The Sun is the star at the center of the Solar System and is by far the most important source of energy for life on Earth. It is a nearly perfect spherical ball of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. Its diameter is about 109 times that of Earth, and it has a mass 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; the rest is mostly helium, 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 spectral class and 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 increasingly hot and dense, eventually initiating nuclear fusion in its core. It is thought that almost all stars form by this process.
The Sun is roughly middle aged and has not changed dramatically for over four billion years, and will remain fairly stable for more than another five billion years. However, after hydrogen fusion in its core has stopped, the Sun will undergo severe changes and become a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury, Venus, and possibly Earth.
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. Earth's movement around the Sun is the basis of the solar calendar, which is the predominant calendar in use today.
UY Scuti is a bright red supergiant and pulsating variable star in the constellation Scutum. It is a leading candidate for being the largest known star by radius and is also one of the most luminous of its kind. It has an estimated average median radius of 1,708 solar radii, or a diameter of 2.4 billion km (1.5 billion mi; 15.9 AU); thus a volume 5 billion times that of the Sun. It is approximately 2.9 kiloparsecs (9,500 light years) from Earth. If placed at the center of the Solar System, its photosphere would engulf the orbit of Jupiter, although the radius is not known for certain and may be larger than the orbit of Saturn.
Explanation from: https://en.wikipedia.org/wiki/Sun and https://en.wikipedia.org/wiki/UY_Scuti