1. Section 3 – pg 608 Lives of Stars
Chapter 15
Section 3 – pg 608
Lives of Stars

2. Pg 609
The Lives of Stars
Stars do not last forever, they go through stages of birth, life, and death

3. Pg 609
A Star is Born
All stars begin their lives as part of nebulas
Nebula: a large cloud of gas and dust spread out in an immense volume
In the densest part of the nebula, gravity pulls gas and dust together heating up
Protostar: a contracting cloud of gas and dust with enough mass to form a star
A star is born when the contracting gas and dust from a nebula become so dense and hot that nuclear fusion starts

4. Lifetimes of Stars Pg 609 How long a star lives depends on its mass
Small stars live longer than big stars because they use up their gas less quickly
Small stars can live up to 200 billion years
Medium stars live for about 10 billion years
Our sun is about 4.6 billion years old
Big stars live only about 10 million years

5. Pg 610
Deaths of Stars
As long as a star produces energy through fusion of hydrogen to helium, it stays in the main sequence
When a star begins to run out of hydrogen, its core shrinks and its outer portion expands
Depending on its mass, it either becomes a red giant or a super giant

6. Pg 610
Once the hydrogen is used up, the helium begins to fuse in the star’s core creating heavier elements such as carbon and oxygen
After a star runs out of fuel, it becomes a white dwarf, a neutron star, or a black hole

7. Pg 610
Planetary Nebulas
Low-mass stars and medium-mass stars take billions of years to use up their nuclear fuel
As they start to run out of fuel, outer layers expand and become red giants
Eventually the outer parts grow apart and drift out into space
Form a glowing cloud of gas called a planetary nebula
Cat’s Eye Nebula

8. Pg 611
White Dwarfs
White Dwarf: the blue-white core left over in a planetary nebula
About the size of Earth, have as much mass as the sun
Are extremely dense
Nuclear fusion no longer happens, but they glow faintly from left over energy

9. Pg 611
Supernovas
High-mass stars quickly evolve into brilliant supergiants
When supergiants run out of fuel, they explode suddenly (supernova)
Produces enough energy to create elements that are heavier than iron, such as lead and gold
These elements are flung into space during the explosion
These elements may become part of a nebula which is later recycled into a new star

10. Pg 612
Neutron Stars
After a supergiant explodes, some material from the star is left behind
Neutron stars: the remains of high-mass stars
Smaller and denser than white dwarfs
May contain 3 times the mass of the sun in the size of a city

11. Pg 612
In 1967, Jocelyn Bell, a British astronomy student, detected an object in space that appeared to give off regular pulses of radio waves
Some astronomers hypothesized that the pulses might be a signal from an extraterrestrial civilization
Named the source LGM (Little Green Men)
Later astronomers concluded that the source was really a rapidly spinning neutron star

12. Pg 612
Pulsars
Pulsars: spinning neutron stars, pulsating radio sources
Emit steady beams of radiation in narrow cones
As they spin, astronomers can briefly detect a flash of radiation

13. Pg 613
Black Holes
The most massive stars may become black holes when they die
Black hole: an object with gravity so strong that nothing, not even light, can escape
When a massive star dies in a supernova explosion, a lot of mass is left over
The gravity of the mass is so much that the gas is pulled inward, into a smaller and smaller space

14. It is not possible to detect a black hole directly since no light, radio waves, or any other form of radiation can get out
Can detect it indirectly
Gas near black holes is pulled so strongly that it revolves faster and faster around the black hole
Friction heats up the gas and we can detect x-rays coming from this gas
Astronomers can calculate the size of black holes from the effect of its gravity on a nearby star
We have detected huge black holes that are millions or billions of times the sun’s mass
Pg 613

15. Chapter 15 Section 3 Homework – pg 613

16. 1A. What is a nebula?

17. 1B. How does a star form from a nebula?

18. 1C. How is a protostar different from a star?

19. 2A. What factor determines how long a star lives?

20. 2B. A star is twice as massive as the sun
2B. A star is twice as massive as the sun. Will its lifespan be longer, shorter, or the same of that of the sun? Explain.

21. 3A. What is a white dwarf? How is it different from a neutron star?

22. 3B. Why do some stars become neutron stars or black holes?

23. 3C. What will happen to the sun when it runs out of fuel? Explain.