Sunspots appear as dark spots on the surface of the sun, even though they are at temperatures of thousands of kelvin. This demonstration shows this phenomenon in the classroom.
In this video you first see that the light bulb's filament appears bright both on top of the projector and when projected onto the overhead screen.
The camera then zooms in for a closer look at the filament projected onto the screen. The variable power supply is then slowly turned down and you see the filament go from nearly white in colour to somewhat orange and then black.
As the camera zooms out, you see that the light bulb is still shining quite brightly. The camera zooms in on the light bulb to show that the filament is not black and is actually still very bright. The projection of the filament appears black since the filament is now at a lower temperature than the surrounding surface of the projector.
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Magnetic fields are also an important factor in the creation of sunspots. The Sun has a magnetic field with a magnetic north and south pole, just like Earth. However, over a cycle of 22 years, the magnetic north and south poles of the Sun will switch places. Various observations have confirmed that sunspots are associated with areas of strong magnetic fields and that their appearance is linked to the 22 year cycle of the Sun's magnetic pole reversal.
Another important concept in explaining sunspots is convection. The Sun is a main sequence star which is burning hydrogen into helium in its core. In certain areas of the Sun, the energy created by the hydrogen burning is carried to its surface by convection, similar to a pot of boiling water. Convection is the main mechanism for transporting energy to the surface of the Sun. However, strong magnetic fields can inhibit the motion due to convection, so energy is unable to reach the surface in these areas. This leads to areas with temperatures cooler than that of the rest of the surface of the Sun.
The dark colour of the sunspots is due only to the significant difference between their temperature and the temperature of the surface of the Sun. The temperature of sunspots can be as low as 3900K, whereas the Sun's surface temperature is 5777K. Flux is related to the amount of light energy emitted from an object, and using the surface flux equation, F = σT4, we see that the flux from a sunspot is a factor of 4.8 lower than that of the surrounding surface:
Fsun/Fsunspot = σ(5777)4/σ(3900)4
1. Plug the light bulb into the variable power source and place it on the overhead projector. Placing dark fabric over the remainder of the overhead makes viewing the light bulb and its filament easier on the screen.
2. Turn the overhead projector and the power supply on. Turn the power supply up until the light bulb filament appears bright on the overhead projector. View the brightness of the light bulb as it sits on the overhead projector.
3. Slowly turn down the power supply just until the image of the light bulb's filament appears dark on the projection screen.
4. Notice that the filament is not actually dark, just the projection of it is. This is because the temperature of the filament is now less than that of the surrounding screen visible through the light bulb.