Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake,
forming areas of reduced surface temperature. Although they are at
temperatures of roughly 3,000â€“4,500 K (2,727â€“4,227 Â°C), the contrast
with the surrounding material at about 5,780 K leaves them clearly
visible as dark spots, as the intensity of a heated black body
(closely approximated by the photosphere) is a function of temperature
to the fourth power. If the sunspot were isolated from the surrounding
photosphere it would be brighter than an electric arc.
Sunspots expand and contract as they move across the surface of the Sun
and can be as large as 80,000 kilometers (49,710 mi) in diameter,
making the larger ones visible from Earth without the aid of a telescope. They may also travel at relative speeds ("proper motions") of a few hundred m/s when they first emerge onto the solar photosphere.