Sunspots look dark because they are roughly 2,000 degrees cooler than the surrounding solar surface. They’re still incredibly hot, around 3,800 to 4,300 Kelvin (about 6,400°F), but the photosphere around them blazes at 5,800 Kelvin. That temperature gap makes sunspots appear nearly black by contrast, even though a sunspot in isolation would glow brighter than an electric arc welder.
Magnetic Fields Block the Heat
The root cause is magnetism. Sunspots sit where intense magnetic fields punch through the solar surface, with field strengths reaching several thousand gauss. For perspective, that’s thousands of times stronger than Earth’s magnetic field. These fields are strong enough to physically suppress the normal churning motion that carries heat from the Sun’s interior to its surface.
On the rest of the photosphere, superheated plasma rises in convection cells (visible as a granular, bubbling texture), delivers its energy, cools slightly, and sinks back down. Inside a sunspot, the magnetic field lines are so tightly packed and powerful that they essentially freeze the plasma in place. The gas can’t move freely across field lines, so the normal upward flow of hot material gets choked off. With less fresh heat arriving from below, the sunspot region keeps losing energy through radiation without being replenished, and it cools down.
This process feeds on itself. As the gas cools, it becomes denser and sinks, pulling in more magnetic flux and strengthening the field further. The result is a self-sustaining pocket of relative cold on an otherwise scorching surface.
How Much Dimmer a Sunspot Really Is
The brightness difference is more dramatic than the temperature gap alone suggests. The energy radiated from a hot surface scales with the fourth power of its temperature. So a modest temperature drop produces a much larger drop in brightness. A patch of normal photosphere at 5,780 Kelvin radiates about 63 million joules per second per square meter. A sunspot at 4,280 Kelvin radiates roughly 19 million. That means the surrounding surface is about 3.3 times brighter than the sunspot.
Your eyes perceive this ratio as a stark dark-versus-bright contrast, which is why sunspots look almost black in photographs. If you could somehow peel a sunspot off the Sun and view it against the dark sky, it would be blindingly luminous on its own. The darkness is entirely relative.
The Anatomy of a Sunspot
Sunspots aren’t uniform dark blobs. Each one has a layered structure with distinct zones that reflect how the magnetic field is oriented.
- Umbra: The innermost core, and the darkest part. Here the magnetic field is most intense and points nearly straight up (perpendicular to the surface). Convection is almost completely shut down, making the umbra the coolest region.
- Penumbra: A lighter, striated ring surrounding the umbra. The magnetic field here is weaker and tilts more horizontally. Because the field isn’t as dominant, some convective motion still occurs, so the penumbra is warmer and brighter than the umbra, though still dimmer than the normal photosphere.
Not every sunspot develops a penumbra. Smaller ones, sometimes called pores, consist of just a dark umbral core a few thousand kilometers across. Larger, more developed sunspots can span tens of thousands of kilometers with a well-defined penumbral ring.
How Scientists Confirmed the Magnetic Connection
The link between sunspots and magnetism was confirmed in 1908, when astronomer George Ellery Hale observed a telltale signature in the light coming from sunspots. When light passes through a strong magnetic field, its spectral lines split into multiple components, a phenomenon called the Zeeman effect. Hale detected exactly this splitting in sunspot spectra, providing the first direct evidence that sunspots are regions of powerful magnetic activity. He went on to derive the polarity patterns that sunspots follow across solar cycles.
Sunspots and the Solar Cycle
Sunspot numbers rise and fall over roughly 11-year cycles. The Sun is currently in Solar Cycle 25, which NOAA’s Space Weather Prediction Center predicted would peak around July 2025, with a maximum monthly sunspot number of about 115. The actual peak could fall anywhere between late 2024 and early 2026.
During solar maximum, it’s common to see dozens of sunspots on the solar disk at once, some large enough to be visible (with proper eye protection) without a telescope. During solar minimum, weeks can pass with no sunspots at all. The cycle matters because sunspots are linked to solar flares and coronal mass ejections, which can affect satellite communications, power grids, and produce vivid auroras on Earth. More sunspots generally means more space weather activity.