🌞Why Do Sunspots Appear Dark?

The sunspot is formed due to the varying magnetic field on the surface of the sun. The strong magnetic force in the sunspot region creates its own pressure. Therefore, the gases in these regions have to exert less pressure to maintain equilibrium. As the gas pressure decreases, their temperature also decreases which creates a temperature difference in the sunspot and its surrounding area. This temperature difference of about 2000 K makes them cooler than the rest of the surface and hence the sunspot appears dark.

What is a sunspot?

Sunspots are the temporary dark regions that are observed on the sun’s surface.

These dark spots have an irregular shape whose diameter size might vary from 10 miles (16 km) to 100,000 miles (160,000 km).

Do You Know?

In 1947, scientists spotted the biggest sunspot group that was about 18 times the surface area of the Earth.

Different sunspots can last for different time periods, anywhere from a few days to a few months. During their appearance, they can change their size by contracting and expanding as they are move over the surface of the sun.

They can be present in groups whose population can increase or decrease over time.

Sunspots complete their solar cycle over a period of 11 years. In this 11 years cycle, sunspot activities go from minimum to maximum and then again from maximum to minimum. The highest point of sunspot activity during the solar cycle is known as solar maximum.

Why these sunspots are dark?

Sunspots on the photosphere of the sun appear dark because the temperature of the sunspot region is comparatively cooler than the temperature of the sun’s surrounding areas. This reduced temperature region that is sunspot, radiates only one-fourth of the light emitted by the rest of the sun’s surface. Thus, the sunspot region appears dark compared to its surrounding region. The temperature of the sunspot can vary in the range of 3000 to 4000 K which is about 2000 K cooler than the temperature of the sun’s surrounding surface (5,778 K).

According to Stefan-Boltzmann Law, the rate of radiation emitted by an object is proportional to the fourth power of their absolute temperature.

E = σT4


E is the maximum rate of radiation emitted per square meter

σ is the Stefan-Boltzmann constant whose value is 5.67 x 10-8W/m2K4

T is the surface temperature of the object in Kelvin

Using Stefan-Boltzmann law to calculate the radiations emitted by sunspot and its surrounding area, we can find out that the radiations emitted by the sunspot are lower than the rest of the sun’s surface. This difference in emitted radiations causes the sunspot to appear dark.

The dark appearance of the sunspots does not mean that they cannot glow brightly. Actually, if we can take out these sunspots from the photosphere and keep them alone in the space, they can glow brightly. They appear dark just due to the relative difference in the temperature of sunspots and the surrounding temperature.

Now we know that the sunspot appears dark because of the temperature difference but what causes this difference in temperature.

Credit: gsfc, flickr

How magnetic field causes sunspot to appear dark?

The magnetic field of the sun gets twisted as it spins which causes the magnetic field to vary with time and location across the sun’s surface. Sometimes this dynamic magnetic force rises up from below the Sun’s surface and creates a sunspot. The strength of the magnetic field in the sunspot region is almost 1000 times stronger than the average magnetic field on the sun’s surface. The gases present in the sunspots have to exert less pressure as compared to the rest of the surface, to maintain equilibrium because of the extra pressure produced by the strong magnetic force in the sunspot region. As the pressure applied by the gases is reduced, the temperature of the gases also gets reduced because according to the van der Waals equation, cooler gas exerts less pressure as compared to the hot gas.

This decrease in the temperature of gases lowers the visible radiations emitted from the sunspot. This results in the temperature and radiation difference between the sunspot and the rest of the sun’s surface. Hence the sunspot region appears dark.

Professor Atom

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Professor Atom is a science enthusiast and alumni of IIT Bombay. According to him, every question can be solved with curiosity and mind mapping. ( Curiosity = Asking Questions = Learning )

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