Its radius decreases by 1 or 2 kilometers during periods of high solar activity
Solar Dynamics Observatory/NASA
How big is the sun? Well, that depends on when you’re measuring.
The sun slightly shrinks and expands as it goes through a solar cycle, a roughly 11-year period of high and low magnetic activity, a new study finds. When the sun is the most active, its radius decreases by 1 or 2 kilometers, two researchers report in a paper accepted in the Astrophysical Journal. Given that the sun’s full radius is about 700,000 kilometers, that’s a tiny change.
Unlike a planet, the sun has no solid surface, so defining the absolute size isn’t simple. “It’s a slippery concept: What does it mean, the radius of the sun?” says astronomer Jeff Kuhn of the University of Hawaii in Maui. One way of defining the sun’s width is based on how the brightness of the sun decreases from its center. In 2010, Kuhn and colleagues