1. Stellar Evolution

2. Basic Structure of Stars Mass and composition of stars determine nearly all of the other properties of stars Mass and composition of stars determine nearly all of the other properties of stars More massive a star is, the greater the gravity is, and the hotter and denser the star is inside More massive a star is, the greater the gravity is, and the hotter and denser the star is inside Temperature inside stars determines rate of nuclear reactions, which in turn affects the energy output, or luminosity of the star Temperature inside stars determines rate of nuclear reactions, which in turn affects the energy output, or luminosity of the star

3. Fusion Density and temperature increase toward the center, where energy is generated by nuclear fusion (hydrogen into helium) Density and temperature increase toward the center, where energy is generated by nuclear fusion (hydrogen into helium) Stars not on main sequence either fuse different elements in their core, or do not undergo fusion at all Stars not on main sequence either fuse different elements in their core, or do not undergo fusion at all

4. Stellar Evolution and Life Cycle A star changes as it ages because its internal composition changes as nuclear fusion reactions convert one element into another A star changes as it ages because its internal composition changes as nuclear fusion reactions convert one element into another As the star’s core composition changes, its density increases, its temperature rises, and its luminosity increases As the star’s core composition changes, its density increases, its temperature rises, and its luminosity increases When nuclear fuel runs out, star’s internal structure and mechanism for producing pressure must change to counteract the force of gravity When nuclear fuel runs out, star’s internal structure and mechanism for producing pressure must change to counteract the force of gravity

5. Star Formation Begins with cloud of interstellar gas and dust, called a nebula that collapses in on itself as a result of its own gravity Begins with cloud of interstellar gas and dust, called a nebula that collapses in on itself as a result of its own gravity

6. Star Formation As the nebula cloud contracts, its rotation forces it into a disk shape with a hot condensed object at the center, called a protostar As the nebula cloud contracts, its rotation forces it into a disk shape with a hot condensed object at the center, called a protostar The condensed object will become a new star The condensed object will become a new star

7. Star Formation Eventually, temperature inside protostar is hot enough for nuclear fusion reactions to occur Eventually, temperature inside protostar is hot enough for nuclear fusion reactions to occur Once fusion of hydrogen to helium occurs, the star becomes stable because it has sufficient internal heat to produce the pressure needed to balance the pressure of gravity Once fusion of hydrogen to helium occurs, the star becomes stable because it has sufficient internal heat to produce the pressure needed to balance the pressure of gravity

8. Life Cycle of Average Stars (Our Sun) What happens during a star’s life cycle depends on its mass What happens during a star’s life cycle depends on its mass As a star like the Sun converts hydrogen to helium in its core, it gradually becomes more luminous because the core temperature and density rise and increase the fusion reaction rate As a star like the Sun converts hydrogen to helium in its core, it gradually becomes more luminous because the core temperature and density rise and increase the fusion reaction rate Takes 10 billion years for average-sized star to convert all the hydrogen in its core into helium Takes 10 billion years for average-sized star to convert all the hydrogen in its core into helium

9. Life Cycle of Average Stars (Our Sun) Only innermost 10% of a star’s mass can undergo nuclear reactions because temperatures outside the core never get hot enough for nuclear reactions to occur Only innermost 10% of a star’s mass can undergo nuclear reactions because temperatures outside the core never get hot enough for nuclear reactions to occur When all the hydrogen in a star’s core is gone, star has a helium center and the outer layers have mostly hydrogen gas When all the hydrogen in a star’s core is gone, star has a helium center and the outer layers have mostly hydrogen gas Some hydrogen will continue to undergo reactions in the outermost layer of the helium core; energy produced in this layer forces out layers of the star to expand and cool Some hydrogen will continue to undergo reactions in the outermost layer of the helium core; energy produced in this layer forces out layers of the star to expand and cool The star has now become a red giant The star has now become a red giant

10. Life Cycle of Average Stars (Our Sun) While the star is a red giant, it loses gas from its outer layers While the star is a red giant, it loses gas from its outer layers Core becomes hot enough to for helium to react and form carbon Core becomes hot enough to for helium to react and form carbon Star contracts to a more smaller size where it is more stable Star contracts to a more smaller size where it is more stable Star never becomes hot enough for carbon to react, so star’s energy production ceases at this point Star never becomes hot enough for carbon to react, so star’s energy production ceases at this point Outer layers of gas expand and are driven off Outer layers of gas expand and are driven off This outer layer of gas is called a planetary nebula This outer layer of gas is called a planetary nebula Core of star becomes exposed as a small, hot object the size of Earth called a white dwarf Core of star becomes exposed as a small, hot object the size of Earth called a white dwarf White dwarf remains

11. Life Cycle of Massive Stars More massive star begins its life same way as average sized stars, but the star’s lifetime is much shorter because the star is so luminous and uses its fuel up quickly More massive star begins its life same way as average sized stars, but the star’s lifetime is much shorter because the star is so luminous and uses its fuel up quickly Undergoes more reaction phases and produces more elements in its interior Undergoes more reaction phases and produces more elements in its interior

12. Life Cycle of Massive Stars Star becomes a red giant several times as it expands following the end of each fusion reaction Star becomes a red giant several times as it expands following the end of each fusion reaction As more shells are formed, the star expands to larger size and becomes a supergiant (EX: Betelgeuse) As more shells are formed, the star expands to larger size and becomes a supergiant (EX: Betelgeuse) Light Echo Illuminates Dust Around Supergiant Star V838 Monocerotis (V838 Mon)

13. Size Comparison Size Comparison of Stars Size Comparison

14. Life Cycle of Massive Stars Some massive stars lose enough mass to become white dwarfs Some massive stars lose enough mass to become white dwarfs Stars that don’t lose that much mass come to a more violent end Stars that don’t lose that much mass come to a more violent end Once reactions in the core have produced iron, no future reactions can occur and the core the star quickly collapses on itself Once reactions in the core have produced iron, no future reactions can occur and the core the star quickly collapses on itself Neutron star is formed while outer gas layers are blown off in a massive explosion called a supernova Neutron star is formed while outer gas layers are blown off in a massive explosion called a supernova

15. Life Cycle of Massive Stars Neutron Star Supernova explosion

16. Life Cycle of Massive Stars When they are formed neutron stars rotate in space. As they compress and shrink, the rotation occurs faster. Those bodies that are still spinning rapidly may emit radiation that from Earth appears to blink on and off as the star spins, like the beam of light from a turning lighthouse. This "pulsing" appearance gives some neutron stars the name pulsars. When they are formed neutron stars rotate in space. As they compress and shrink, the rotation occurs faster. Those bodies that are still spinning rapidly may emit radiation that from Earth appears to blink on and off as the star spins, like the beam of light from a turning lighthouse. This "pulsing" appearance gives some neutron stars the name pulsars.pulsars The white dwarf in the AE Aquarii system is the first star of its type known to give off pulsar-like pulsations that are powered by its rotation and particle acceleration. Credit: Casey Reed

17. Life Cycle of Massive Stars Some stars too massive to form neutron stars. The core of these stars collapses forever, compacting matter into smaller and smaller volumes Some stars too massive to form neutron stars. The core of these stars collapses forever, compacting matter into smaller and smaller volumes The small, extremely dense object that remains is called a black hole The small, extremely dense object that remains is called a black hole Called a black hole because its gravity is so great that nothing, not even light, can escape Called a black hole because its gravity is so great that nothing, not even light, can escape

18. Black Holes Estimated that black holes will consume other stars at a rate of about once every ten thousand years in a typical galaxy Estimated that black holes will consume other stars at a rate of about once every ten thousand years in a typical galaxy Journey to a Black Hole! Journey to a Black Hole!Black HoleBlack Hole What happens if you fall into a black hole? What happens if you fall into a black hole? What happens if you fall into a black hole? What happens if you fall into a black hole?

19. Life Cycle of Stars – Summary