The sun is the largest and the most massive object in the solar system. It is a medium-size star among the hundreds of billions of stars in the Milky Way galaxy. Its radius is about 696,000 kilometers (432,450 miles), which makes its diameter about 1.392 million kilometers (864,938 miles). Scientific prediction is that in about 5 billion years the sun will start to use up all of the hydrogen at its center. It will expand into a red giant and passing the orbits of the inner planets, including Earth.
The sun is the closest star to Earth, at a distance from our planet of about 150 million kilometers (93 million miles). This distance is known as an astronomical unit (abbreviated AU), and sets the scale for measuring distances all across the solar system. The sun has six regions: the core, the radiative zone, the convective zone in the interior, the visible surface (the photosphere), the chromosphere, and the outermost region - the corona. The sun has no solid surface.
At the core, the temperature is about 15 million degrees Celsius (about 27 million degrees Fahrenheit), which is sufficient to sustain thermonuclear fusion. The energy produced in the core powers the sun and produces essentially all the heat and light we receive on Earth. The sun surface, the photosphere, is 500 kilometers (300 miles), from which most of the sun's radiation escapes outward and is detected as the sunlight we observe here on Earth about eight minutes after it leaves the Sun. The temperature of the photosphere is about 5,500 degrees Celsius (10,000 degrees Fahrenheit). Above the photosphere lie the chromosphere and the corona (crown). Here, the temperature increases with altitude, reaching as high as 2 million degrees Celsius (3.5 million degrees Fahrenheit).