1. Stellar Evolution Describe how a protostar becomes a star.
Explain how a main-sequence star generates energy.
Describe the evolution of a star after its main- sequence stage.

2. Classifying Stars
Main sequence the location on the H-R diagram where most stars lie; it has a diagonal pattern from the lower right to the upper left.
One way scientists classify stars is by plotting the surface temperatures of stars against their luminosity. The H-R diagram is the graph that illustrates the resulting pattern.
Astronomers use the H-R diagram to describe the life cycles of stars. Most stars fall within a band that runs diagonally through the middle of the H-R diagram. These stars are main sequence stars.

3. Classifying Stars

4. Star Formation (1st stage)
Nebula a large cloud of gas and dust in interstellar space; a region in space where stars are born.
1. A star beings in a nebula.
2. When the nebula is compressed, some of the particles move close to each other and are pulled together by gravity.
3. As described in Newton’s law of universal gravitation, as gravity pulls particles of the nebula closer together, the gravitational pull of the particles on each other increases.
4. As more particles come together, regions of dense matter begin to build up within the cloud.

5. Star Formation
Protostars
5. As gravity makes dense regions within a nebula more compact, these regions spin and shrink and begin to form a flattened disk. The disk has a central concentration of matter called a protostar.
6. The protostar continues to contract and increase in temperature for several million years. Eventually the gas in the region plasma.
Plasma – when the gas becomes so hot that its electrons are stripped from their parent atoms, it becomes a separate state of matter.

6. Star Formation (1st Stage)
The Birth of a Star
A protostar’s temperature continually increases until it reaches about 10,000,000°C.
At this temperature, nuclear fusion begins.
The onset of nuclear fusion marks the birth of a star. Once this process begins, it can continue for billions of years.

7. Star Formation A Delicate Balancing Act
The outward pressures of the radiation and the hot gas resist the inward pull of gravity.
This equilibrium makes the star stable in size.

8. Star Formation, continued
Reading Check How does the pressure from fusion and hot gas interact with the force of gravity to maintain a star’s stability? The forces balance each other and keep the star in equilibrium. As gravity increases the pressure on the matter within a star, the rate of fusion increases. This increase in fusion causes a rise in gas pressure. As a result, the energy from the increased fusion and gas pressure generates outward pressure that balances the force of gravity.

9. The Main-Sequence Stage (2nd Stage)
The second and longest stage in the life of a star is the main-sequence stage. During this stage, energy continues to be generated in the core of the star as hydrogen fuses into helium.
A star that has a mass about the same as the sun’s mass stays on the main sequence for about 10 billion years.
Scientists estimate that over a period of almost 5 billion years, the sun has converted only 5% of its original hydrogen nuclei into helium nuclei.

10. Leaving the Main Sequence (3rd stage)
Giant Stars
Giant - a very large and bright star whose hot core has used most of its hydrogen.
A star enters its third stage when almost all of the hydrogen atoms within its core have fused into helium atoms.
A star’s shell of gases grows cooler as it expands. As the gases in the outer shell become cooler, they begin to glow with a reddish color. These stars are known as giants.

11. Leaving the Main Sequence
Supergiants
Main-sequence stars that are more massive than the sun will become larger than giants in their third stage.
These highly luminous stars are called supergiants. These stars appear along the top of the H-R diagram.

12. Leaving the Main Sequence, continued
Reading Check Where are giants and supergiants found on the H-R diagram? Giants and supergiants appear in the upper right part of the H-R diagram.

13. The Final Stages of a Sunlike Star
Planetary Nebulas
As the star’s outer gases drift away, the remaining core heats these expanding gases.
The gases appear as a planetary nebula, a cloud of gas that forms around a sunlike star that is dying.

14. The Final Stages of a Sunlike Star
White Dwarfs
As a planetary nebula disperses, gravity causes the remaining matter in the star to collapse inward.
The matter collapses until it cannot be pressed further together.
A hot, extremely dense core of matter - a white dwarf - is left. White dwarfs shine for billions of years before they cool completely.
The gases appear as a planetary nebula, a cloud of gas that forms around a sunlike star that is dying.

15. The Final Stages of a Sunlike Star
Novas and Supernovas
Nova - a star that suddenly becomes brighter
Some white dwarfs revolve around red giants. When this happens, the gravity of the whit dwarf may capture gases from the red giant.
As these gases accumulate on the surface of the white dwarf, pressure begins to build up.
This pressure may cause large explosions. These explosions are called novas.

16. The Final Stages of a Sunlike Star
Novas and Supernovas
A white dwarf may also become a supernova, which is a star that has such a tremendous explosion that it blows itself apart.
The explosions of supernovas completely destroy the white dwarf star and may destroy much of the red giant.

17. The Final Stages of Massive Stars
Supernovas in Massive Stars
Massive stars become supernovas as part of their life cycle.
After the supergiant stage, the star collapses, producing such high temperatures that nuclear fusion begins again.
When nuclear fusion stops, the star’s core begins to collapse under its own gravity. This causes the outer layers to explode outward with tremendous force.

18. The Final Stages of Massive Stars, continued
Reading Check What causes a supergiant star to explode as a supernova? Giants and supergiants appear in the upper right part of the H-R diagram.

19. The Final Stages of Massive Stars
Neutron Stars
neutron star a star that has collapsed under gravity to the point that the electrons and protons have smashed together to form neutrons
Stars more massive than the sun do not become white dwarfs.
After a star explodes as a supernova, the core may contract into a neutron star.

20. The Final Stages of Massive Stars

21. The Final Stages of Massive Stars
Pulsars
pulsar a rapidly spinning neutron star that emits pulses of radio and optical energy
Some neutron stars emit a beam of radio waves that sweeps across space and are detectable here on Earth.
These stars are called pulsars. For each pulse detected on Earth, we know that the star has rotated within that period.

22. The Final Stages of Massive Stars
Black Holes
Black hole - an object so massive and dense that even light cannot escape its gravity
Some massive stars produce leftovers too massive to become a stable neutron star.
These stars contract, and the force of the contraction leaves a black hole.