The Cosmos

Development of the Universe


Over 13 billion years ago, the universe began to expand out from a center point following an explosion.  This model is called the Big Bang Theory.




Galaxies are massive groups of stellar, or star-like, objects.  Our galaxy is the Milky Way, a spiral-shaped galaxy similar to the one pictured.  We have no pictures of the entire Milky Way because we are part of it, but it is sometimes visible in the night sky as a large streak.  Our solar system is on one of the arms of the spiral.




Starts are massive spheres of hot plasma.  The Sun is the star at the center of our solar system.




Groups of stars in the sky are called constellations.  Constellations are used by astronomers to identify and keep track of stars. 


Solar Systems


Stars are very large and have a gravitational pull that causes planets to revolve around them.  A star with planets around it is called a solar system.


Star Sequence


A star goes through several stages in its life.  It starts out as a protostar:  gravitational instability causes a star to form, which takes about 10-15 million years.  The second stage is the main sequence, which is about 90% of a star's life.  Main sequence stars convert hydrogen to helium.  The third stage is the red giant stage.  After the hydrogen is depleted, the radius of the star increases, but the mass decreases and the star begins burning helium.  The fourth stage is the red supergiant.  The star now burns neon, silicon, carbon and oxygen.  The fifth stage is collapse.  The core of the star suddenly collapses into a white dwarf and a supernova (pictured) is created that sends out a right shockwave.  Larger stars can form a black hole after collapse.


The Sun


The Sun is the star at the center of our solar system.  It is a main sequence star, meaning it gives off heat from a constant nuclear reaction of hydrogen being converted to helium.


Planets of our Solar System


Our solar system has eight planets:  Mercury, Venus, Earth, Jupiter, Saturn, Uranus, and Neptune.  The first four are smaller planets made of solid material and the outer four are large, gaseous planets.  The solar system also has asteroids and several dwarf planets that orbit as well.




Many planets have moons.  Our planet has a moon that orbits it.  The Moon is kept in rotation around the Earth by the Earth’s gravity, and the Moon affects the Earth by reflecting the sun’s light and causing tides.


Comets and Asteroids


Comets and asteroids are objects passing through space.  Asteroids are smaller than planets and are usually in the inner part of the solar system.  Comets are icy asteroids that occasionally pass close to the sun and appear to be bright or have a tail.


Earth’s Motion


The Earth revolves around the Sun, and takes one year for a full revolution.  At the same time, the axis of the Earth tilts, causing changes in seasons.  When the Northern Hemisphere is tilted toward the sun, they experience summer, while the Southern Hemisphere experiences winter.  The Earth also rotates, causing night on the part not facing the sun and day on the part facing it.  One rotation takes 24 hours. 




The Earth, Moon and Sun being in perfect alignment cause eclipses.  A lunar eclipse is the Earth being between the Sun and Moon causing the Moon to be blacked out by Earth’s shadow.  A solar eclipse is caused by the Sun being blocked by the Moon


Age of the Earth


The Earth is about 4.54 billion years old according to radiometric dating.  Radiometric dating measures the amount of decay in radioactive materials.  It is also used to date fossils and landforms like mountains.




Eclipse Photo:  Luc Viatour