Sen—The Sun has shown some unpredictable behaviour in recent years. But professional astronomers expect its activity to reach an explosive climax later this year.

Our home star does not behave in a constant manner. It has quiet years and more busy years in a cycle of activity that goes from maximum to minimum and back to maximum in around 11 years.

Around maximum, we see many more sunspots, those blotches on the visible surface, or photosphere, that are really just slightly cooler zones which look dark in contrast to the general surface glow.

This 11-year pattern was first noticed in the mid 19th Century. Later, in 1908, leading observer George Ellery Hale, at Mount Wilson, California, discovered that the spots were produced in powerful magnetic fields. He also noted that the directions of the spots' north-south polarities reversed with each 11-year cycle.

During a solar cycle, the position of these spots migrates from the Sun's higher latitudes towards its equator as activity diminishes, and around minimum, few if any spots are seen at all.

This was especially true during the last solar minimum which was surprisingly prolonged with no spots showing for many months on end. The Sun stayed relatively quiet when so-called Cycle 24 was supposed to have reached a stage when many spots would be visible regularly.

It led some astronomers to wonder if the new scheduled cycle of solar activity, Cycle 25, might happen at all. Others speculated whether the Sun's slowdown might herald a new "mini ice age" like one that happened three centuries ago following a prolonged quiet spell termed the Maunder Minimum.

But over the last couple of years, solar activity has picked up again with several sunspot groups making a healthy appearance, flares and regular eruptions known as coronal mass ejections (CMEs) which can spark auroral displays over the Earth's polar regions.

The Sun shows an outbreak of sunspots soon after the last maximum. Credit: NASA

The maximum of this cycle is expected to be reached in the summer or autumn of 2013 after which activity would be expected gradually to dwindle again.

One of the UK's leading solar astronomers is Dr Lucie Green, of the Mullard Space Science Laboratory in Surrey. She told Sen: "The Sun has shown some unexpectedly low levels of activity in the recent years. Despite this though, the current cycle is progressing with a solar maximum due this year.

"This is an exciting time as we have a fleet of spacecraft and ground-based telescopes watching the Sun, giving us data that will ultimately tell us why solar cycles vary in strength and duration as well as allowing us to forecast stormy solar weather which is headed our way."

Warning: On no account look at the Sun directly unless you are using professional solar filters and know exactly what you are doing. Ordinary dark glasses offer no protection and you could be blinded!