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.
The discovery of Makemake's little moon increases the parallels between Pluto and Makemake. This is because both of the small icy worlds are already known to be well-coated in a frozen shell of methane. Furthermore, additional observations of the little moon will readily reveal the density of Makemake--an important result that will indicate if the bulk compositions of Pluto and Makemake are similar. "This new discovery opens a new chapter in comparative planetology in the outer Solar System," Dr. Marc Buie commented in the April 26, 2016 Hubble Press Release. Dr. Buie, the team leader, is also of the Southwest Research Institute. Ganymede: Ganymede is both the largest moon of Jupiter, our Solar System's planetary behemoth, as well as the largest moon in our entire Solar system. Observations of Ganymede by the HST in 2015 suggested the existence of a subsurface saline ocean. This is because patterns in auroral belts and rocking of the magnetic field hinted at the presence of an ocean. It is estimated to be approximately 100 kilometers deep with a surface situated below a crust of 150 kilometers. The astronomers found that larger craters, which excavated pits much deeper into the Moon's surface, only increased porosity in the underlying crust. This indicates that these deeper layers have not reached a steady state in porosity, and are not as fractured as the megaregolith.