The group is the biggest seen on the surface of our star for more than a decade.
Two major flares have erupted on the surface of the Sun in the region of the giant sunspot group designated Noaa 9393.
The group is the biggest seen on the surface of our star for more than a decade.
One of the flares (explosions in the Sun's atmosphere) was so energetic that it ejected a cloud of superhot gas into space.
Solar observers say that the cloud, called a coronal mass ejection (CME), is heading towards the Earth and will reach us within the next 24 to 36 hours.
Scientists are predicting a "geomagnetic storm" when it reaches us. There may be radio and communications interference, and possibly a fine display of the Northern, and Southern, Lights, visible from lower latitudes than is usually possible.
Very excited
Geoff Elston, director of the solar section of the British Astronomical Association (BAA), has been studying the monster sunspot, Noaa 9393, for several days.
"It is definitely the largest we have seen for a long time, probably for many
decades," he told BBC News Online. "It has overtaken the size of the most recent large one that occurred in March 1989."
Elston said observers did not think the spot would get any bigger.
"Over the past day or so, it has shrunk a little and this trend is likely to continue. It's probably past its best."
Noaa 9393 has provided astronomers with an insight into the complicated way magnetic fields behave on the Sun's surface.
Record holder
Researchers were able to observe the dynamic interface between two regions of the sunspot that had opposite polarity.
It was along the dividing line between the spot's north-pole and south-pole regions that the big flare took place. Magnetic energy was released along the interface and turned into heat. This caused the explosion and the CME.
Although very large by normal standards, Noaa 9393 is way short of the all-time record holder. That title is held by a spot group which appeared in 1947. It was three times larger than Noaa 9393.
Sunspots are magnetic regions on the Sun's surface. More of them appear on the Sun during the time of solar maximum, which happens every 11 years. Astronomers believe that 2000-1 is the time of the current maximum.