Solar Cycle

The Sun has been a calm, steady main-sequence star, burning hydrogen into helium for more than 4.5 billion years. And that's a relief, because life on Earth depends on the Sun being steady and constant. But there are regular variations that happen over a shorter period of time that keep the Sun active and interesting. The periodic rise and fall of solar activity is called the solar cycle.

Astronomers aren't sure why, but the Sun's magnetic field flips polarity every 11 years. In other words, the Sun's north pole becomes its south pole every 11 years. It takes a full 22 years for the solar cycle to come full circle, so that the Sun's polarity returns to its original position.

The changing magnetic field can cause a tremendous amount of solar activity, increasing the number of sunspots, solar flares and coronal mass ejections. The amount of solar activity also rises and falls in an 11-year cycle. At one point, the solar maximum, the Sun's surface is dotted with sunspots, and flares are a regular occurrence. Halfway through the cycle, the activity settles down, and there are very few (and even no) sunspots. And then, 11 years later, the Sun returns to its maximum state.

Astronomers track these solar cycles and even give them numbers. Solar Cycle 23 began in the late 1990s and lasted until 2008. Solar Cycle 24 began in 2008 and is expected to last until around 2019-2020.

We have written many articles about the solar cycle. Here's one about Solar Cycle 24 finally beginning in 2008. And here's an article about how some of the activity of the Sun during the solar cycles.

One of the best places to track the activity on the Sun is at SpaceWeather.com.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

Filed under: Astronomy