Stellar Evolution

1. Unit 1: Space The Study of the Universe Stellar Evolution

2. Basic Structure of Stars • Mass governs a star’s temperature, luminosity, and diameter. • Mass Effects: • The more massive the star, the greater the force of gravity towards its center of mass (the core). • As a result, a star needs to be hotter and denser to counteract its own gravity. • The balance between gravity squeezing inward and outward pressure is maintained by heat due to nuclear reactions and compression.

3. Stellar Evolution • Star formation • The formation of a star begins with a cloud of interstellar gas and dust called a nebula. • Provided the cloud is big enough, it will begin collapsing in on itself as a result of gravity. • As it continues to contract, its rotational forces it into a disk shape with a hot and dense center. • This is called a protostar.

4. Stellar Evolution • Fusion Begins. • When the temperature at the core of the protostar becomes hot enough, nuclear fusion reactions begin. • The first fusion reactions always begin with the conversion of hydrogen into helium. • Once this happens, the star becomes stable and it takes its place along the main sequence according to its mass.

5. Stellar Evolution

6. Stellar Evolution • Life Cycles of medium-low mass stars. • A star like the Sun will gradually become more luminous because the core density and temperature rise slowly and increase the reaction rate. • It takes about 10 billion years for a star like the Sun to convert all of the hydrogen in its core to helium.

7. Stellar Evolution • Red Giant Phase • Once a star begins burning helium in its core, it grows to become a red giant. • Red giants are so large because hydrogen continues to react in a thin layer at the edge of the helium core. The energy produced in this layer forces the outer layers of the star to expand.

8. Stellar Evolution • Red giants are so large that their cores are a great distance from the outer layers. As a result, surface gravity is low and some of the outer layers can be released by small expansions of the star due to instability. • Meanwhile, the core of the star becomes hot enough (100 million K) for helium to react and begin forming carbon. • At this point, the star contracts again and becomes more stable.

9. Stellar Evolution • The final stages • Stars that are about the same mass as the Sun never become hot enough to fuse carbon and so energy production ends. • The outer layers expand again and are expelled by pulsations that develop in the outer layers. • This shell of gas is known as a planetary nebula.

10. Stellar Evolution • White Dwarfs • A white dwarf is made of carbon and it is stable despite its lack of nuclear reactions. • It counteracts the effects of gravity with the resistance of electrons being squeezed so closely together. • This electron pressure does not require ongoing reactions so it can last indefinitely. • Eventually, the white dwarf cools and loses its luminosity: it has become a black dwarf.

11. Stellar Evolution Not this kind of black dwarf.

12. Stellar Evolution • Life Cycles of Massive Stars • Stars that are much larger than our Sun have a different life cycle. • These star may begin in the same way but, because its initial mass is greater, it is more luminous and its main sequence lifetime is much shorter.

13. Stellar Evolution • Supergiant • Supergiants can undergo many more reaction phases and, therefore, they can produce far more elements in their interior. • These stars can become red giants several times as they expand following the end of each reaction phase. • Supernova Formation • A star that begins with a mass that is 8-20 times greater than the Sun’s mass will end up with a core that is too massive to be supported by electron pressure. • Once core reactions have produced iron, no further energy producing reactions can occur.

14. Stellar Evolution

15. Stellar Evolution • Neutron Star • The iron core collapses in on itself and protons and electrons merge to form neutrons. • The neutrons resist being so near to each other and this creates a tremendous amount of pressure. • The core becomes a collapsed star remnant – a neutron star. • A neutron star can have a mass of 1.5 to 3 times the Sun’s mass but with a radius of only about 10km. • Some neutron stars are unique in that they have a pulsating pattern of light. These pulsating stars are known as pulsars.

16. Stellar Evolution • Supernova • A neutron star forms quickly while the outer portions of the star are still falling inward. • The falling gas rebounds quickly after it strikes the hard surface of the neutron star. • The entire outer portion of the star is blown off in a massive explosion called a supernova. • This explosion is the only way that elements heavier than iron are produced in the universe.

17. Stellar Evolution • Black Holes • Some stars are too massive to form neutrons stars. • The resistance of neutrons being squeezed is not great enough to counteract the force of gravity (collapse). • Matter gets compacted into an increasingly small volume. • The small, extremely dense object is known as a black hole. • In a black hole, gravity is so immense that nothing, not even light, can escape it.

18. Stellar Evolution • Homework • Answer questions 4 and 8 on page 349 • Complete the Inquiry Investigation (8-B) on page 352: • Answer questions 1-5 and 7 on page 353.