The lighter-colored region around the dark center of a sunspot is called the penumbra.

Here's a breakdown:

* Sunspot: A cooler, darker area on the Sun's surface.

* Umbra: The darkest, central part of a sunspot.

* Penumbra: The lighter, filamentary region surrounding the umbra.

The penumbra is lighter because the magnetic field lines are less concentrated there, allowing for slightly hotter gas to flow into the region.