Chapter 11: Our Sun: The Nearest Star

The Sun's hot corona.

Much of modern astronomy deals with stars — how they generate their energy, what kinds of light they radiate, how they form, and how they evolve. How can we study such remote objects as the stars? We can study the closest one, which is nearer to the Earth than five of the planets. Humans pondered the stars for many centuries before they realized that the Sun is just another star, and the stars are all suns. Light from its surface reaches us in only eight minutes. Our eyes are dazzled by it. The Earth is bathed in its flow of radiation, washed by the winds of its outer atmosphere, blasted by seething swarms of atoms blown out of it, bombarded by bursts of X-rays and radio waves emitted by it. It is our Sun, a ball of hydrogen and helium whose diameter is roughly 100 times the Earth's size.

The Sun as seen by the SOHO Satellite

Our Sun is the nearest star. Despite being separated by an almost perfect vacuum, the Sun and the Earth are intimately linked because almost all life on Earth depends on the Sun's energy. The source of that energy lies deep within the Sun, far below the surface we see. Energy is produced from reactions among the nuclei of atoms in the Sun's core.

A schematic of the different layers of the Sun.

Although the Sun is 150 million kilometers from Earth, it is 300,000 times closer than the next nearest star. We have observed it in great detail. We now understand the physical conditions in this great ball of gas and the ways in which energy travels from the core to the surface. The Sun and the Earth are linked in more subtle ways and small changes in the Sun produce profound changes in the Earth's climate. In human terms, the Sun has a virtually inexhaustible supply of energy. This has implications for the management of our own energy resources.

Solar spectrum showing the dark absorption lines.

The search for understanding of the Sun illustrates how science works to understand remote objects. Nobody has traveled to the Sun. No space probe has ever brought back a gas sample. No optical telescope can see past the surface layer. So how do we know so much? Astronomers use the technique of spectroscopy to identify the Sun's chemical composition. They also use the Sun's electromagnetic radiation beyond the visible spectrum to make images of the tenuous outer atmosphere. They even use tiny high-energy particles to "see" deep into the Sun's core. Astronomers can probe the interior structure with a wide range of observations and make physical and mathematical models of many kinds to describe the data.

SOHO extreme ultraviolet composite showing the number of sunspots through almost an entire solar cycle.

The Sun also exemplifies the application of physical laws to understand the universe. Atomic nuclei fuse to make solar energy, a process that we have observed and tested in making hydrogen bombs. The processes of radiation and convection carry the energy away as heat, which is then radiated into space. We can use the inductive method of science to apply this knowledge to suns much more distant than our own. It's informative to have a star so close because we see details and subtleties that are not apparent in all the much more distant stars, which as just points of light. The Sun represents our "ground truth" on how stars work. In the past few decades, we have found out that the universe contains billions of galaxies. Fusion powers all the billions of stars in each of these galaxies.