ABOUT THE SUN
The Sun is our nearest star. Its light and heat make life on Earth possible.
When did the Sun form?
Scientists calculate the Sun and solar system formed at approximately the same time, 4.55 billion years ago. This is based on the ages of the oldest objects that we have sampled from our solar system, meteorites.
How did the Sun form?
The solar nebula theory describes how most scientists think the Sun formed. A cloud of hydrogen and helium gas and dust existed in space. It began to compress and eventually gravitational forces pulled the gas and dust together and the cloud collapsed. The collapsing cloud began spinning and flattening into a disk. Much of the material was concentrated in the center of the spinning mass, where compression resulted in a “protosun” of increasing density and temperature. Eventually the heat and pressure increased to the point where nuclear fusion of hydrogen occurred and the Sun ignited. By exploring our universe with tools such as the Hubble Space Telescope, scientists have discovered stars in various stages of formation predicted by the solar nebular theory.
How much longer will the Sun shine?
Scientists predict the Sun will shine for another 7 billion years! They arrive at this estimate by calculating how fast the hydrogen in the Sun's core is being converted to helium. Approximately 37% of the Sun's hydrogen has been used since the time of its formaton, 4.55 billion years ago. (Lang, 1999)
How big is the Sun?
The Sun's diameter is 1,391,020 kilometers, or about 109 times the diameter of Earth.
Structure of the Sun 1
Like Earth, the Sun has many different layers. Unlike Earth, the Sun is made of gas!
The Sun's energy is generated in its core. Gravitational pressures compress and heat the material in the core to over 15 million degrees Celsius!
Energy passes from the core into the cooler radiative zone (5 million degrees Celsius). Here the energy (radiation) moves randomly from atom to atom, with some of the energy moving toward the Sun's surface.
As energy moves out of the radiative zone, it enters the convective zone. Here the atoms do not pass the energy from particle to particle; the atoms themselves move, carrying the heat with them. The hotter material near the radiative zone rises to the cooler surface of the convective zone. As it reaches the top of the convective zone, it cools and sinks.
The photosphere (“sphere of light”) is the “surface” of the Sun; because the Sun is made of gas, it does not have a solid surface. The photosphere has temperatures that reach about 5800 degrees Kelvin and is the layer that releases most of the light that reaches Earth.
The surface of the Sun has continuously changing dark regions or sunspots. The spots are dark because they are cooler than the surrounding gas (about 3230 degrees Celsius). Sunspots can persist for an hour to several months. The number of sunspots increases and decreases in an 11-year cycle, the solar cycle.
The photosphere and sadanslyng, unspots can be viewed safely with special solar telescopes, but not directly with the human eye!
The chromosphere (“sphere of color”) is a 2000-kilometer-thick layer of gas that reaches temperatures between 6000 and 50,000 degrees Celsius. Most of the energy from the chromosphere is released as red light, which means that the chromosphere can be viewed with special telescopes that filter out the other wavelengths. The chromosphere is dynamic; convection cells swirl the surface, and material shoots off the surface as flame-like features.
The corona is a thin outer layer of the Sun that is seen during a solar eclipse. The corona emits energy at many different wave lengths. Loops and arches of matter are often seen extending out from the corona along lines of the Sun's magnetic field. This material flows away from the Sun as the solar wind. Some of the particles reach Earth's atmosphere and interact with atmospheric particles to create the aurora.

What is the Sun made of?
While approximately 60 different elements make up the Sun, hydrogen accounts for about 92% of the atoms (almost three-fourths of the mass) and helium makes up most of the rest (7.8% of the atoms). This is similar to the composition of our universe; hydrogen is the most abundant element, with some helium and trace amounts of all other heavier elements (such as carbon, nitrogen, and oxygen, and silicon). We do not have any direct samples from the Sun but scientists can identify the elements by observing the solar spectrum. The Sun, like other stars, emits light, and in some cases more light in one color than another (and some colors are not emitted at all, because they are absorbed). Gases of different elements have distinct patterns of emission or absorption that can be determined in the laboratory. Once scientists know which pattern matches which element, they can determine the composition of our star, or other stars in the universe, by examining the pattern of the spectrum.
Where does the Sun's energy come from?
Stars like the Sun generate their power by turning mass into energy through the process of nuclear fusion. Essentially, hydrogen is converted into helium in the Sun's core, and a little bit of energy is produced every time the reaction occurs. Gravitational pressure compresses and heats the core material to over 15 million degrees. In these extreme conditions atoms cannot exist — hydrogen atoms split apart into protons and electrons. Four hydrogen protons get fused into a single helium nucleus in a process that releases energy. The resulting helium atom has a smaller mass than the four hydrogen atoms. This mass difference is released as energy. Every second, 600 million tons of hydrogen are converted to radiant energy. The energy is carried by high-energy gamma rays. The gamma rays collide with the electrons in the core, losing energy and becoming photons of visible light. While the Sun emits energy across much of the electromagnetic spectrum, approximately half the solar radiation is in the visible part of the spectrum and much of the rest is infrared radiation. Because of our distance from the Sun, the amount of energy reaching Earth is small, only about one two-billionth compared to the amount emitted by the Sun. The top of Earth's atmosphere receives about 1,365 watts per square meter.