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Why Do Sunspots Appear Darker Than Their Surroundings

by Lyndon Langley
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Why Do Sunspots Appear Darker Than Their Surroundings

Why Do Sunspots Appear Darker Than Their Surroundings

You probably noticed it when you looked at the sun during a total solar eclipse — that tiny dark spot amid all those bright spots (or maybe next to one of them). It was a sunspot! And if you saw it again later, you might notice something else about these spots: They’re darker than everything around them. Why is this?
The answer lies not on Earth but on our star. As with any other sunspot, there’s an inner part that gets brighter while the rest of the surface gets dimmer. But why do sunspots get darker than their surroundings? The answer has to do with how the temperature changes across the sunspot.
Sunspots are “dark” because they are cooler than their surroundings. Let’s say you had two identical spaceships parked side-by-side in space. One would be hot inside; the other cold. In between them would be a thin layer of air that wasn’t quite hot enough for either ship to reach. That gap of air exists everywhere on the Sun except where a sunspot forms. There, the cooling effect from the sun’s rays causes the upper layers of the atmosphere to cool down so much that they can no longer absorb heat from sunlight. This leaves behind only a dense core of plasma at the center of each sunspot.
This is what gives sunspots their distinctive appearance. A sunspot isn’t really black or white as we tend to think of darkness and light. Instead, the contrast between the sunspot’s brightness and its surrounding areas makes it look like something more along the lines of purple, blue or green.
Sunspots also have a lighter outer section called the penumbra, and a darker central region named the umbra. Sunspots are caused by disturbances in the Sun’s magnetic field welling up to the photosphere, the Sun’s visible “surface.” These disturbances cause the magnetic field to become distorted. Magnetic fields exert pressure on particles within the Sun’s plasma interior. When the magnetic field becomes twisted and tangled, energy stored in the system is released through electrical currents. The resulting energy heats the area underneath the disturbance, which looks like a sunspot.
In addition, sunspots often form pairs, which scientists call groups. Groups are made up of multiple individual sunspots that appear connected via long strands of magnetic flux. Some people believe that sunspots may even merge together over time into larger superstructures.
As far back as 1610, Galileo Galilei observed sunspots and noted that they appeared redder than the background color of the Sun. He didn’t know exactly why that happened. It took nearly 400 years before someone proposed that sunspots were actually darkened regions of the Sun itself. During this time, other researchers tried to explain away this observation using explanations such as magnetism, convection, coronal loops and prominences.
It wasn’t until 1843 that German astronomer Richard Carrington finally put forth his theory that sunspots are just regions of cooler temperatures on the Sun. His work helped inspire Joseph von Neumann and Edward Norton Noyes, who developed equations describing how temperature affects the intensity of sunlight reaching the surface of the Sun. Using these equations, they calculated that sunspots should produce less intense light than the surrounding photosphere.
So why does the sunspot look darker than the surrounding area? To understand this, let’s take a closer look at how the various factors affect the amount of sunlight reaching us. Light waves travel faster through hotter objects. So when sunlight passes through a medium, such as glass or water, the light slows down. But when it hits a colder object, such as ice, the light speed doesn’t slow down as much. Therefore, when sunlight reaches a sunspot, it takes longer to make it through. This means the sunspot will receive less direct sunlight than the surrounding photosphere.
But since the sunspot is cooler than its surroundings, it absorbs less light. This explains why sunspots appear darker. While sunlight travels through the hotter parts of the sunspot, it exits through the cooler parts. Since the exiting light is slower, the shorter wavelengths travel farther. Because short wavelengths are absorbed first, the sunspot appears darker.
This phenomenon is similar to how paint is applied differently depending on whether it is being used on a wall or on paper. If you apply paint directly onto paper, the paint will soak into the paper without spreading out very much. On the other hand, if you spray paint onto walls, the paint will spread out and cover a wider area. The same thing happens with light. Direct exposure to sunlight produces a uniform distribution of light throughout the sunspot. However, indirect exposure allows the light to exit through the cooler parts of the sunspot. Once outside, the light spreads out and covers a larger area.
On average, sunspots are located near the equator. You’ll find sunspots mostly during active periods of Solar Cycle 24, which began in April 2014. Active periods happen roughly every 11 years, lasting approximately 12 months apiece. Scientists don’t fully understand why the cycles occur, but some theories propose that they serve as a way for the Sun to release energy built up over time.
Solar astronomers use advanced telescopes to study the Sun. For example, NASA’s Solar Dynamics Observatory launched in 2010 is designed to help provide answers to questions ranging from how the Sun works to its influence on the environment here on Earth.

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2 comments

Zane Verrecchia June 18, 2022 - 6:30 am

I think you have observed some very interesting points, regards for the post.

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zorivareworilon June 29, 2022 - 5:16 am

Way cool, some valid points! I appreciate you making this article available, the rest of the site is also high quality. Have a fun.

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