How Hot Is the Sun? Colder, the Closer You Get!

the sun, solar surface, solar atmosphere — There's some unexpected physics at work in the solar atmosphere. Universal History Archive/UIG/Getty Images

The temperature of the sun has fascinated scientists for centuries. Early on, estimations ranged from mere heat to comparisons with terrestrial flames.

As our understanding deepened, the scientific community has realized the sun's core holds a searing inferno where nuclear fusion reigns supreme. Modern instrumentation and innovative observations have refined our knowledge, unveiling the intricacies of its temperature gradients and layers. So, just how hot is the sun, exactly?

Let's dive in.

The Temperature Enigma

Picture the sun as a cosmic cauldron of extremes. At its core, a relentless nuclear fusion dance rages, creating an inferno that reaches a staggering 15 million degrees Celsius (27 million degrees Fahrenheit). This blazing heart powers the sun's luminous existence.

Traveling upward, the sun's visible surface, known as the photosphere, emerges as a relatively cooler realm, where temperatures hover between 4,000 and 6,000 degrees Kelvin. This luminous layer is akin to the gentle warmth of a campfire, casting its radiant glow across the cosmos.

This is where energy generated in the core reaches the surface and is radiated out as visible light, making the photosphere the visible "surface" of the sun that we see from Earth.

Yet, the sun's enigma deepens when we ascend further out toward its crown jewel: the corona. Against all expectations, this outermost layer flares to over a million degrees Celsius (1.8 million degrees Fahrenheit), a region of searing intensity. The contrast between the corona's blazing heat and the photosphere's comparative coolness remains a puzzle.

How Do Scientists Measure the Sun's Temperature?

It's literally the hottest star in the solar system, but you can't just use a thermometer to figure out how hot it actually is. Instead, scientists use a number of tools and indirect methods to figure this hot math out:

Spectroscopy: These instruments split sunlight into its component colors, creating a spectrum. By analyzing this spectrum, scientists can identify absorption lines and their shifts, which provide information about the temperature of different layers of the sun's atmosphere.
Spectrometers: These instruments measure the intensity of light at different wavelengths. The shape of the spectrum, particularly in regions of strong absorption lines, can indicate the temperature of the sun's surface and atmospheric layers.
Pyrometers: These instruments measure the intensity of thermal radiation emitted by the sun. By analyzing the wavelength distribution of this radiation, scientists can determine the temperature of the sun's surface and its layers.
Coronagraphs: These devices block out the bright disk of the sun, allowing scientists to observe its much fainter outer atmosphere, the corona, during solar eclipses or through special instruments. This provides insights into the temperature and dynamics of the corona.
Solar telescopes: Ground- and space-based telescopes designed to observe the sun in different wavelengths of light help scientists study specific layers and phenomena associated with different surface temperatures and atmosphere temperatures.
Solar satellites: Satellites, like the Solar Dynamics Observatory (SDO), provide continuous observations of the sun across multiple wavelengths, allowing scientists to monitor changes in temperature and activity over time.
Computer models: Advanced computer simulations and models help scientists predict the temperature profiles of the sun's layers based on theoretical understanding of solar physics and observational data.

How Does the Sun Produce Heat?

The sun generates its intense heat through a process called nuclear fusion. In its blazing core, hydrogen atoms collide under immense pressure and temperature, fusing together to form helium atoms.

This fusion releases an incredible amount of energy in the form of light and heat. The core's extreme conditions, with temperatures reaching around 15 million degrees Celsius (27 million degrees Fahrenheit), enable these nuclear reactions to occur.

The energy produced travels outward through the sun's layers, taking millions of years to reach the surface, or photosphere, where it is released as sunlight. This ceaseless fusion reaction, like an eternal cosmic furnace, is what fuels the sun's brilliance and provides the life-sustaining energy for our solar system.

Colder on the Inside

One of the weird things about space is that things don't always conform to what would seem like common sense. Take the sun, for example. You'd think that its surface would be hotter than its outer atmosphere, since the surface is closer to the nuclear furnace at the sun's core. After all, when you're sitting in front of a fireplace, it feels hotter when you get closer to it, right?

But the sun doesn't work that way.

The photosphere, as the solar surface is called, is indeed pretty hot: between 6,700 and 11,000 degrees Fahrenheit (3,700 to 6,200 degrees Celsius). But the further you get from the sun's surface, the hotter the atmosphere seems to get. At the sun's corona — the outermost atmospheric layer about 1,200 miles (2,100 km) from the surface — the temperature soars to an astonishing 900,000 degrees Fahrenheit (500,000 degrees Celsius).

It's All in the Waves

Besides the sun, some other stars exhibit this curious pattern as well, and for a long time, scientists struggled to figure out why. They developed a hypothesis, in which magnetohydrodynamic (MHD) waves distribute energy from below the photosphere directly up to the corona, almost like an express train with no local stops.

In 2013, British researchers used advances in imaging technology to examine the chromosphere, the layer between the photosphere and the solar corona, and actually examined the MHD waves. Their calculations confirmed that the waves could be responsible for transporting energy to the corona and heating that layer.

"Our observations have permitted us to estimate the amount of energy transported by the magnetic waves, and these estimates reveal that the waves' energy meets the energy requirement for the unexplained temperature increase in the corona," Richard Morton, a scientist for the U.K.'s Northumbria University, explained when announcing the discovery.

The Pioneering Probe

The Parker Solar Probe is a pioneering NASA spacecraft designed to venture closer to the sun than any previous mission [source: NASA]. Launched in August 2018, its mission is to study the sun's outer atmosphere (the corona) and gain insights into the solar wind, a continuous stream of charged particles emanating from the massive star.

Named after solar physicist Eugene Parker, the probe employs cutting-edge technology to withstand the extreme heat and radiation near the sun. It aims to answer critical questions about the nature of solar winds, how they are accelerated and why the corona is much hotter than the sun's surface.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.