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

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

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

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

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

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

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

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

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

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

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

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

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

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

Chapter 15 Section 3 Homework – pg 613

1A. What is a nebula?

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

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

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

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.

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

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

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