How hot is the Sun? Our solar system's fiery star explained

As tempting as it is to quote a single number, there is no one number that can describe the temperature of the entire Sun. Its layers are at different temperatures because they are doing very different things. The Sun is a gigantic dynamo powered by hydrogen fusion within its plasma core. At pressures of trillions of pounds per square inch, the Sun's core averages about 15 million Kelvin (15 million Celsius, 27 million Fahrenheit). It's hard to describe temperatures like these in understandable terms because they are so far outside of anything we humans experience. The core of a star reaches temperatures — energy levels, really — that we don't see anywhere else outside of fusion reactors.

The visible surface layer of the Sun, called the photosphere, has a pleasant temperature of 5,800 Kelvin (about 5,600 Celsius or 10,000 degrees Fahrenheit). But the average temperature of the Sun's surface layer crown It can reach temperatures of up to 300,000 K and 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. When you move away from a campfire, you feel less of its heat because you're receiving less energy due to the inverse square law. Why is the more distant corona so much hotter than the parts of the Sun closer to its core? We just don't know yet, but that's one of the questions NASA tried to answer with the Parker Solar Probe.

How do we study the Sun?

In general, the Earth's atmosphere blocks photons with a wavelength shorter than ultraviolet light, making it difficult to study the inner workings of the Sun directly from Earth. However, we have our methods. Many ground-based telescopes They use a sensor array 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 Image Generator on the Parker Solar Probe. These sensitive instruments can detect photons from infrared to gamma rays because of the way the photons interact with electrons in the sensors. (CCDs also power many consumer digital cameras.)

Compared to the brilliant force of sunlight, the light from the Sun's corona is almost imperceptible. Like lens flare in a digital photograph, a star's ultra-bright light can wash out light from other parts of the frame, such as its corona. Some coronagraphs have a round, opaque spot in the middle of their viewing aperture. The spot 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 (They were our best chance to observe the corona.) Another type, called a vortex coronagraph, uses the shape of the observing lens to physically redirect scattered light into a “optical vortex” destructive interference.

Trace elements from the Sun contribute their own spectral characteristics to the light produced by the Sun's fusion engine. The presence of certain elements, such as Extremely over-ionized iron (Fe¹³+) in our Sun's atmosphere also tells us what the Sun's temperature must be.

Solar observatories

The Sun's influence extends far beyond the orbit of its planets, so we launch spacecraft like Voyager 1 and 2 and NASA's various solar orbiters to study the Sun's effects on the rest of the solar system. Voyager 1 and 2 are on a one-way trip, sent to study the boundary between our solar system and interstellar space. In 2012, Traveler 1 began sending home readings indicating it had reached the heliopause, the region of magnetic turbulence where the solar wind begins to slow and decline. Traveler 2 crossed into interstellar space on November 5, 2018.

Meanwhile, NASA Solar dynamics observatory (SDO) and its STEREO The Solar Terrestrial Relations Observatory satellites orbit Earth. These spacecraft use magnetometers, electrical antennas, prismatic spectrometers, and instruments designed to sample the relatively cool plasma of the solar wind.

The STEREO A/B and SDO spacecraft 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 bursts of energy. STEREO, meanwhile, is a pair of satellites that gave us a stereoscopic view of the Sun in the same way that stereoscopic cameras allow us to film movies in native 3D. In 2011, their orbital separation allowed us to see the entire Sun at once, for the first time in history.

NASA also collaborated with the European Space Agency to develop ESA's Solar Orbiter (SolO), which studies the Sun from a distance so close that Icarus will be jealous. In 2022, SolO delivered us the The highest resolution image of the solar corona ever taken.

What's next?

Researchers have not left aside the Coronal heating problemFortunately, there are more solar observatories than ever before, and most are synchronized to the same atomic clock. This means we have multispectral, time-stamped images and readings of the Sun across multiple solar cycles—solar flares, coronal holes, and everything in between. Whether it's magnetic stress, micro-eruptions from the convection granules that fill the photosphere, or something else entirely, heliophysicists are determined to determine why the corona is such a spicy meatball.

Machine learning is… play a role in the heliophysics of the future. Cooperation on solar research between NASA and other public and private institutions has also created a huge set of open data accessible to anyone who wants to Explore it.

But the real cutting edge in solar science may be one of the satellites themselves. SolO's orbital radius dips down to 0.284 astronomical units (AU), just about sixty solar radii, putting it within Mercury's orbital perihelion. One of its primary scientific goals is to investigate the Connection between solar flares and coronal mass ejections, or CMEs. Another: photographing the Sun's poles, hitherto unnoticed. Icarus would be truly jealous.