The Sun, our star, is at the center of the solar system and critical to our existence. Without the energy supplied by the Sun, Earth would be a cold and inhospitable planet — it would have no liquid water, no weather, and no life. The Sun contains approximately 99% of the mass of the entire solar system and its gravity holds the solar system together. More than one hundred Earths could line up across the Sun's diameter, and over one million Earths could fit inside it.

However, compared with other stars, the Sun is rather ordinary. It is about in the middle of the ranges for star size and brightness. Many of the stars that you can see in the night sky are actually bigger and brighter than the Sun — they only appear smaller because they are much farther away. However, there are also many stars dimmer than the Sun that are too faint to be seen from Earth. In fact, these dim stars are by far the most common stars in the galaxy. Thus, despite the Sun being in the middle range for all possible types of stars, when considering the total population of stars in the galaxy, the Sun is larger and brighter than most.

Like all stars, the Sun is made of hot gases. It doesn't have a solid surface, but its surface is defined by the photosphere — distinctly visible from a distance — which has a temperature of about 5,800 K (9980°F). The photosphere looks granular because of convection cells of hot gas within the Sun, similar to those on the surface of boiling water. The photosphere sometimes displays dark blemishes called sunspots, which are caused by the Sun's magnetic field. Above the photosphere are a thin layer called the chromosphere and the outermost layer of the Sun, the corona, a wispy and extremely hot layer with a temperature of a few million Kelvin. The Sun is very active and produces events such as solar flares, coronal holes, and coronal mass ejections.

The Sun is composed of about 75% hydrogen, 25% helium, and trace amounts of heavier elements. At its core, the Sun's temperature reaches over 15 million K (27 million°F) and its pressure is over 200 billion times the pressure at Earth's surface. At these temperatures and pressures, hydrogen is converted into helium in the process of nuclear fusion. The energy created in the core travels outward toward the Sun's surface and is radiated as light. This energy takes about 8.5 minutes to reach Earth, which orbits about 150 million kilometers (93 million miles) from the Sun.

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