Life Cycle of Stars

Stars are born in Nebulae Vast clouds of gas and dust Composed mostly of hydrogen and helium Some cosmic event triggers the collapse of a portion of the nebula Gravity condenses the matter into a central area Heat and pressure build in the core Protostar is formed

Eagle Nebula Helix Nebula

Protostar

A Star is Born When fusion of hydrogen begins in the core, the protostar becomes a star High Mass Star: a star whose mass is >3 solar masses (>3 times the mass of our sun) Low Mass Star: A star whose mass is <3 solar masses

Nuclear Fusion

Hertzsprung-Russell Diagram

A Star’s Balancing Act A star stays on the Main Sequence as long as there is a balance of inward and outward forces: –Inward Force = Gravity –Outward Force = Heat Pressure generated by nuclear fusion

Nuclear fusion Star Equilibrium – Balance of Forces

Life Span – Low Mass Star Hydrogen fusion in the core continues for billions of years, forming helium Once the hydrogen in the core is used up, the star begins to collapse and moves to the next stage in its life. Heat and pressure in the core cause the helium to fuses rapidly, causing a “helium flash.” Outer layers of the star begin to expand

Final Stages – Low Mass Star Outer layers expand and star becomes a Red Giant Eventually the outer layers disperse, leaving a very hot, dense core (maybe the size of Earth) called a White Dwarf

Red Giant White Dwarf Red Giant

Life Span – High Mass Star Hydrogen fuses to helium in the core but is used up much faster – may last only 100 million years Once the hydrogen in the core is used up, the star begins to fuse heavier elements. Heavier elements continue to fuse until iron (element 26) is formed. Iron cannot be fused in the core of a star – fusion stops and gravity takes over, collapsing the star

Final Stages – High Mass Star With decreasing outward pressure, gravity causes the star to collapse. The collision of atoms in the core causes a Supernova explosion. All the elements heavier than iron are formed in a supernova explosion. Remnants of a supernova depends on the star’s mass: –Neutron Star = a dense star body made of neutrons –Black Hole = an object so dense even light cannot escape.

High Mass Star Supernova Black Hole