Notes

Umbra is the central portion of sunspot …

Umbra is the central portion of sunspot. It is darker and its temperature is between 4000 K and 4500 K. It is surrounded by penumbra where temperature may be only about 100 K to 300 K lower than surrounding photosphere. Outside diameter of penumbra is around 2 to 3 times the diameter of umbra. The shape of both may, of curse, be much different from circular.