1. 25.2 – Stellar Evolution – Part II

2. Do Now
Will all stars eventually run out of fuel? Even our sun?

3. Do Now Will all stars eventually run out of fuel? Even our sun?
Yes and yes

4. Key Words
Consume
Exhausted
Implosion
Remnant

5. Vocab Words
Supernova
White Dwarf
Neutron Star
Pulsar
Black Hole

6. Burnout and Death
Most of the events of stellar evolution that were previously talked about are well documented, the rest are based more on theory.
We do know that all stars, regardless of their size, eventually run out of fuel and collapse due to gravity.
We hypothesize that stars of different masses have different final stages.

7. Burnout and Death

8. Death of Low-Mass Stars
Stars less than one half the mass of the sun consume their fuel at a fairly slow rate.
Can remain on the main-sequence star stage for up to 100 billion years.
The interior of a low-mass star never reaches high enough temperatures and pressures to fuse helium, its only energy source is hydrogen.
They will never evolve into red giants.
They remain as stable main-sequence stars until they consume their hydrogen fuel and collapse into a white dwarf.

9. Burnout and Death

10. Death of Medium-Mass Stars
Stars with masses similar to the sun evolve in essentially the same way.
During their giant phase, sunlike stars fuse hydrogen and helium fuel at a fast rate. Once this fuel is exhausted, these stars also collapse into white dwarfs.
During their collapse from red giants to white dwarfs, medium-mass stars are thought to cast off their bloated outer layer, creating an expanding round cloud of gas.
The remaining hot, central white dwarf heats the gas cloud, causing it to glow.
These are called planetary nebulae.

11. Planetary Nebula

12. Burnout and Death

13. Death of Massive Stars
Stars with three times the mass of the sun have relatively short life spans.
These stars end their lives in a brilliant explosion called a supernova.
During a supernova, a star becomes millions of times brighter than its prenova stage.
If one of the nearest stars to Earth produced such an outburst, it would be brighter than the sun.
Supernovae are rare.
None have been observed in our galaxy since the invention of the telescope, although Tycho Brahe and Galileo each recorded one about 30 years apart.
An even larger supernova was recorded in 1054 by the Chinese, and the remnant of this great outburst is the Crab Nebula.

14. Crab Nebula

15. Death of Massive Stars
A supernova event is though to be triggered when a massive star consumes most of its nuclear fuel.
Without a heat engine to generate the gas pressure required to balance its immense gravitational field, the star collapses.
This implosion, or bursting inward, is huge, resulting shock wave that moves out from the star’s interior.
This energetic shock wave destroys the star and blasts the outer shell into space, generating a supernova event.

16. Burnout and Death

17. H-R Diagrams and Stellar Evolution
As stars go through their different stages, they also will change their absolute magnitude and temperature, so we still can use the H-R diagram to study and classify stars throughout all their stages.

18. Life-cycle of a Sunlike Star

19. Stellar Remnants
Eventually, all stars consume their nuclear fuel and collapse into one of three states:
White Dwarf
Black Dwarf
Neutron Star
Black Hole
Although different in some ways, these small, compact objects are all composed of incomprehensibly dense material and all have extreme surface gravity.

20. Stellar Remnants

21. White Dwarf Stars
They are the remains of low-mass and medium-mass stars.
They are extremely small stars with densities greater than any known material on Earth.
As a star contracts into a white dwarf, its surface becomes very hot sometimes exceeding 25,000 K.
The smallest white dwarf stars are the most massive and the largest white dwarf stars are the least massive.
A more massive star, because of its greater gravitational force, is able to squeeze itself into a smaller, more densely packed object than a less massive star.
Most white dwarf stars will end up as black dwarf stars because there is no source of energy and the star becomes cooler and dimmer to a point where we can no longer see it in space.

22. White Dwarf Stars

23. Neutron Stars Remnants of supernova events.
Smaller, more massive than white dwarfs.
In a white dwarf, the electrons are pushed close to the nucleus, where in a neutron star, the electrons are forced to combine with the protons to produce neutrons.
If Earth were to collapse to the density of a neutron star, it would have a diameter equal to the length of a football field.
They can be thought of as large atomic nuclei.

24. Neutron Stars

25. Supernovae
During a supernova, the outer layer of the star is ejected, while the core collapses into a very hot neutron star about 20 kilometers in diameter.
As the star collapses, it will rotate faster, and radio waves are generated.
Pulsars were discovered inside remnants of supernovae, and they are a source that radiates short bursts or pulses of radio energy.

26. Black Holes
During a supernova event, remnants of stars three times more massive than the sun apparently collapse into objects even smaller and denser than neutron stars, called black holes.
They are very hot, their gravity is so strong that not even light can escape their surface.
Anything that moves too near a black hole would be swept in by its gravity and lost forever.

27. Black Holes

28. Group Activity
On page 710, Figure 11, copy and label stellar evolution.