The Sun’s corona is visible only during a total solar eclipse or when the brightest portion of the star is blocked by an instrument called a coronagraph, as shown here. A coronal mass ejection of hot plasma appears at lower left. Coronal gases reach temperatures of 1,800,000 degrees Fahrenheit (1,000,000 C) or more. The photosphere, or visible surface of the Sun, typically measures up to 10,000 F (5,540 C).