A red giant is a star that has exhausted the supply of hydrogen in its core and has begun thermonuclear fusion of hydrogen in a shell surrounding the core. They have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is lower in temperature, giving them a reddish-orange hue. Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size. Red-giant-branch stars have luminosities up to nearly three thousand times that of the Sun (L☉), spectral types of K or M, have surface temperatures of 3,000–4,000 K, and radii up to about 200 times the Sun (R☉). Stars on the horizontal branch are hotter, with only a small range of luminosities around 75 L☉. Asymptotic-giant-branch stars range from similar luminosities as the brighter stars of the red giant branch, up to several times more luminous at the end of the thermal pulsing phase.
This important measurement was made using Cassini's INMS instrument, which detects gases with the goal of determining their composition. INMS was designed to sample the upper atmosphere of Saturn's large, smoggy moon Titan. However, after Cassini's surprising discovery of a tall plume if icy spray erupting from cracks on Enceladus in 2005, planetary scientists turned its detectors to that small moon. Dr. Carolyn Porco, a planetary scientist and leader of the Imaging Science team for Cassini, explained to the press in March 2012 that "More than 90 jets of all sizes near Enceladus's south pole are spraying water vapor, icy particles, and organic compounds all over the place. Cassini has flown several times now through this spray and has tasted it. And we have found that aside from water and organic material, there is salt in the icy particles. The salinity is the same as that of Earth's oceans." Moons, Myths, Etc. Our bewitching, and sometimes bewildering Moon, has long been the inspiration for magical myths, weird legends, bedtime stories, and beautiful poetry. Earth's Moon is a very ancient symbol of femininity, as well as for wild bouts of strange madness and romantic love. Some ancient, traditional legends and childhood stories tell of a man's face etched out on its shining surface, while others tell strange tales of a "Moon Rabbit." Lovely, ancient myths, tales, and bedtime stories aside, Earth's Moon is a very real object. It has been a companion-world to our Earth almost from that very ancient era, when our Solar System was first forming, about 4.5 billion years ago.