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How do sunspots form?

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Quick answer

Sunspots form when strong magnetic fields generated inside the Sun rise and break through the visible surface. These fields block the upward flow of hot plasma, leaving a cooler, darker patch. Because the surrounding photosphere is hotter and brighter, the cooler region appears as a dark spot.

Sunspots form where intense bundles of magnetic field poke through the Sun's visible surface, cooling those patches by blocking the flow of hot plasma from below. Because they are cooler than their surroundings, they look dark against the brighter photosphere.

Step by step

  1. The Sun's hot plasma churns and rotates, generating a tangled magnetic field.
  2. Bundles of magnetic field lines rise toward the Sun's visible surface, the photosphere.
  3. Where the field is strongest, it suppresses the convection that normally carries heat upward.
  4. The blocked region stays cooler than the surrounding photosphere, forming a dark spot.
  5. Sunspots grow, drift with the Sun's rotation, then fade as the magnetic field rearranges.

Key things to understand

  1. The Sun's interior churns plasma, and this motion, combined with the Sun's rotation, generates a tangled magnetic field.
  2. Where magnetic field lines become concentrated, they form loops that burst through the photosphere, the Sun's visible surface.
  3. These strong magnetic fields suppress the convection that normally carries hot plasma upward, so the region below stays cooler.
  4. A sunspot is therefore a patch of the photosphere roughly 1,500 to 2,000 degrees Celsius cooler than the surrounding surface.
  5. Sunspots appear dark only by contrast; if isolated, they would still glow brighter than the full Moon.

Labelled diagram

  1. Convection zone

    Hot plasma churns and generates magnetic fields deep inside the Sun.

  2. Magnetic bundle

    A concentrated tube of magnetic field rises toward the surface.

  3. Photosphere

    The field breaks through the visible surface, forming a dark patch.

  4. Cooler spot

    Suppressed convection leaves the area about 1,500 degrees Celsius cooler.

  5. Umbra and penumbra

    The dark core and lighter outer ring make up a mature sunspot.

How a sunspot forms

Real-world examples

  • A large sunspot group can span several Earth diameters and is visible from Earth with a safe solar filter.
  • During solar maximum, dozens of sunspots may appear on the Sun's disk at once.
  • The 1859 Carrington Event was linked to a huge sunspot group that produced a powerful solar storm.
  • Modern solar observatories track sunspots daily to forecast space weather that can affect satellites.

Common mistake

✗ Myth: Sunspots are holes or dark objects on the Sun's surface.

✓ Fact: They are not holes. They are regions of the photosphere that are cooler because strong magnetic fields block the flow of hot plasma from below, making them look dark by contrast.

People also ask

What causes sunspots?
Sunspots are caused by intense magnetic activity inside the Sun. The Sun's churning plasma and rotation generate a tangled magnetic field. Where bundles of this field break through the visible surface, they suppress the convection that carries heat upward. That leaves a cooler, darker patch we call a sunspot.
Why are sunspots cooler than the rest of the sun?
Strong magnetic fields in a sunspot block the convection that normally brings hot plasma to the surface. Without that heat supply, the region stays cooler, roughly 1,500 to 2,000 degrees Celsius below the surrounding photosphere. The temperature difference is what makes sunspots appear dark.
How long do sunspots last?
Small sunspots may last only a few hours or days, while large ones can persist for weeks. They change as the magnetic field evolves, growing, splitting and fading. Their numbers rise and fall over an 11-year cycle tied to the Sun's magnetic activity.
Why do sunspots look black?
They are not actually black; they are just cooler than the surrounding photosphere. A sunspot glows at roughly 4,000 degrees Celsius, while the photosphere around it is about 5,500 degrees Celsius. The brightness difference makes the cooler patch look dark against the brighter surface.
Do sunspots move?
Yes, they move because the Sun rotates. Sunspots are carried across the visible disk as the Sun turns, taking about 27 days for one full rotation at the equator. They also grow, change shape and fade over days to weeks as the magnetic field evolves.
What is the sunspot cycle?
Sunspot numbers rise and fall in a cycle of roughly 11 years. During a solar maximum, many sunspots appear; during a solar minimum, few or none are visible. The cycle is linked to the Sun's magnetic field flipping polarity, and it affects space weather near Earth.
Are sunspots dangerous to Earth?
Sunspots themselves are not dangerous, but the magnetic activity that creates them can trigger solar flares and coronal mass ejections. These can disturb satellites, radio communication and power grids. Auroras are a visible sign of that activity reaching Earth's magnetic field.

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