1. 25.2 – Stellar Evolution – Part I

2. Do Now
Where are stars said to be “born”?

3. Do Now Where are stars said to be “born”?
Dark, cool interstellar clouds, called nebulae.

4. Key Words
Development
Expansion

5. Vocab Word
Protostar

6. Stellar Evolution
Determining how stars are born, age, and die was difficult because the life of a star can span billions of years. However, by studying stars of different ages, astronomers have been able to piece together the evolution of a star.
Different stages of a star:
Birth
Protostar Stage
Main-sequence Stage
Red-Giant Stage
Burnout and Death

7. Star Birth Nebulae are the birthplace of stars.
Consists of 92% hydrogen and 7% helium, and less than 1% heavier elements.
The nebula becomes dense enough to begin to contract then gravity squeezes particles in the nebula, pulling every particle toward the center. As the nebula shrinks, gravitational energy is converted into heat energy.

8. Protostar Stage The initial contraction spans a millions years or so.
As time passes, the temperature of this gaseous body slowly rises until it is hot enough to radiate energy from its surface in the form of long-wavelength red light.
This large red object is called a protostar.
A developing star not yet hot enough to engage in nuclear fusion.
When the core of a protostar has reached about 10 million K, pressure within is so great that nuclear fusion of hydrogen begins, and a star is born.
Heat from hydrogen fusion causes the gases to increase their motion. Then the balance from the inside pressure balances the gravity pushing on the star.
This balance then mean the star has become a main-sequence star.

9. Protostar Stage

10. Main-Sequence Stage
From this point in the evolution of a main-sequence star until its death, the internal gas pressure struggles to offset the unyielding force of gravity.
Hydrogen fusion will continue for billions of years to support the star from gravitational collapse.
Different stars age at different rates.
We have hypothesized that a yellow star, such as the sun, remains a main-sequence star for about 10 billion years.
An average star spends 90% of its life as a hydrogen- burning, main-sequence star.
Once the hydrogen fuel in the star’s core is depleted, it evolves rapidly and then dies.

11. Red-Giant Stage
Once all the hydrogen in the core is consumed, no fusion takes place in the core and leaves behind a helium core.
Without a source of energy, the core no longer has enough pressure to support itself and the core begins to contract.
It grows hotter by converting gravitational energy into heat energy, and it expands outwards. As it expands it continues to heat in the core until the core collapses and the surface grows and cools.
Then as all usable energy is used up the star will then begin the burnout and death stage.

12. Life-cycle of a Sunlike Star

13. Group Activity
On page 709, Figure 10, copy and label the life cycle of a sunlike star.