9.1: Interesting Facts About the Sun

Our Sun, a star in the Milky Way galaxy, formed about 4.6 billion years ago. It began as a protostar, a whirl of gases—namely helium and hydrogen—that collapsed into a spinning ball. Increased temperature and pressure within the ball over the course of some 50 million years eventually ignited hydrogen fusion, a reaction that sustains the release of energy from the Sun even today. Under extreme gravitational forces, hydrogen atoms fuse into helium atoms, resulting in the release of enormous quantities of energy. The start of nuclear fusion represents the transition from protostar to full-fledged star (e.g., NASA 2023b). It is the moment when we may say, “Let there be sunlight, the sum of all wavelengths of light emitted by the Sun.” From that moment on, light from the Sun—sunlight—filled space.

The Solar Wind

In addition to light, the Sun also emits charged particles—electrons, protons, and others—in a steady stream known as the solar wind. These particles interact with Earth’s magnetic field and gases in Earth’s atmosphere to create spectacular light shows, the aurora borealis and aurora australis, the northern and southern lights, respectively. Coronal mass ejections (CMEs)—intense outbursts of particles from the Sun—occasionally shift the lights toward the equator, offering opportunities for people living at lower latitudes to witness auroras (e.g., NASA 2019b).

CMEs can also produce dangerously strong geomagnetic storms, disruptions of Earth’s magnetic field. In September 1859 the solar wind was so powerful it produced an aurora visible around the world. Electrical pulses from the event flickered through telegraph systems, shocking telegraph operators and setting their papers on fire (e.g., Muller 2014). While the damage that day was limited, scientists estimate that if such a storm were to occur today, the cost could reach trillions of dollars (Riley et al. 2018). NASA and other agencies monitor and forecast solar activity, what’s known as space weather. Like atmospheric weather forecasts, space weather forecasts allow officials to inform civilian and military populations of possible disruptions to satellites, electrical power, computers, and other electrical devices during strong solar activity.

Classification of the Sun

Different stars in the Universe have different characteristics. When you look at stars in the sky, they may appear white, yellow, red, or blue. It’s not an optical illusion. Different stars emit different colors of light. Astronomers can use these colors (and other properties) to classify stars—kind of like sorting your socks or dress shirts by color. Star classification serves as a kind of shorthand for describing the characteristics of a star. Based on the properties of the light and particles emitted, astronomers classify our Sun as a G2V star, a yellow dwarf star, and a main sequence star (NASA 2021). These classifications have great significance for astronomers, but for our purposes, the important point is that the Sun emits white-yellow light.

Of course, the Sun is not the only star in our neighborhood. Peer into the night sky and you will see thousands of twinkling stars. On a really dark night, far from the lights of industrial civilization, you might see as many as 4,500 stars. With a pair of binoculars or a telescope, you can see as many as a half a million stars and galaxies—collections of star systems bound together by gravity. Our own galaxy—the Milky Way—contains some 200 billion stars. And beyond the Milky Way, in the vast expanse of the Universe, lie perhaps as many as two trillion other galaxies, each with its own billions and billions of stars. It’s said there are more stars in the Universe than grains of sand on the world’s beaches. Of course, an accurate count of the number of stars in the Universe or grains of sand on Earth’s beaches can never be known for sure. But it’s fun to think about.

The Solar System

Around the time that our Sun lit up, gases and cosmic materials in nearby space coalesced into the celestial bodies orbiting the Sun, our solar system. The rocky planets—Mercury, Venus, Earth, and Mars—and the gas planets—Jupiter, Saturn, Uranus, and Neptune, formed around this time. So did the dwarf planet, Pluto, and hundreds of moons and asteroids. At the far reaches of the solar system, a collection of icy objects, the Kuiper Belt and the Oort Cloud, appeared as well. These icy bodies occasionally send traveling snowballs our way, the comets (NASA 2019a).

The dimensions of our Sun are impressive. It makes up 99.98 percent of all the matter in our solar system. It has a diameter that is 109 times the diameter of Earth. If the Sun were a hollow sphere, you could put 1.3 million Earth spheres inside of it. Of course, if you put Earth inside the Sun, you would experience heat like you’ve never felt before. The core of the Sun has a temperature of about 27 million degrees Fahrenheit (F), about 15 million degrees Celsius (C); °F or °C, respectively. Out at its edge, it’s cooler, only 10,000°F (5,500°C; e.g., NASA 2021).

People sometimes ask why the Sun looks so small if it’s that big. Well, if you were 93 million miles away, you’d look small, too. This distance—the distance from the Earth to the Sun—represents an astronomer-designated unit called the astronomical unit, or AU. Earth, by definition, is one AU from the Sun. But because the Earth orbits the Sun in a slightly elliptical path, the Earth–Sun distance varies. In January, the Sun is slightly closer to Earth, at perihelion, about 0.98 AU (91.4 million miles). In July, it’s slightly farther away, at aphelion, about 1.02 AU (94.5 million miles). On average, it works out to about 93 million miles (NASA 2001).

This brief introduction to the star of our solar system serves as a backdrop for the more important consideration of how the Sun—specifically, energy from the Sun—interacts with Earth and drives so many important Earth processes. Let’s take a look now.