The Sun, A Star
A star is a large ball of gas, mostly hydrogen and helium, which is held together by gravity. The sun is the only star in our solar system around which all planets orbit. The sun is also the largest part of our solar system. Ninety-nine percent of the matter in our solar system is found in the sun.
Interior of a Star
Energy is produced in the core of a star. The energy produced in the core takes millions of years to move to the outer layer of a star. First, the energy passes from the core into a shell of cooler hydrogen, called the radiative zone. Next, the energy reaches the convective zone. Within the convective zone, hot gases move toward the surface and cooler gas moves deeper into the interior.
Atmosphere of a Star
The photosphere is the lowest layer of a star’s atmosphere and the layer from which light is given off. It takes about 8.3 minutes for light to travel from the sun to Earth. Temperatures in the photosphere are around 5,500 degrees Celsius.
Above the photosphere is the chromosphere. This layer is an area of rising temperatures. The temperature ranges from 6,000 °C (at lower altitudes) to 50,000 °C (at higher altitudes). This layer is a few thousand miles (or kilometers) thick.
The corona is the uppermost layer of the sun’s atmosphere. It extends millions of kilometers in space. The temperatures here are tremendous, reaching one million °C! The corona is only visible during a solar eclipse.
The Sun’s Changing Features
From our viewpoint on Earth, the sun’s surface appears to be smooth, but this is actually not the case. The sun’s photosphere is too bright to see many of the features but with special telescopes we can see them.
Sunspots are areas of the Sun’s surface that appear darker because they are cooler than the surrounding areas. Sunspots can move and disappear over a period of several days to several months. An average sunspot is about the size of the Earth.
Prominences are huge arching columns of gas that erupts from the surface of the sun. Prominences can loop hundreds of thousands of miles into space. Prominences are held above the Sun's surface by strong magnetic fields and can last for many months.
A solar flare is a sudden violent eruption on the sun’s surface. They are very bright and often are found near sunspots and prominences. They release huge amounts of high-energy particles and gases and are tremendously hot (from 3.6 million to 24 million °F). They are ejected thousands of miles from the surface of the Sun.
Coronal Mass Ejections (CMEs) are like solar flares but are much larger. They are huge bubbles of gas ejected from the corona. Material from CMEs can reach Earth, occasionally knocking out radio signals or causing malfunctions in orbiting satellites.
These are current images of the sun from NASA. They are updated daily or even hourly!