At more than one million degrees, the sun's atmosphere—the corona—is incredibly hot; but not everywhere. Time and again, huge structures of significantly cooler solar plasma—about 10,000 degrees—appear within the corona. These structures are known as prominences. They span up to several thou sand kilometers and often resemble flickering flames that can take on a wide variety of shapes. Despite their delicate appearance, they are massive "chunks of matter": their density exceeds that of the surrounding corona by more than a hundred. ...read more read less