Giant planet landing on a giant planet may or may not be possible, depending on the size and composition of its core. Gas giants consist mostly of hydrogen and helium. The Solar System's gas giants, Jupiter and Saturn, have heavier elements making up between 3 and 13 percent of their mass. Gas giants are thought to consist of an outer layer of molecular hydrogen, surrounding a layer of liquid metallic hydrogen, with a probable molten core with a rocky composition. Jupiter and Saturn's outermost portion of the hydrogen atmosphere has many layers of visible clouds that are mostly composed of water and
Blue giant both cooler and more luminous. Intermediate-mass stars will continue to expand and cool until they become red giants. Massive stars also continue to expand as hydrogen shell burning progresses, but they do so at approximately constant luminosity and move horizontally across the HR diagram. In this way they can quickly pass through blue giant, bright blue giant, blue supergiant, and yellow supergiant classes, until they become red supergiants. The luminosity class for such stars is determined from spectral lines that are sensitive to the surface gravity of the star, with more expanded and luminous stars being given I (supergiant) classifications