1. Components of our Universe
Stars and Galaxies

2. Light Year
Astronomers measure distances to objects beyond the solar system using a larger distance of unit the light year.
A light year is the distance light travels in one year.

3. Big Bang Theory
According to the big bang theory, the universe began from one point billions of years ago and has been expanding ever since.

4. Star
A star is a large ball of gas held together by gravity with a core so hot that nuclear fusion occurs.

5. Life Cycle of a Star:
Stars formed deep inside clouds of gas and dust called a nebula (plural nebulae).
Gravity causes the densest parts to collapse forming regions call Protostars.  Over thousands of years, the energy produced by the protostars heats the gas and dust surrounding them.  
Eventually, the surrounding gas and dust blows away and the protostar becomes visible as a star.  

6. Life Cycle of a Star:
 A star becomes a main sequence star as soon as it begins to fuse hydrogen into helium in the core.   
When a star's hydrogen supply is nearly gone, the star moves off the main sequence. Massive stars become red giants, then larger red giants.

7. Death of a Star:
Average mass stars- After helium in their core is gone, the star casts off their gases exposing their cores.  The core becomes a white dwarf, a hot, dense, slowly cooling sphere of carbon.

8. Death of a Star
More Massive Stars- become red supergiants. The red supergiant explodes into something called a super nova.
The supernova can then form a neutron star which is a dense core of neutrons that remain after the supernova.

9. Hurtzsprung-Russell Diagram
Stars can be classified by their temperature, color, and mass.
The Hertzsprung-Russell diagram, or HR diagram, is a graph that plots luminosity versus temperature of stars.

10. Galaxies Galaxies are huge collections of stars.
There are three major types of galaxies: spiral, elliptical, and irregular.
Spiral galaxies look like flat discs with spiral arms and bulges in their center core.

11. Galaxies Elliptical galaxies are more rounded, like a football or egg.
Galaxies that appear neither spiral nor elliptical are classified as irregular because they do not have a definite shape.