Sun

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, i.e. 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally referred to as a yellow dwarf. It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.

The Sun is roughly middle-aged; it has not changed dramatically for more than four billion years, and will remain fairly stable for more than another five billion years. The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the predominant calendar in use today.

Heliosphere
The Sun drives a consistent magnetic system that surrounds it, Earth, and the planets, and which extends further than the solar system, known as the heliosphere. While sunlight fundamentally enables and sustains life, the Sun can also produce radiation and magnetic energy that can disrupt planets’ atmospheres, satellites, and even life. Study of the space environment is required, in order to send spacecraft and astronauts through it safely. Space weather can interfere with communications, artificial satellites and power grids. The study of the sun and space helps to understand the forces that forged the planets in the beginning and even other star systems.

The heliosphere is the subject of intense study in NASA's. Heliophysics research of the heliosphere includes: the sun’s connection to Earth and other planets at both small and large scales, the sun's 11-year solar cycle; giant solar explosions such as solar flares and coronal mass ejections; the constant stream of solar particles called the solar wind; the magnetic environment near Earth; what drives change in the charged particles that surround Earth and inhabit the ionosphere; and the boundaries of the solar system as it travels through our interstellar neighborhood.