant physiology and also holds a Master's in Environmental Science, she is currently based in Nottingham, U.K.

With contributions from