Sun: The Summation of Our Solar System

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley, Ph.D., Johns Hopkins University

The Sun is, by far, the biggest object in the solar system. It’s so massive that one could say that the Sun IS our solar system, and the rest is mere dust. The Sun’s primary forces, such as gravity and heat, not only define our solar system, but also make it what it is: a living, sparkling wonder.

An illustration of our solar system with the stars set as its background.
The Sun comprises 99.9% mass of our solar system. (Image: Sergey Nivens/Shutterstock)

The Sun is, by far, the biggest and most massive object in the solar system. In fact, at 99.9% of the total mass of the system, one could say the Sun is the solar system. Imagine what would happen if one magically removed the Sun from the solar system. Darkness would reign, temperatures would plummet, and planets would all go flying off in straight lines from wherever they were in their orbits.

The Formation of Planets

But the Sun’s importance goes even further. Without the Sun, the planets would never have formed. The planet formation process relied on dust and gas being confined to a disk orbiting the proto-Sun. That disk formed because material was collapsing onto the proto-Sun. Thankfully, some material had a bit more rotation, making it end up in a disk orbiting around the Sun, rather than onto the Sun itself.

Observational History of the Sun

Observations of the Sun and its motions in the sky were recorded by ancient astronomers from many cultures. Chinese astronomers even made observations of sunspots during the Han Dynasty over 2,000 years ago. But it wasn’t until the 17th century that scientists began to take seriously the idea that the Sun is a star, just like all the stars we see in the night sky.

People had suggested it previously—Giordano Bruno was even burnt at the stake in the year 1600 by the Roman Catholic Church for saying it. But later in the 17th century, a series of scientific discoveries lent support to the notion that the Sun is a star. Galileo’s telescope led him to conclude that stars must be very far away since they still looked like points of light rather than resolved planets through his telescope.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.

Modern Observational History of the Sun

Kepler’s laws of planetary motion followed Copernicus in putting the Sun at the center of the solar system, and determined that the orbits of the planets around the Sun had an elliptical shape, not circular. Then, Newton’s theory of gravity explained why planets orbited the Sun, and demonstrated that the gravity we feel on Earth was the same as the gravity elsewhere in the solar system.

An illustration of our solar system and the sun's gravity.
All of the planets are gravitationally bound to the Sun. (Image: Zonda/shutterstock)

And then, people began calculating distances to the stars. Christiaan Huygens calculated the distance to the star Sirius assuming it had the same brightness as the Sun. And he found that the distance was extremely far. Much later, in 1838, Friedrich Bessel used a new technique called parallax to determine that the distance to a star called 61 Cygni was what we would now call 10 light-years away from Earth.

This demonstrated that stars must be extremely bright, as bright as the Sun, in order for us to see them so far away. This ultimately led to the understanding that the Sun is not a unique entity in the universe. There are many more suns, or stars, out there. But the Sun is a star that we can study up close.

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The Gravity of the Sun

As the most massive body in the solar system, the Sun has the deepest gravity well. A gravity well is ultimately defined by how much energy an object needs in order to escape the gravitational pull of that larger body. The more massive the pulling body, the more energy one would need to overcome the attractive gravitational force pulling one towards the body.

All of the planets are gravitationally bound to the Sun, in the sense that they don’t have enough energy to escape the Sun’s gravity well. But that doesn’t mean they will fall directly into the Sun. Instead, planets orbit the Sun. They essentially are in constant freefall towards the Sun, but their velocity is tangential to their orbit, and that keeps them from ever falling in.

The Relative Gravity of Objects

But just because the Sun has the largest gravity well in the Solar System doesn’t mean everything in the solar system orbits the Sun. The strength of gravity is a function of distance. Moons that orbit planets, for example, happen to be close enough to the planet that the planet’s gravity at that location dominates the Sun’s gravity.

The Sun’s gravitational influence extends to far distances, as evidenced by the Oort cloud out at orbital distances up to 50,000 astronomical units. But at the distance called the Hill sphere, the gravitational force of nearby stars takes over. That’s at around 100,000 astronomical units. But gravity isn’t the only way the Sun shapes the solar system. The Sun is a major source of heat and light for the solar system as well.

The Heat of the Sun

People are taught at an early age not to look directly at the Sun because of the potential damage to the eyes. Luckily, solar telescopes can photograph the Sun through various filters in different wavelengths of light. When one chooses a particular wavelength of light to study the Sun, one is looking at different parts of the Sun’s surface and atmosphere.

The wavelength is related to the temperature, which is related to specific parts of the Sun. For example, the yellowish light we see in the visible part of the spectrum comes from the opaque ‘surface’ of the Sun, where radiation has a temperature of about 5800 Kelvin.

The Nuclear Fusion of the Sun

Waves emitting from the sun.
The energy of the Sun comes from a nuclear fusion. (Sahara Prince/Shutterstock)

People would have to wait until the early 20th century for Einstein’s famous E = mc2 equation to ascertain that matter can be converted to energy. That’s what happens at the center of the Sun. The pressures and temperatures are so high in the center of the Sun that hydrogen atoms are squeezed together, fusing to make helium atoms.

All the energy from the Sun comes from nuclear fusion occurring in the Sun’s core. This is possible because the four hydrogen atoms are slightly more massive than the resulting helium atom. The difference in mass between one helium and four hydrogen atoms is converted into energy; that energy equals the mass difference times the speed of light squared. The inner 25% of the Sun converts over 600 million metric tons of hydrogen into helium every second.

Temperatures need to reach about 14 million Kelvin in the solar core for fusion to ignite because the hydrogen atoms have to have enough kinetic energy to overcome the repulsive forces between them. It’s this fusion that makes the Sun a star rather than a planet.

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The Determination of the Sun’s Composition

The fact that the Sun is composed primarily of hydrogen and helium was first determined in 1925 by Cecilia Payne-Gaposchkin. Before then, the reigning belief was that the Sun’s composition was quite similar to Earth’s.

But Payne-Gaposchkin showed that many features in the Sun’s spectrum could be better explained if the Sun were predominantly hydrogen and helium at very high temperatures. This showed that the amount of hydrogen and helium in the Sun was far greater than the amounts of other elements. The Sun’s composition is about 75% hydrogen and 24% helium. The remainder, a mere 1.3%, contains all the heavier elements.

Do Planets Also Have Nuclear Fusion?

No planets have fusion running on in their interiors. Even Jupiter, the largest planet in the solar system, doesn’t have the pressures and temperatures needed at its center to start hydrogen fusion. In Jupiter’s core, temperatures are tens of thousands of degrees and pressures are around 100 million bars. But in the Sun, central temperatures range around 14 million Kelvin with pressures exceeding beyond 200 billion bars.

Planets do have internal heat sources. For example, Earth’s interior is heated from both radioactive decay of unstable elements in its interior and the heat that got buried in the planet from all the collisions that occurred while it was forming. There’s just no nuclear fusion inside a planet.

Common Questions about Sun: The Summation of Our Solar System

Q: How were planets formed in our solar system?

The planet formation process relied on dust and gas being confined to a disk orbiting the proto-Sun. That disk formed because the gravity of the proto-Sun was attracting material on to it. Some material, luckily, had a bit more rotation, hence it ended up in a disk orbiting around the Sun.

Q: Is the Sun a planet?

No, the Sun is not a planet. It is the lone star of our solar system.

Q: How hot is the sun?

The central temperatures on the Sun are around 14 million Kelvin.

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