What Are Sunspots? | The Old Farmer's Almanac

What Are Sunspots and Why Do They Form?

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The dark patches on the Sun explained.

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The Sun has spots. We don’t mean blemishes. We’re talking about dark spots on the surface of the Sun—which are actually intense storms. Learn how sunspots form, why they are dark, and how sunspots help us predict weather.

What Are Sunspots?

Sunspots are dark areas on the surface of the Sun (the photosphere) caused by strong magnetic activity from deep within the Sun.

Let’s back up:

The Sun is a magnetic star. Think of the Sun as a very very hot thermonuclear furnace. It’s churning electrically-conducting plasma just as a stove boils water. And, like water boiling on a stove, activity bubbles up from the hot interior. This happens because magnetic fields that are deep within the Sun get twisted and bundled up during its rotation. 

When a sunspot appears, the magnetic force is concentrated about 1000 times more than it is on the rest of the Sun’s surface (photosphere) and breaks through.

Just as planet Earth has storms, the Sun and space has weather. These magnetic storms, aka sunspots, occur naturally and have been forming for many hundreds of years. Historically, we know that the ancient Chinese recorded sunspots back in 400 BC. After the invention of the telescope, Galileo and many other scientists made further records. 

Sunspots consist of two parts. The first part is a dark center called the umbra. The second part is a brighter area around the umbra called the penumbra. The penumbra is a type of exterior structure. Sometimes, several sunspots together have a single penumbra around their umbrae.

Sunspots consist of two parts: Umbra and Penumbra

Why are Sunspots Dark?

Sunspots appear as darker regions because they are cooler than the surrounding areas. The concentrated magnetic fields are so strong that they keep the heat from reaching the Sun’s surface. Since the heat can’t escape, sunspots are a few thousand degrees cooler than the surface of the Sun.

The temperature of a sunspot is still very hot, though!  For perspective, the surface of the Sun, called the photosphere, has a temperature of 5,800 degrees Kelvin. Sunspots have temperatures of about 3,800 degrees K.

They look dark only in comparison with the brighter and hotter regions of the photosphere around them. A single sunspot would still glow brighter than the Full Moon!

How Do Sunspots Form?

Think of the Sun’s magnetic fields as rubber bands from the North to South Poles. As the Sun rotates, these rubber bands (magnetic fields) get wound up, tangled, and crossed like a rubber band, twisting and pulling and twisting, building energy. Eventually, the magnetic fields “snap,” rise and break the surface. 

Another analogy is a soda pop bottle being shaken. The pressure builds and builds inside the soda can until … pop! The energy explodes!

Solar Flares

Sunspots are areas on the Sun’s surface where magnetic field lines become tangled or cross. When this happens, it can cause a solar flare, which is a sudden release of energy. Sometimes, solar flares accompany a coronal mass ejection (CME for short) that explodes into space.

Think of that rubber band snapping! Energy is released, and plasma explodes into space.

The Sun’s radiation can sometimes reach Earth’s atmosphere, but Earth’s magnetic field mostly protects it from the Sun’s emissions. This can temporarily knock out Earth’s satellites, electricity, and communications, and pose a hazard to astronauts.

How Sunspots Move

The Sun rotates around its axis, similar to Earth, and the sunspots follow the Sun’s rotation. A sunspot region appears to travel from east to west as seen from Earth.

A sunspot region near the equator takes around two weeks to move across the sun’s surface from east to west. The further away a sunspot region is from the equator the longer it takes move across the face of the Sun.

How Long Do Sunspots Last

Sunspots on the Sun are temporary, similar to storms on Earth. Some can last a few hours or a few days. Usually, they last a week or so. There have been large sunspots that have lasted a month or more.

Video: Two weeks in the life of a sunspot. Watch it rotate into view on the Sun and out of Sun. Credit: NASA Goddard.

Sunspot Size

Some are as big as Earth, while others are more than ten times that! They can get as large as 80,000 km in diameter. Sunspots expand and contract as they move across the surface of the Sun.

How Long is a Sunspot Cycle

Historical records from the 1700s show that the sunspot cycle is about 11 years, but it can vary in length. Scientists refer to the highest activity of the solar cycle as “solar maximum” or “solar max.” Solar minimum is the term used to describe the lowest points.

Learn more about solar cycles and why they matter.

How Do Sunspots Affect Earth?

The sunspot itself doesn’t affect Earth. However, as mentioned above, intense sunspots are where much space weather originates, such as solar flares and coronal mass ejections. 

  • The charged solar particles can disrupt infrastructure, such as satellites, navigation, communications, and the electric power grid.
  • Low sunspot activity is believed to cause colder temperatures in certain areas on Earth. It also impacts the strength of the Sun’s energy.

But just how much this affects Earth’s climate is in scientific debate. For the moment, human activities have a much more demonstrable effect. Find out how the Almanac predicts weather.

About The Author

Catherine Boeckmann

Catherine Boeckmann loves nature, stargazing, and gardening so it’s not surprising that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann

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