A giant sunspot complex is marching across the Sun's face this week, much to the delight of avid skywatchers. The spot group is so large it is visible without magnification — all you need to see it is a sunny day and a safe solar filter. Of course, a telescope makes the view all the more spectacular.
The spot group, designated active region 10030 by the U.S. National Oceanic and Atmospheric Administration (NOAA), is one of the largest of the current solar cycle. It features a complex, twisted magnetic field and has spawned numerous solar flares, including a very energetic ("X-class") flare earlier today. The chain of large and small spots spans about 220,000 kilometers (136,000 miles), or 17 Earth diameters.
Sunspots are dark blemishes on the Sun's photosphere, or visible surface. They form where the solar magnetic field traps ionized gas and allows it to cool. Whereas the photosphere's average temperature is 5,700° Kelvin, in the dark center ("umbra") of a sunspot, where the field is strongest, it's about 2,000°K cooler. If you could view a sunspot by itself, it would appear blazingly bright. But in contrast with its even brighter surroundings, it appears dark. The umbra of a large spot is typically surrounded by a lighter penumbra, where the magnetic field isn't quite as strong.
The Sun turns on its axis roughly once a month, so active region 10030, having just crossed the center of the solar disk, should remain visible for several more days. Solar astronomers are monitoring it closely using ground-based telescopes and orbiting satellites, since powerful flares could produce geomagnetic storms and disrupt radio communications here on Earth.