The following paragraphs and images are those from the Wikipedia page on the Sun, the links therein direct to Wikipedia articles on the corresponding item. At the bottom of the page is a video clip from NASA Goddard Space Flight Center showing a brief period in the present life of this our closest star!
The Sun (in Greek: Helios, in Latin: Sol) 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 4.567 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 four billion years, and will remain fairly stable for another four 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 the 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.
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They correspond to concentrations of magnetic field flux that inhibit convection and result in reduced surface temperature compared to the surrounding photosphere. Sunspots usually appear in pairs, with pair members of opposite magnetic polarity. The number of sunspots varies according to the approximately 11-year solar cycle.
Individual sunspots may endure anywhere from a few days to a few months, but eventually decay. Sunspots expand and contract as they move across the surface of the Sun with a size ranging from 16 kilometres (10 mi) to 160,000 kilometres (100,000 mi) in diameter. The larger variety are visible from Earth without the aid of a telescope. They may travel at relative speeds, or proper motions of a few hundred metres per second when they first emerge.
Indicating intense magnetic activity, sunspots accompany secondary phenomena such as coronal loops (prominences) and reconnection events. Most solar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars other than the sun are commonly called starspots and both light and dark spots have been measured.
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(credit: NASA’s Goddard Space Flight Center, published on Nov 1, 2015)
It’s always shining, always ablaze with light and energy that drive weather, biology and more. In addition to keeping life alive on Earth, the sun also sends out a constant flow of particles called the solar wind, and it occasionally erupts with giant clouds of solar material, called coronal mass ejections, or explosions of X-rays called solar flares. These events can rattle our space environment out to the very edges of our solar system. In space, NASA’s Solar Dynamics Observatory, or SDO, keeps an eye on our nearest star 24/7. SDO captures images of the sun in 10 different wavelengths, each of which helps highlight a different temperature of solar material. In this video, we experience SDO images of the sun in unprecedented detail. Presented in ultra-high definition, the video presents the dance of the ultra-hot material on our life-giving star in extraordinary detail, offering an intimate view of the grand forces of the solar system.
This video is public domain and can be downloaded at: http://svs.gsfc.nasa.gov/12034