The Sun is the Solar System's parent star, and far and away its chief component. Its large mass gives it an interior density high enough to sustain nuclear fusion, releasing enormous amounts of energy, most of which is radiated into space in the form of electromagnetic radiation including visible light. It is classed as a moderately large yellow dwarf; however, this name is misleading, as on the scale of stars in our galaxy, the Sun is rather large and bright. Stars are classified based on their position on the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperatures. Generally speaking, the hotter a star is, the brighter it is. Stars which follow this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. This has led many astronomy textbooks to label the Sun as "average;" however, stars brighter and hotter than it are rare, whereas stars dimmer and cooler than it are common. The vast majority of stars are dim red dwarfs, though they are under-represented in star catalogues as we can observe only those few that are very near the Sun in space. The Hertzsprung-Russell diagram. The main sequence is from bottom right to top leftThe Sun's position on the main sequence means, according to current theories of stellar evolution, that it is in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more inefficient elements such as helium and carbon. The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today.[20] Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through its life cycle, and will eventually begin moving off the main sequence, becoming larger and brighter but also cooler and redder, until, about five billion years from now, it too will become a red giant. The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe's evolution. As such, it contains more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars such as those found in globular clusters.[21] Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could be enriched with them. For this reason, the very oldest stars contain very few metals, while stars born later have more. This high metallicity is thought to have been crucial in the Sun's developing a planetary system, because planets form from accretion of metals.[22] The heliospheric current sheetThe Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds greater than 2 million kilometres per hour,[23] creating a very tenuous "atmosphere" (the heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the interplanetary medium. Small quantities of cosmic dust (some of it arguably interstellar in origin) are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet.[24] Earth's magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.