Every star starts of as a stellar nebula. Then they can become an average star or massive star. If they became an average star then they would become a red giant, then a planetary nebula and lastly become a white dwarf.  If they became a massive star then they would become a red supergiant, then a supernova and lastly either become a neutron star or black hole.

A stars size determines the route a star takes.

Stars are born within nebulae and the suns life cycle follows the following order. It starts of a nebula, to an average star, to a red giant, to a planetary nebula, to a white dwarf.

The purpose of the HR Diagram is to classify stars and understand the relationship between star brightness and temperature. Brightness(luminosity) and temperature being the two variables found on the X and Y-axis.

 The 4 groups of stars found on the H-R Diagram are Main Sequence stars, Red Giants, Supergiants, and White Dwarfs. Our Sun belonging in the Main Sequence group.

In main sequence stars the temperature on the X-axis is proportional to the luminosity in the y-axis. Making temperature on the X-axis unique.

In the top left corner of the diagram the stars are hot and bright, in the top right the stars are cool and bright, in the bottom left the stars are hot and dim, and lastly in  in the bottom right the stars are cool and dim.

A galaxy is a group of stars, dust, gas and dark matter that are together by gravity.

We classify galaxies by their shape and structure and the 3 main types of galaxies are spiral galaxies, elliptical galaxies and irregular galaxies.

 Spiral galaxies are rich in gas and dust. They have old stars and new stars, the new stars are located in the arms of the rotating disk shape it has and the old stars are more compacted in the middle. Elliptical galaxies have very little gas or dust and is made up of a bunch of old stars. They can be dense in the middle but fades away in the edges. Irregular galaxies can be rich in gas and dust and has a mix of different star ages. Irregular galaxies don't have a regular shape and is chaotic.