The Sun is a sphere, but unlike the Earth and the Moon, is not solid. Most atoms in the Sun exist as plasma, or a fourth state of matter made up of superheated gas with an electrical charge. Our Sun consists almost entirely of the elements hydrogen and helium. Because the Sun is not solid, it does not have a defined outer boundary. It does, however, have a definite internal structure. There are several identifiable layers of the Sun:
The core is the innermost or central layer of the Sun. The core is plasma, but moves similarly to a gas. Its temperature is around 27 million degrees Celsius. In the core, nuclear reactions combine hydrogen atoms to form helium, releasing vast amounts of energy in the process. The energy released then begins to move outward, towards the outer layers of the Sun.
The radiative zone is just outside the core, which has a temperature of about 7 million degrees Celsius. The energy released in the core travels extremely slowly through the radiative zone. Particles of light called photons can only travel a few millimeters before they hit another particle in the Sun, are absorbed and then released again. It can take a photon as long as 50 million years to travel all the way through the radiative zone.
The convection zone surrounds the radiative zone. In the convection zone, hot material from near the Sun's center rises, cools at the surface, and then plunges back downward to receive more heat from the radiative zone. This movement helps to create solar flares and sunspots, which we'll learn more about in a bit. These first three layers make up what we would actually call "the Sun". The next three layers make up the Sun's atmosphere. Of course, there are no solid layers to any part of the Sun, so these boundaries are fuzzy and indistinct.
The photosphere is the visible surface of the Sun (Figure 24.18). This is the region of the Sun that emits sunlight. It's also one of the coolest layers of the Sun—only about 6700°C. Looking at a photograph of the Sun's surface, you can see that it has several different colors; oranges, yellow and reds, giving it a grainy appearance. We cannot see this when we glance quickly at the Sun. Our eyes can't focus that quickly and the Sun is too bright for us to look at for more than a brief moment. Looking at the Sun for any length of time can cause blindness, so don't try it! Sunlight is emitted from the Sun's photosphere. A fraction of the light that travels from the Sun reaches Earth. It travels as light in a range of wavelengths, including visible light, ultraviolet, and infrared radiation. Visible light is all the light we can see with our eyes. We can't see ultraviolet and infrared radiation, but their effects can still be detected. For example, a sunburn is caused by ultraviolet radiation when you spend too much time in the Sun.
The chromosphere is the zone about 2,000 kilometers thick that lies directly above the photosphere. The chromosphere is a thin region of the Sun's atmosphere that glows red as it is heated by energy from the photosphere. Temperatures in the chromosphere range from about 4000°C to about 10,000°C. Jets of gas fire up through the chromosphere at speeds up to 72,000 kilometers per hour, reaching heights as high as 10,000 kilometers.
The corona is the outermost layer of the Sun and is the outermost part of its atmosphere. It is the Sun's halo or "crown". It has a temperature of 2 to 5 million degrees Celsius and is much hotter than the visible surface of the Sun, or photosphere. The corona extends millions of kilometers into space. If you ever have the chance to see a total solar eclipse, you will be able to see the Sun's corona, shining out into space.
In the Sun's core, nuclear fusion reactions generate energy by converting hydrogen to helium. Fusion is a process where the nuclei of atoms join together to form a heavier chemical element. Fusion reactions in the Sun's core produce energy, which we experience as heat and light. The rest of the Sun is heated by movement of heat energy outward from the core. Light energy from the Sun is emitted from the photosphere. It travels through space, and some of it reaches the Earth. The Sun is the source of almost all the energy on Earth and sunlight powers photosynthesis, as well as warming and illuminating our Earth
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