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Credit: ESA & NASA/Solar Orbiter/EUI team; Data processing: E. Kraaikamp (ROB)
As tempting as it is to cite one, no single number can describe the temperature of the whole Sun. Its layers are at different temperatures because they’re doing very different things. The Sun is a gigantic dynamo powered by hydrogen fusion within its plasma core. At pressures in the trillions of pounds per square inch, the Sun’s core averages about 15 million Kelvin (15 million Celsius, 27 million Fahrenheit). It’s tough to describe temperatures like these in relatable terms because they’re so far outside everything humans experience. The core of a star reaches temperatures—energy levels, really—we don’t see anywhere else outside fusion reactors.
The visible surface layer of the Sun, called the photosphere, is a toasty 5,800 Kelvin (about 5,600 Celsius, or 10,000 degrees Fahrenheit). But the average temperature of the Sun’s corona is up to 300,000 K, and it can reach temperatures in the millions of degrees during high-energy solar flares.
Credit: NASA/Sally Bensusen
Despite being so hot, the corona is less than a millionth as bright as the Sun. As you walk away from a bonfire, you feel less of its heat because you receive less energy due to the inverse square law. Why is the more distant corona so much hotter than parts of the Sun closer to its core? We simply don’t know yet—but that’s one of the questions that NASA launched the Parker Solar Probe to answer.
How Do We Study the Sun?
Mostly, the Earth’s atmosphere blocks out photons with a shorter wavelength than ultraviolet light. This makes it difficult to study the Sun’s inner workings from Earth directly. However, we do have our ways. Many ground-based telescopes use a grid of sensors also known as a charge-coupled device, or CCD. Some space telescopes use a complementary metal-oxide-semiconductor (CMOS) active-pixel sensor, such as the WISPR imager on the Parker Solar Probe. These sensitive instruments can detect photons from the infrared to gamma rays because of how photons interact with the electrons in the sensors. (CCDs also power many consumer digital cameras.)
Compared with the brilliant force of sunlight, the light from the Sun’s corona is almost imperceptible. Like lens flare in a digital photo, ultra-bright light from a star can wash out the light from elsewhere in frame, such as its corona. Some coronagraphs have a round, opaque dot in the middle of their observing aperture. The dot obscures the body of a star, much like an annular eclipse, allowing scientists to observe its much fainter corona. (For a long time, solar eclipses were our best chance at observing the corona.) Another type, called a vortex coronagraph, uses the shape of the observing lens to physically redirect stray light into an “optical vortex” of destructive interference.
Trace elements in the Sun contribute their own spectral characteristics to the light produced by the Sun’s fusion engine. The presence of certain elements, such as the wildly over-ionized iron (Fe¹³+) in our Sun’s atmosphere, also tells us about what the Sun’s temperature must be.
Solar Observatories
The Sun’s influence extends well beyond the orbit of its planets, so we launched spacecraft like the Voyager 1 and 2 probes and NASA’s various solar orbiters to study the Sun’s effects on the rest of the solar system. Voyagers 1 and 2 are on a one-way trip, sent forth to study the boundary between our solar system and interstellar space. In 2012, Voyager 1 started sending home readings indicating that it had reached the heliopause, the region of magnetic turbulence where the solar wind starts to slow down and trail off. Voyager 2 crossed into interstellar space on Nov. 5, 2018.
Meanwhile, NASA’s Solar Dynamics Observatory (SDO) and its STEREO (Solar TErrestrial RElations Observatory) satellites are in orbit around Earth. These spacecraft use magnetometers, electrical antennae, prismatic spectrometers, and instruments designed to sample the relatively cool plasma of the solar wind.
The STEREO A/B and SDO spacecraft both enjoy significantly improved resolution, compared to their predecessor, SOHO.
Credit: NASA Solar Dynamics Observatory
Launched in 2010, the Solar Dynamics Observatory is a mission to study the aspects of the Sun that directly affect our life on Earth: solar wind, solar flares, and other outbursts of energy. STEREO, for its part, is a pair of satellites that gave us stereo vision of the sun in the same way that stereoscopic cameras let us film movies in native 3D. In 2011, their orbital separation allowed us to see the entire Sun at the same time, for the first time ever.
NASA also collaborated with the European Space Agency to develop the ESA’s Solar Orbiter (SolO), which studies the Sun from close enough to make Icarus jealous. In 2022, SolO delivered to us the highest-resolution image of the Sun’s corona ever taken.
What’s Next
Researchers haven’t let go of the coronal heating problem. Happily, there are more solar observatories than ever, and most are synchronized to the same atomic clock. This means we have multispectral, timestamped images and readings for the Sun through multiple solar cycles: solar flares, coronal holes, and all. Whether it’s magnetic tension, micro-flares from the convection granules filling the photosphere, or something else entirely, heliophysicists are determined to determine why the corona is such a spicy meatball.
Machine learning stands to play a role in future heliophysics. Cooperation in solar research between NASA and other public and private institutions has also created a huge body of open data accessible to anyone who wants to explore it.
But the real vanguard in solar science might be one of the satellites itself. SolO’s orbital radius drops to 0.284 astronomical units (AU), only about sixty solar radii, bringing it within the orbital perihelion of Mercury. One of its chief scientific objectives is investigating the connection between solar flares and coronal mass ejections, or CMEs. Another: photographing the Sun’s poles, heretofore unobserved. Icarus would be jealous indeed.