 A star begins its formation in a cloud of hydrogen and helium gas referred to as a nebula.

 The material blown off by a supernova explosion ultimately scatters throughout space, and some of it is incorporated in clouds of dust and gas which later form new suns and planets.

The material from which the solar system formed, must have been the cloud of dust and gas left from a supernova and gradually pulled together by gravity. Astronomers have observed (using the Hubble telescope, among others), disks of swirling dust which presumably mark the birth of other planetary systems.

This is an important question, and one that is difficult for scientists to understand. The creation of our Solar System took place billions of years before there were any people around to witness it. Scientists believe that the Solar System evolved from a giant cloud of dust and gas. They believe that this dust and gas began to collapse under the weight of its own gravity. As it did so, the matter in this could begin moving in a giant circle, much like the water in a drain moves around the center of the drain in a circle.

At the center of this spinning cloud, a small star begin to form. This star grew larger and larger, as it collected more of the dust and gas that were collapsing into it. Further away from the star that was forming in the center were smaller clumps of dust and gas that were also collapsing. The star in the center eventually ignited forming our Sun, while the smaller clumps became the planets, minor planets, moons, comets, and asteroids.

Once ignited, the Sun's powerful solar winds began to blow. These winds, which are made up of atomic particles being blown outward away from the Sun, slowly pushed the remaining gas and dust out of the Solar System.

It's a crucial question, because light and heat from the sun are the basis of (almost) all life on earth. Sunlight drives plant life via photosynthesis, and animals survive by eating plants. Almost all microscopic forms of life (bacteria, protozoa, etc.) survive by using the energy of sunlight.

Heat is the motion of atoms and molecules: the higher the temperature, the greater is their velocity and the more violent are their collisions. When the temperature at the center of the newly-formed Sun became great enough for collisions between nuclei to overcome their electric repulsion, nuclei began to stick together and protons were combined into helium. This released nuclear energy and kept up the high temperature of the Sun's core, and the heat also kept the gas pressure high, keeping the Sun puffed up and stopping gravity from pulling it together any more.

 That, in greatly simplified terms, is the "nuclear fusion" process which still takes place inside the Sun.

 Nuclear fusion is how stars produce their light, heat, and energy. Through this process, they “burn” a fuel known as hydrogen.  The result is that they create another type of matter known as helium.  However, stars do not burn in the same way that a fire does, because stars are not on fire.

With no more gas or dust, the planets, minor planets, moons, comets, and asteroids stopped growing. You may have noticed that the four inner planets are much smaller than the four outer planets. Why is that?

 Because the inner planets are much closer to the Sun, they are located where the solar winds are stronger.  As a result, the dust and gas from the inner Solar System was blown away much more quickly than it was from the outer Solar System.  This gave the planets of the inner Solar System less time to grow.

 Astronomers like to categorize the planets into two categories.  The first category is the terrestrial planets.  The second category is the Jovian planet

 The terrestrial planets include Mercury, Venus, Earth, and Mars.  These planets have solid surfaces, with the last three also having an atmosphere.  Their size, radius, and density are all similar.

The outer planets have a lot of atmosphere. Below the surface, hydrogen and helium are in a liquid form. At the center is a rocky core. Each of these huge planets has its own system of moons. Rings around Saturn have been seen for many years. They are probably made mostly of water, ice, and other icy particles. Rings have been discovered around two more giant planets — Jupiter and Uranus. Scientists expect to find rings around Neptune, too.

 Another important difference is that the outer planets are made up largely of gas, and water, while the inner planets are made up almost entirely out of rock and dust.  This is due to the same reasons. As the outer planets grew larger, their gravity had time to accumulate massive amounts of gas, water, as well as dust.

There is a very great distance between the inner and outer planets. This region is called the Asteroid Belt and large chunks of rock swarm around and around in it. Except for Pluto, the outer planets are alike in a lot of ways. They are much bigger than the inner planets. They are made mostly of hydrogen and helium. The hydrogen and helium are in the form of gas in the planets' atmospheres.

The Jovian planets include Jupiter, Saturn, Uranus, and Neptune. These planets are also called gas giants. They are much larger in radius than the terrestrial planets, and the majority of their volume is composed of gas mostly hydrogen and helium gas. Small amounts of other gases also appear and are likely the reason for the various Jovian colors. The structure of all Jovian planets, is layered. The central layer is a solid core. Surrounding the core is a mantle of liquid metallic hydrogen. Outside this mantle is a layer of hydrogen and helium gas.

Terrestrial planets are small, rocky, low surface gravity and have short years. Jovian planets are large, gaseous, high surface gravity and have long years.

 Moon rock solidified from magma before Earth's crust had formed. A rock's age is determined from its point of solidification.

Geologists would use some form of radiometric dating.

EARTH CHANGES Earths process include all of the forces that combined to form the landform. These processes might be weather, pressure, water, wind, moving sheets of ice, etc. All landforms are being changed by the many different forces that are constantly at work on them.

The Earth as been evolving and changing for many hundreds of millions of years, even billions of years. The changes that take place in the Earth’s landscape are usually very slow by human standards. It takes many generations of lifetimes before any noticeable changes have taken place in the landscapes of a particular location. Yet, in geologic time, the Earth is constantly changing and evolving. What we see today will be very different in the future. The mountains, rivers, lakes, and land formations of today will not exist at some point in the distant future.

 What is a nebula?  What is the source of the sun's energy?  What is the process which still takes place inside the Sun.  Where did the material come from to make a solar system?  What is a supernova?  The four inner planets are also known as the ________ planets.  Why are the four inner planets much smaller than the four outer planets?  Name the terrestrial planets  Name the outer planets  Why are the outer planets larger?  What planets are also called gas giants?  What are the 4 main differences between the terrestrial and Jovian planets?  There is a very great distance between the inner and outer planets. What lies between this vast space?  Why are moon rocks older than earth rocks?