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According to the big bang theory, the universe started in a cataclysmic explosion, known as the big bang, and has been expanding ever since. Distant galaxies move away from earth at immense speeds based off the expanding universe theory. All matter and energy were initially packed into an infinitesimally small point which exploded approximately 13.7 billion years ago.


After the explosion, there were extremely high temperatures which eventually cooled, allowing atoms to form. The process, known as nucleosynthesis, could produce small atoms, such as hydrogen during this time. As the universe continued to cool, chemical bonds such as H2 formed. The patchy cloud of gas made up of atoms, mostly hydrogen and helium, that resulted from the big bang, is known as a Nebula. Over time, gravity remolded the universe permanently. The amount of gravitational pull depends on the mass of the object. Therefore, as objects in space pull each other closer, they get denser and their ability to attach objects increased.

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The accretion disk is the rotation of condensing portions of the nebula which evolved into a spinning disk. The accretion became so big that the surrounding of the nebula collapsed, and it was pulled into a dense ball. The kinetic energy of the gas falling into this ball was transformed into heat. Furthermore, the squeezing together of gas atoms increased the temperature. Eventually, the central ball became hot enough to glow. The product is called a protostar. As the protostar continued to grow the temperature increased. This allowed fusion reactions of the hydrogen nuclei to occur and they produced helium.


The Protostar’s body ignited, and a star was formed. Older stars are often very large because there was more matter available in space when they were being formed in the past. Also, large stars burn faster which means they run out of fuel earlier than smaller stars. When a star runs out of fuel it dies by exploding into a supernova.


All 92 naturally occurring elements arose from stars which is why stars are considered elementary factories. This method is known as stellar nucleosynthesis. Large atoms were fashioned out of smaller atoms. This process depends on the mass of a star because the temperature and density of stars are greater as size increases. This is important for element formation because a higher temperature means the particle’s velocity increases and nuclei can be driven together. High mass stars can form elements up to 26 which is iron. Larger elements form during supernova explosions. The total atoms emitted from a star during its lifetime is known as a stellar wind.


Planets are large, spherical solid objects orbiting a star and they may have one or more moons. There are eight planets in our solar system. Four are terrestrial planets, the ones that are closest to the sun and Earth-like. These include Mercury, Venus, Earth, and Mars. Additionally, there are four gas-giant planets which are outer planets and their chemical composition is mostly gas. These include Jupiter, Saturn, Uranus, and Neptune. Moons are objects locked in orbit around a planet.


Asteroids and comets are left-over material from the big bang. Asteroids are relatively small chunks of rock and or metal and comets are small blocks of “ice” which orbit the sun. Ice is the solid version of materials that could be gaseous under earths atmosphere.

99.8% of the solar systems mass comes from the Sun and Jupiter is 99.5% of all non-solar mass in the solar system.


Meteorites are solid objects falling from space that land on a planet and leave craters. They also contributed to heating the planet. About 4.53 Ga, a Mars sized protoplanet hit Earth and created a cloud of molten rock around the Earth. The rock accreted into the moon.

The Solar system was formed 4.56 Ga, 9 billion years after the big bang. Ga stands for giga annum or billion years ago.



The Protoplanetary disk is another name for an accretion disk. The central ball of our protoplanetary disk developed into the proto-sun and the remainder evolved into a series of concentric rings. The disk was hotter in the center, causing a higher concentration of dust, and cooler on the outer rings, thus, high ice concentration. Dust is clumps of the molecules that make up rock or metal. Eventually, planetesimals, which are masses whose diameter exceeds one kilometer, formed. Their mass allowed them to attract debris and they grew into protoplanets, bodies the size of planets today. This process may have taken a few hundred thousand years.


All planets are spherical because when a protoplanet gets big enough, gravity can change its shape. Since the inside of a planet is warm, the rock is soft enough to flow in response to gravity. As the gravitation force becomes stronger protrusions are pulled inwards towards the center and the planetesimal reforms into a shape that permits the force of gravity to be the same at all points on its surface. Therefore, the planet forms a sphere to allow mass to distribute around the center.


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