Galaxies / Stages of a Star's Life Cycle

Galaxies 

• A galaxy is a huge system of millions or billions of stars, together with gas and dust held together by gravity 
• Galaxies are classified by their shape 
• There are three types of galaxies - Spiral. Elliptical, and Irregular 

Spiral Galaxies 

• A flat, disk-shaped galaxy with a central bulge and "arms" that extend outward 
• We live in a spiral galaxy – The Milky Way

"Milkyway"

Elliptical Galaxies

• The most abundant type of galaxy that has a smooth, circular for an oval) shape Irregular Galaxies 

Irregular Galaxies

• A random, scattered type of galaxy that does not form a distinct shape or pattern 

Stars and Nebulas 

Nebula- a huge cloud of dust and gas in space held together by gravity 

Protostar - when nuclear fusion begins and a star gives off light for the first time 

Main Sequence -90% of stars (including the Sun) are in this stage of their life cycle 

Red Giant - the stage after the main sequence for a small to medium mass star; fuel begins to run out 

Red Supergiant- the stage after the main sequence for a high mass star 

White Dwarf - formed when a low-mass star has exhausted all its nuclear fuel and lost its outer layers Black Dwarf - a star that no longer gives off light or heat 

Supernova - an explosion of a high mass star 

Neutron Star-on extremely dense star that results from the gravitational collapse of a massive star after a supernova 

Black Hole - a region of space having a gravitational field so intense that no matter or radiation can escape 

Planetary Nebula - a ring-shaped nebula formed by an expanding shell of gas around an aging star 

Stages of a Star's Life Cycle 

• All stars go through the same first three stages - Nebula, Protostar, and Main Sequence 
• After Main Sequence stage: Small to average mass stars like the Sun - Red Giant, Planetary Nebula, White Dwarf, Black Dwarf
• After the Main Sequence stage: High Mass Stars - Red Supergiant, Supernova, Neutron Star
• After Main Sequence stage: Extremely Massive Stars - Red Supergiant, Supernova, Black Hole

The mass of a star is the main factor that determines the path a star will take during its life cycle!