SUN FACTS
“The Great Eclipse,” Rebecca Boyle’s informative article about April 8’s total solar eclipse, describes materials of different temperatures in the sun’s corona in the graphic “Portrait of Our Sun.” Among them, it refers to “hot flaring plasma at or above 20 million K [kelvins].” The box text notes that the sun’s core is 15 million K, and I wonder how that plasma can be about five million K hotter when the core is the main source of the sun’s energy production.
ROBERT WALTY STEPHENS CITY, VA.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Boyle says “a solar eclipse is one of the best ways for scientists to study the solar corona.” I thought satellite-based instruments were able to study the solar corona by masking out the sun’s disk. Are there Earth-based instruments that are not currently available on satellites?
DAVID LUNN DEVON, ENGLAND
BOYLE REPLIES: In response to Walty: The sun’s core is the most consistently hot part of our star. But interactions among the sun’s magnetic field lines create intense bursts of energy in the solar corona, or outer atmosphere, called solar flares, and they can indeed reach temperatures of 20 million K or higher. The corona itself is about one million to two million degrees Celsius overall, and it’s hard to understand why it is so much hotter than the sun’s surface, which is only 6,000 degrees C. Magnetic activity is probably the source of the sun’s superheated atmosphere.
To answer Lunn: A solar eclipse is the best way to study the sun’s atmosphere. Telescopes on Earth and satellites can use a coronagraph, an instrument that blocks the sun’s disk while leaving much of its atmosphere visible. But coronagraphs can block important parts of the solar atmosphere near the surface. In the atmosphere’s “transition region,” the area that is closest to the surface, it heats up dramatically, and the nature of the sun’s plasma changes. The region is key to understanding major solar phenomena such as the solar wind, not to mention clouds of charged particles called coronal mass ejections. (Although a handful of spacecraft have been able to study the region, Earth-based observations during solar eclipses are very useful and in some ways more accessible.) What’s more, most Earth-based coronagraphs are washed out by the daytime sky, which obscures many of the corona’s features.
An eclipse can also serve as a calibration tool, allowing us to compare data on the sun or background stars during the event with “regular” observations. Previous eclipses unveiled helium in the sun’s atmosphere in 1868 and helped to demonstrate the gravitational bending of light predicted by Einstein’s general theory of relativity in the early 20th century. During the April 2024 eclipse, NASA scientists measured the sun’s radio waves as the moon slipped in front of it. By calculating the difference in radio signals, scientists hope to understand the strength of solar magnetic fields that produce them, which could provide insight into sunspots.
LASER SCOPE
A caption for the photograph in “The Era of Monster Telescopes,” by Phil Plait [The Universe], mentions that the Extremely Large Telescope (ELT) in Chile will use lasers to create artificial stars. The article does not provide any more information about this. Are the lasers part of the calibration system or for some other use?
JONATHAN LAMB CHARLESTON, S.C.
PLAIT REPLIES: The lasers are used to fine-tune the telescope’s resolution. They emit a very specific color that’s absorbed by sodium atoms in a thin layer in the atmosphere about 90 kilometers above Earth’s surface. The atoms respond by glowing, creating a very small point of light—a bright artificial star. The ELT measures the changes in the spot as its light passes through our atmosphere and uses those data to rapidly deform a mirror that’s in the light’s path. The change in the mirror’s shape counteracts the “twinkling” of the star, focusing the telescope better and allowing it to see finer details in cosmic objects. This process is called adaptive optics and is pretty common now. It should enable the ELT to have incredibly sharp vision, even better than that of the Hubble Space Telescope.
“We must actively remove CO2 from the air to keep the global average temperature below two degrees Celsius above the preindustrial average.”
—Volker Sick University of Michigan
CARBON CAPTURE
In “The False Promise of Carbon Capture” [Observatory], Naomi Oreskes offers a valid criticism of what she refers to as “carbon capture” as it is typically practiced today. But humanity has a lot to lose if we give up on where the approach is headed.
Oreskes wisely underscores that most of the efforts being categorized as carbon capture right now are fueling rather than fighting climate change. The prevalence of the greenwashed practice of “enhanced oil recovery” is obscuring a technique that is essential to correcting our carbon trajectory in the short term and eventually ushering in a new circular carbon economy: carbon capture and utilization (CCU).
We’ve been using “carbon” as an abbreviation for “carbon dioxide.” But what utilization does is chemically convert that CO2 to usable carbon, which is fundamental to products we rely on every day. Currently most of that carbon comes from fossil sources. To achieve net zero, we’re going to need to get it somewhere else, and biomass cannot meet global carbon demands. Direct air capture, while expensive right now, offers a solution for the long run. Utilization brings the net costs down: instead of being sequestered, the captured carbon becomes a feedstock to make valuable products such as fertilizer, aviation fuel, plastics and even concrete.
Although reducing carbon emissions and scaling up renewable energy must be priorities, they simply won’t be enough on their own. We must actively remove CO2 from the air to keep the global average temperature below two degrees Celsius above the preindustrial average. Carbon capture must be part of the solution.
VOLKER SICK PROFESSOR, MECHANICAL ENGINEERING, UNIVERSITY OF MICHIGAN, AND DIRECTOR, GLOBAL CO2 INITIATIVE
ORESKES REPLIES: My argument isn’t against trying to find uses for carbon. If we could make CCU work, that would be great. Folks have been talking about it for decades, however, and no one has yet found ways to do it profitably. My argument is about focus and particularly about how we use taxpayer dollars.
If private-sector actors have ideas for capturing, storing or using carbon, I’m all for that. But they should invest their own money. We should focus taxpayer funds on what we know works: replacing fossil fuels with energy efficiency and renewables. We are paying now for climate damages. We should not also be expected to pay for the industry’s waste products.
ERRATA
“The Great Eclipse,” by Rebecca Boyle, incorrectly referred to the sun’s outermost layers as including “the chromosphere (the transition region).” The chromosphere and the transition region are separate layers.
“A Safe and Just Earth,” by Joyeeta Gupta and InfoDesignLab, should have described the World Health Organization’s standards for fine-particulate pollution in terms of micrograms per cubic meter, not micrograms per square meter.
In “When We Find Earth 2.0, What’s Next?” by Phil Plait [The Universe, May], the illustration of an Earth-size exoplanet should have been credited to Aaron Alien/Alamy Stock Photo.