THE SUN

Living with a Star

The Sun is a star and the biggest source of heat and light in our solar system. It’s one of at least several hundred billion stars in the Milky Way Galaxy. Without it, life might not exist, and that makes it very important to us. To early people the Sun was something to worship. Ancient Greeks venerated Helios as the sun god, but they had insatiable scientific curiosity about it. They had lively debates over the true nature of this bright thing in the sky. In the 1600s, Italian astronomer Galileo Galilei (1564–1642) speculated about what the Sun could be. So did Johannes Kepler (1571–1630) a few decades later. In the 1800s, astronomers developed scientific instruments to measure the Sun’s properties, which marked the beginning of solar physics as a scientific discipline.

Structure of the Sun

The Sun is essentially a big sphere of superheated gas. An imaginary voyage into its heart shows its structure.

• First, we have to traverse the outer solar atmosphere, called the corona. It’s an incredibly thin layer of gas superheated to temperatures well over a million degrees.
• Once we’re through the corona, we’re in the chromosphere. It’s a thin, reddish-hued layer of gases, and its temperature changes from 3,500° C (6,300°F) at the base to about 34,726 C (62,500°F) where it transitions up to the corona.
• Beneath the chromosphere is the photosphere, where temperatures range from 4,226°C (7,640°F) to 5,700°C (10,340°F). When you look at the Sun, the photosphere is what you actually see. The Sun is actually white, but it appears yellowish because its light travels through our atmosphere, which removes blue and red wavelengths from the incoming light.
• After we dive below the photosphere, we’re in a layer called the convective zone. If you’ve ever boiled water or syrup, you might recall seeing little granular bubbles. The Sun’s convective zone has these granular areas, too. They form as hot material from deep inside rises to the surface. These bubbles are actually currents moving through the Sun.
• The next layer down is the radiative zone. This is a very descriptive name because this region truly does radiate heat from the center of the Sun up to the convective zone.
• Finally, beneath the radiative zone is the solar core. This is the inner sanctum—a huge nuclear furnace. It’s the place where nuclear fusion takes place. How does that happen? The temperature in the core is about 15 million degrees Celsius (27 million degrees Fahrenheit). The rest of the Sun pressing down on it provides a pressure 340 billion times Earth’s atmospheric pressure at sea level. Those extreme conditions create a huge pressure cooker in which atoms of hydrogen slam together to form atoms of helium. The Sun fuses about 620 metric tons of hydrogen to helium each second, and that’s what provides all of the heat and light.

The Sun’s influence extends throughout the solar system. Its heat and light travel out to the planets. Mercury is baked while the most distant worlds get a small fraction of the Sun’s warmth. Our star also exerts another influence called the solar wind. This constantly blowing stream of charged particles extends out about 100 astronomical units (a hundred times the distance between Earth and the Sun) and creates a huge bubble that surrounds the solar system. The bubble’s inner edge is called the heliopause. Beyond that lies interstellar space, where other stars go through the process of nuclear fusion just as our Sun does.

Exploring the Sun

Both professional and amateur astronomers study our star using observatories on Earth’s surface and from space. Ground-based telescopes measure all aspects of the Sun’s surface and atmosphere. Some radio telescopes and radar detectors track the Sun’s influence on Earth’s upper ionosphere. Their data is used in models that help predict space weather events. Solar physicists peer inside the Sun, using a set of special instruments called the Global Oscillations Network Group (GONG), which focus on sound waves moving through the Sun, a science called helioseismology. The Solar Dynamics Observatory is also equipped to do this work.

NASA has a fleet of space-based instruments that study the Sun and how it influences Earth:

• The Solar Terrestrial Relations Observatory (STEREO), two orbiting satellites that give a constant three-dimensional view of solar activity
• The Solar Dynamics Observatory (SDO), which gives real-time imagery of the Sun and its outbursts and does helioseismology
• The Solar Heliospheric Observatory (SOHO), which focuses on the Sun’s upper layers and corona