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.
An Icy Nest Of Space Eggs. Methone is actually only one member of an icy nest of Space eggs, which also includes the very strange and smooth moons of Saturn, Pallene and Aegaeon. Aegaeon is a very, very small moonlet that also twirls around between Mimas and Enceladus. Like Methone, Aegaeon displays a mysteriously unblemished surface.
Titan: Titan, the tormented, hydrocarbon-slashed largest moon of Saturn--and the second largest moon in our Solar System, after Ganymede--could possess a subsurface, salty ocean that may well be as salty as the Dead Sea on Earth. The salty water could begin approximately 31 to 62 miles beneath Titan's icy shell, according to recent estimates. Meanwhile, on Titan's smog enshrouded surface, "life as we do not know it" could swim in alien lakes and rivers that flow with liquid methane and ethane hydrocarbons--instead of water.
The scientists also ruled out the possibility that the mysterious features actually exist on Titan's surface in the form of frozen methane rain or icy lava erupted from cryovolcanoes. Such surface features would show a different chemical signature and would be visible for much longer periods of time than the bright features observed in this study. The bright features were visible from time spans of only 11 hours to five weeks.