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Sunspots have been monitored on the Sun since 1610, shortly after the invention of the telescope. They provide the longest-running direct measurement of our star's activity.

The variation in sunspot numbers has revealed the Sun's 11-year cycle of activity as well as other, longer-term changes.

In particular, it has been noted that between about 1645 and 1715, few sunspots were seen on the Sun's surface. This period is called the Maunder Minimum after the English astronomer who studied it.

It coincided with a spell of prolonged cold weather often referred to as the "Little Ice Age". Solar scientists strongly suspect there is a link between the two events - but the exact mechanism remains elusive.

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A sunspot group aimed squarely at Earth has grown to 20 times the size of our planet and has the potential to unleash a major solar storm.

Sunspots are areas of intense magnetic energy, cooler and darker than the surrounding surface of the thermonuclear furnace. Sometimes the magnetic fields let loose and huge amounts of radiation and charged particles are hurled into space.

The Sun's last bout of intense storminess occurred last fall, when a string of 10 major flares over two weeks knocked out satellites, damaged others, and forced the FAA to reroute airlines away from exposed polar routes.

"The implications of this spot have scientists on the edge of their seats," NASA said in a statement Friday. "If the active region generates coronal mass ejections (CMEs), massive explosions with a potential force of a billion megaton bombs, it will be a fairly direct hit to Earth and its satellites and power grids."

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