What are the umbra and penumbra of a sunspot?

In the study of the Sun, a mature sunspot is characterized by two distinct regions defined by their appearance and magnetic properties:1

• Umbra: This is the dark central core of the sunspot. It appears dark because it is significantly cooler—approximately 3,000 K—compared to the 5,800 K temperature of the surrounding photosphere. In this region, the magnetic flux loop emerges vertically from below the surface.1
• Penumbra: This is the less-dark, outer ring of fibrils that surrounds the umbra. In this area, the magnetic field spreads outward horizontally, allowing for detectable horizontal flows of material.1

While the umbra is too magnetically intense to permit much motion, the penumbra is the site of various organized activities, such as the Evershed effect, which involves horizontal flows of material. Together, these structures represent regions of extremely strong magnetic fields on the Sun's surface.1