Limb darkening is an effect we see on the sun. The sun’s atmosphere consists of layers like Earth’s. On the sun, the outermost layer that we can see during solar eclipses is the corona. The next layer down is the chromosphere, followed by the deeper photosphere. When you look at the edge or limb of the sun, you can only see some of the photosphere, but not all the way to the base. When you look closer to the center of the sun, you can see the entire depth of the photosphere, from its top to bottom. We see a deeper, hotter layer, so it’s brighter compared to the cooler material on the limb.
Visible light and other stars
This limb darkening effect is specific to the photosphere when we observe the sun in visible light. When we look in a more energetic light, such as the extreme ultraviolet, the high part of the atmosphere we see on the limb is reversed and now hotter. So, in those images, the limb is brighter.
The photosphere is an important region on not just the sun, but on all stars. And limb darkening doesn’t just apply to the sun; it’s true for any star. When we talk about how hot a star is, we’re usually talking about the temperature of its photosphere. And when you’re looking at sunspots, you’re looking at a cool island amid the hot sea of the photosphere.
Bottom line: Limb darkening is an effect we can see on the sun where the edges of the sun are darker than the central portion. This is due to the amount of photosphere we’re looking at.
C. Alex Young is a solar astrophysicist studying the Sun and space weather. Alex is passionate about sharing science with diverse audiences. This led him to start The Sun Today with his designer wife, Linda. First through Facebook and Twitter then adding an extensive website thesuntoday.org, the two work together to engage the public about the Sun and its role in our solar system. Alex led national engagement efforts for the 2017 total solar eclipse. He is the Associate Director for Science in the Heliophysics Science Division at NASA Goddard Space Flight Center.
Kelly Kizer Whitt has been a science writer specializing in astronomy for more than two decades. She began her career at Astronomy Magazine, and she has made regular contributions to AstronomyToday and the Sierra Club, among other outlets. Her children’s picture book, Solar System Forecast, was published in 2012. She has also written a young adult dystopian novel titled A Different Sky. When she is not reading or writing about astronomy and staring up at the stars, she enjoys traveling to the national parks, creating crossword puzzles, running, tennis, and paddleboarding. Kelly lives with her family in Wisconsin.
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