25.2 - Stellar Evolution

• Astronomers have studied stars of different ages to determine the life cycle of stars

• We have learned that stars age just like we do

What’s between the Stars?

25.2 - Stellar Evolution

• Star Birth:

• Stars are born in dark, interstellar clouds called nebulae

• The cloud begins to contract and gravity squeezes the center until heat is produced

25.2 - Stellar Evolution• Over millions of years, the

temperature continues to rise because gravity continues to contract the core

• A protostar is a developing star not yet hot enough to start nuclear fusion

• A star is said to be “born” when the core reaches a temperature of about 10 million K and fusion of hydrogen begins

25.2 - Stellar Evolution• Main-Sequence Stage:

• Nuclear fusion has begun

• An average star will spend 90% of its life in this stage

• A stable main-sequence star is balanced between two forces:

● gravity, which is trying to squeeze it into a smaller sphere

● gas pressure, which is trying to expand it