The Sun formed about 4.567 billion[a] years ago from the gravitational collapse of a region within a large molecular cloud. Most of the matter gathered in the center, whereas the rest flattened into an orbiting disk that would become the Solar System. The central mass became increasingly hot and dense, eventually initiating thermonuclear fusion in its core. It is thought that almost all stars form by this process. The Sun is a G-type main-sequence star (G2V) based on spectral class and it is informally designated as a yellow dwarf because its visible radiation is most intense in the yellow-green portion of the spectrum. The Sun’s color is white, with a CIE near (0.3, 0.3), when viewed from space or when high in the sky; when low in the sky, atmospheric scattering renders the Sun yellow, red, orange, or magenta. Despite its typical whiteness, most people mentally picture the Sun as yellow; the reasons for this are the subject of debate. In the spectral class label, G2 indicates its surface temperature, of approximately 5778 K (5505 °C, 9941 °F), and V indicates that the Sun, like most stars, is a main-sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses about 620 million metric tons of hydrogen each second.