The life cycle of a star starts as a Nebula. It then moves on to becoming an Average Star or a Massive Star. Whichever it becomes determines the rest of the star's life. This picture shows the star's life cycle before it is done and starts life for new stars to form. 

A star's mass determines the star's temperature, luminosity, and how quickly it burns its life. With this information you can determine which path the star will take in its life cycle. 

The sun was born as a stellar nebula like all stars.  It then goes in the Main Sequence as a average star, which is where it is now in its life cycle. Later, it will turn into a red giant when the core collapses and heats up. Next it becomes a Planetary Nebula, until it reaches its final stage, a white dwarf.  

The HR diagram is here to classify stars and analyze their evolution. The star's temperature is on the X-axis and the luminosity is on the Y-axis. 

The four groups of stars found on the HR Diagram are the Main Sequence, White Dwarfs, Giants, and Supergiants. Our sun is in the main sequence. 

For the main sequence stars, the hotter the star gets, the brighter it becomes. The x-axis showing temperature on the HR diagram, is unique because it goes the opposite way of most graphs. The further left you go, the hotter the star is, and the further right you go, the cooler the star is. The top left is hot and bright, the top right is cool and bright, the bottom left is hot and dim, and the bottom right is cool and dim. 

A galaxy is a gravitationally bound system that holds stars, planets, and other stellar matter.  There are three main types of galaxies; spiral, elliptical, and irregular, and they are primarily classified by their shape. 

A spiral galaxy is the most common type of galaxy. It holds older yellow stars in the bulge and younger blue stars on the spirals arms. A spiral galaxy has  a bright light in the center and then swirls of color around it (pictured left).  Spiral galaxies are rich in gas and dust, giving the galaxy ongoing star formation. The Milky Way, where Earth is, is an example of a spiral galaxy.

An elliptical galaxy contains some of the oldest, low mass, yellow-red stars. It looks like a burst of yellow, white, orange, or blue/green light, and doesn't form a certain formation seen by a human eye (pictured left). This type of galaxy contains very little gas and dust so star formation rates are very low. 

An irregular galaxy mixed with older stars and young (bright blue) stars, meaning active star formation is happening. Irregular galaxies have a few different types, and a slight change in appearance, but, the most common look of an irregular galaxy is chaotic and shapeless (pictured left).  Unlike elliptical galaxies, irregular galaxies contain lots of gas and dust providing material for new star formation.