This is a continuation of my recommendations for the First Great Lesson. The story is usually given at the beginning of the school year, and it provides an important framework for children. Elementary children are assembling their worldview, and it is important to give them the best information we have. A story that you received as part of your teacher education may not be the current story.

In my last blog on the First Great Lesson (https://big-picture-science.myshopify.com/blogs/news/putting-more-truth-in-the-first-great-lesson), I ended with the chemical elements being formed inside stars.

About 4.6 billion years ago, there was a huge collection of stars in a galaxy. It was one of billions of galaxies. This one had spiral arms curving out from its center. On one of these arms, a giant cloud of gas and dust formed. It was the result of stars reaching the end of their lives and blowing up or puffing out their outer layers. The giant cloud was mostly hydrogen and helium, like the rest of the universe, but the stars that came before it had left metals like iron and nickel and compounds of silicon that made rocky particles. There were also small molecules like water, ammonia, and methane. The rocky dust particles and metals are very important in the Earth’s story, but first we will look at the Sun. 

How did our Sun come to be? It started with the huge, huge cloud of gas and dust, which astronomers call the solar nebula. This cloud was cold, not hot and energetic. It is very hard to pack a hot, highly energetic cloud into a new star.

At the beginning of the Sun’s formation, somewhere relatively nearby in space, there was a disturbance – perhaps a supernova explosion – that caused our giant cloud of gas and dust to collapse on itself. It is hard to imagine how huge that supernova was. The solar nebula began to rotate, and the gravity pulled most of its mass into the center as it flattened into a disk and rotated faster (see demonstration below). Even today, about 4.6 billion years later, the planets orbit the Sun in the plane of the disk, and they move in the direction it turned.

Image from NASA-JPL: Artist's illustration of a young star and its protoplanetary disk.

The pull of gravity depends on mass, and as more mass packed together, it pulled in yet more mass. After about 100,000 years, the mass in the center of the rotating disk became the Sun. It has over 99% of the mass of the solar system. As gravity pulled the Sun’s mass together, it heated up. (See demonstration on compressing gas and temperature rise.) When the temperature of its core reached about 15 million degrees Celsius, its nuclear furnace ignited. Like other young stars, it began joining hydrogen atoms into helium and producing heat and light. This process is called nuclear fusion. Like all stars, the Sun is in a giant tug-of-war between gravity pulling inward and nuclear fusion pushing outward.

The rest of the material in the disk around the Sun, less than one percent of the total, became the planets, moons, and asteroids of the Solar System. A system is a collection of things that interact and form one whole thing. The Sun’s gravity holds the Solar System together.

The disk of gas and dust around the Sun was made of the same materials as the Sun. It was mainly hydrogen and helium. There were little bits of metals, such as iron and nickel. Silicon combined with oxygen and other chemical elements, which made rocky fragments, like bits of sand. There were also small molecules like water, methane, and ammonia. (If you have a molecule modeling set, show the models for these small molecules.)  

As the Sun ignited and began to put out light and heat, it also began to spew out a strong, fast solar wind of particles. This swept the lighter materials like hydrogen and helium away from the part of the disk nearest to the Sun. The planets that formed closest to the Sun are rocky ones made of denser materials that didn’t get blown away. They are Mercury, Venus, Earth, and Mars.

Much farther away, hydrogen, helium, water vapor, and other gases gathered around rocky cores of the gas giant planets, Jupiter and Saturn. Beyond them, the ice planets, Uranus and Neptune, formed. Not all the gas and dust became planets. Some formed other parts of the Solar System, like the asteroids, comets, and dwarf planets.

How big is our Earth compared to the Sun? It is very small, less than one percent of the Sun’s diameter. If this meter stick was the diameter of the Sun, the Earth’s diameter would be about 9 mm. The volume comparison is more impressive – the Sun’s volume is about 1,300,000 times that of Earth. For another perspective on the size of the Sun, if the Earth is placed at the center of the Sun, where would the Moon orbit? About 270 mm from the Earth in this meter-stick model – a bit over half-way to the surface of the Sun! It takes three days for a spacecraft to reach the Moon.

Here are some references and resources for the Sun and Solar System story.

NASA information about the Sun. https://science.nasa.gov/sun/facts/  

NASA information about the planets. https://science.nasa.gov/solar-system/planets/

The Lunar and Planetary Institute page on the Sun. https://www.lpi.usra.edu/education/skytellers/sun/

Demonstration for spinning faster (conservation of angular momentum): You can demonstrate the acceleration as mass shifts to the center. You need a rotating chair with arms and two small weights that a child can hold comfortably at arm’s length. The child sits in the chair, holds the weights, and extends their arms. Rotate the chair, and when it is moving at a steady, slow speed, ask the child to quickly move the weights close to their chest. The rotation should accelerate. The child will feel it, and others can see it. Make sure the child doesn’t slip off the chair.

Demonstration for the heating of compressed gases: You need a bicycle tire and a tire pump. If the tire isn’t low on air, let some out. Pump air into the tire and then feel the tire stem. It should feel warm. The heat comes from the compression of air.

Next, I will write about the formation of the Earth and its Moon.