Stellar Evolution 1

Stellar Evolution . . . What are We Talking About?

• This lecture begins one of two on the topic that astronomers refer to as Stellar Evolution. The word evolution may cause some confusion here. In biology, evolution refers to the change of a species where the individual entity, the organism, does not change.

 
• Stars, of course, don't reproduce, so there is no species to evolve. Instead, in astronomy, Stellar Evolution means:

 

 
 
 
 
 
 
 

• Throughout human history, the stars have always been considered eternal; however, what modern astronomers have discovered is that even the stars themselves (including the Sun) are born, live, and die.

 
• One basic problem that astronomers face when studying stellar evolution is

 

 
 
 
 
 

How is this overcome?
 
 

• One of the principal tools that astronomers use to study stellar evolution is:

 

 
 
 
 
 
 

Thus, over the course of its lifetime, a star will "move" on the HR diagram.
 

• For Example:

 
• The HR diagram for the evolution of a person:

 

 
 




• What determines a person's evolutionary track on the the people HR diagram?

 

 
 
 
 
 
 
 

• What determines a star's evolution and its evolutionary track on a stellar HR diagram?

 

 
 
 
 
 

Stellar Formation

• Stars form in cool clouds of gas and dust known as the interstellar medium that exist throughout the galaxy. For example, the Orion Nebula which is visible with a modest telescope.

 
• How does it happen?

 

 
 




• If undisturbed,

 

 
 
 
 
 
 
 

• Something is required to get the cloud to collapse under its own gravity . . . a region of higher density that will gravitationally attract the rest of the cloud. Perhaps

 

 
 
 
 
 
 
 

• Where would such a shock wave come from?

 

 
 
 
 
 
 
 
 

• Once the cloud begins to collapse, it becomes

 

 
 




• What does this phase look like on the HR diagram?

 
• When the temperature and density get high enough in the center of the protostar:

 

 
 
 
 
 

• Many stars are observed to begin their lives violently as

 

 
 
 
 
 

• After this violent phase, the star reaches middle age and is known as

 

 
 
 
 
 

• What determines the star's position on the main sequence?

 

 
 
 
 
 
 
 

The Main Sequence . . Stellar Middle Age

• Middle age in stars (some might say in humans as well) is not very exciting. The star generally stays on the main sequence, steadily fusing hydrogen to helium in its core; thus providing the pressure necessary to balance further gravitational collapse.

 
• How long will a star remain on the Main Sequence, i.e what is its Main Sequence Lifetime?

 

 
 
 
 
 
 

For example:
 
 

Why is this?
 
 

• Another factor that influences a star's MSLT is what goes on in the interior: