The Sun has dark areas on its surface. We call these small dark areas sunspots. This may lead to some people asking why do sunspots appear dark in pictures of the Sun.
Sunspots are not permanent features on the Sun’s surface. They appear and disappear and move across the face of the Sun. The number of sunspots increases and decreases in a relatively short period of time. These cycles have some consequences for life on our planet.
In this article, we will directly answer the question in the next section. After that, we will go into detail about different aspects of sunspots that may be of interest.
Why do Sunspots Appear Dark in Pictures of the Sun?
Sunspots appear dark in pictures of the Sun because they contain cooler gas than the surrounding gas.
Knowing the reason may cause you to have more questions. An obvious question is why cooler gas emits less light. Similarly, how can the temperature be so low that it doesn’t appear to emit any light?
We need to go a bit deeper to answer these and other questions. First, we will detail exactly what sunspots are before explaining why light differs.
What Are Sunspots?
Sunspots are irregularly shaped regions of relatively low temperature on the Sun’s surface. They may appear in isolation or as part of a group. They vary in size but are typically 10s of thousands of kilometers across. While we stated that they appear on the Sun’s surface, they technically occur in what is called the photosphere. It is this region from which light is emitted.
Sunspots have a dark central core which is known as an umbra. Surrounding the umbra is a brighter border called a penumbra. You can see both of these features in the image above.
The temperature of the gas in the central regions of sunspots is about 4300 K (Kelvin – K is a temperature measurement). The surrounding region is at about 5000 K. This compares to the Sun’s average photosphere temperature of 5800 K. (4300 K = 40260C, 72800F: 5000 K = 47270C, 85400F: 5800 K = 55270C 99800F)
If you block the region outside of sunspots you will find that the central region of sunspots is red. The surrounding region is orange. The reason that you may not see these colors is because the normal regions of the Sun are very bright. Furthermore, the central region of sunspots only emits about 30% of the light areas not affected by sunspots. They appear dark only because the surrounding area is very bright.
Sunspot activity occurs cyclically. The Sun is said to be active when there are many sunspots. As such, sunspots are active Sun features.
Sunspots are the results of concentrated magnetic fields. Due to this, the driver for the sunspot cycle is the Sun’s magnetic field. For more information on the sunspot cycle see our article on The Sunspot Cycle and the video below.
How is Light Emission Related to Temperature?
If you have ever observed metal being heated, you may have seen that it changes color. After an initial period, the metal starts to glow a dull red. As heating continues, it becomes brighter and glows a brighter red. Eventually, it turns orange while continuing to get brighter. If it is heated sufficiently, it will start to glow brightly in white light.
To determine the color of light emitted by a body, we consider it a ‘ black body’. By that, we assume that it will perfectly emit light in direct relation to its temperature. This is often not the case, but it is a starting point.
Wien’s displacement law determines the radiation emitted by a black-body. This law states that the ‘black-body radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature.’ This means that matter at different temperatures emits different colored light. Hotter bodies will emit more shorter wavelength light (i.e., more blue) while cooler bodies will emit more longer wavelength light (i.e., more red light). Incidentally, cooler bodies will stop emitting visible light but can emit infrared light, which we can not see but feel as heat.
The Blackbody Sun
The diagram on the right illustrates the effect of temperature on the light emitted from a body. A body can be a solid or a gas, as no distinction is made.
As mentioned above, the average temperature of the Sun’s surface is 5800 K. This is a little higher than the blue line. It peaks in the middle part of the visible spectrum. As a consequence, we see it as white light.
The central regions of sunspots are approximately 4300 K. This is a little higher than the green line. The peak is, therefore, in the red part of the spectrum, so the light appears red. Also, note that the amount of light emitted is less.
As a result, the central regions of sunspots are red and not as bright as the normal parts of the Sun’s surface. Because of the high contrast, we see them as dark regions and only see red if we block the blinding light.
At this stage, you might be wondering where the energy comes from to keep the surface of the Sun hot. The energy comes from the Sun’s core, which fuses hydrogen to form helium. For information on this process and how the energy migrates to the Sun’s surface, see How Do Stars Shine.
Concluding Thoughts
Hopefully, you understand why sunspots appear dark in pictures of the Sun.
It is not because there is a hole in the Sun. The reason is that the Sun’s concentrated magnetic field restricts the movement of heat to the surface of the Sun. Because the lower temperature results in the emitted light being less bright and red. We can not see the color due to the brightness of the surrounding areas.
We have also discovered that the timing of the solar cycle drives the number of sunspots.
Observing sunspots is a fascinating activity that can be conducted using a small telescope. However, an appropriate filter must be used.
For more information on the effects of sunspot activity you can see Space Weather.
Frequently Asked Questions
What causes sunspots?
As the Sun rotates, it does not do so evenly. Regions nearer the Sun’s equator rotate faster than those nearer the poles. Because of this differential rotation, the Sun’s magnetic fields become twisted. As a consequence, the magnetic field becomes strongly concentrated in some areas. These areas are where sunspots form.
Why are sunspots cooler than other regions?
As stated in the last question, sunspots result from concentrated magnetic fields. A concentrated magnetic field can stop energy from being transmitted. This results in the gas being cooler where sunspots exist.
Where are sunspots located?
Sunspots are not distributed evenly on the Sun’s surface. They are mostly found between the equator and the mid-latitudes of each hemisphere. The video below explains why this is so.
How do we know that sunspots are the result of magnetism?
We can’t directly measure magnetism on the Sun’s surface. However, some techniques indicate the presence of a magnetic field. Light from the Sun’s surface results from changes within gas atoms. Light is emitted when an electron moves to a lower energy level. Because of other influences within the atom when in strong magnetic fields the spectrum of the light is changed in a measurable way. The strength of the magnetic field dictates how much the light is affected. For more information, see the Zeeman effect.
What is the sunspot cycle?
The number of sunspots increases and decreases regularly. On average, there are about 11 years between a minimum and a maximum number of sunspots. The sunspot cycle is driven by the cyclic changes in the Sun’s magnetic field due to the twisting of its magnetic field due to differential rotation.
See the video below for more details on the sunspot cycle.
Is a sunspot a hole in the Sun?
No, sunspots are not a hole in the Sun. They are cooler areas of the Sun, as described above.
How to observe sunspots?
Never view the Sun without using a suitable filter. If using a telescope, the filter must be fitted to the front of the telescope so that no light enters the instrument.
What a suitable filter you can observe sunspots with a small telescope. It is also possible to see larger sunspots without a telescope (again, a suitable filter must be used).
Are sunspots harmful?
No, sunspots are not harmful to life on Earth.
Do sunspots affect our climate?
Yes, sunspots do have a small effect on our climate. Modelling shows that during high solar activity (i.e. when there are many sunspots) the amount of energy we receive from the Sun is slightly higher. This information was considered in light of the observed climate change. However, it was shown that solar activity does not explain the amount of observed heating.
I found astronomy while working in dark rural locations. Initially, I explored the night sky and learnt the constellations before purchasing a pair of binoculars to further my knowledge of the sky.
My first telescope was a 200 mm Newtonian reflector on an equatorial mount. I found that this telescope had a steep learning curve but was a rewarding experience.
As time progressed, I became interested in astrophotography. This resulted in purchasing a 110 mm refracting telescope and a dedicated monochrome-cooled astronomical camera. This resulted in another very rewarding steep learning curve that far surpassed the experience with my first telescope.
I have joined Telescope Guru to share my knowledge of telescopes and astronomy.
