Stellar Evolution

1. Stellar Evolution

2. Stars Defined Review • Star a large celestial body that is composed of gas and that emits light. • Nuclear fusion is the combination of light atomic nuclei to form heavier atomic nuclei • Astronomers learn about stars by analyzing the light that the stars emit.

3. Chapter 30 The Stages of Stars

4. Nebulaa large cloud of gas and dust in interstellar space; a region in space where stars are born. • A star beings in a nebula. • When the nebula is compressed, some of the particles move close to each other and are pulled together by gravity. • As described in Newton’s law of universal gravitation, as gravity pulls particles of the nebula closer together, the gravitational pull of the particles on each other increases. • As more particles come together, regions of dense matter begin to build up within the cloud. Star Formation= 1st Stage

5. Protostars • As gravity makes dense regions within a nebula more compact, these regions spin and shrink and begin to form a flattened disk. • The disk has a central concentration of matter called a protostar. • The protostar continues to contract and increase in temperature for several million years. • Eventually the gas in the region becomes so hot that its electrons are stripped from their parent atoms. • The nuclei and free electrons move independently, and the gas is then considered a separate state of matter called plasma. Star Formation

6. Star FormationSTAR IS BORNhttp://www.discovery.com/tv-shows/other-shows/videos/how-the-universe-works-a-star-is-born.htm The Birth of a Star • A protostar’s temperature continually increases until it reaches about 10,000,000°C. • At this temperature, nuclear fusion begins. • Nuclear fusion is a process in which less-massive atomic nuclei combine to form more-massive nuclei. The process releases enormous amounts of energy. • The onset of nuclear fusion marks the birth of a star. • Once this process begins, it can continue for billions of years.

7. Elements of Stars • Watch the video clip • Complete the video note sheet • Keep the note sheet with you.

8. Nuclear Fusion Nuclear Fusion • Nuclear fusion the process by which nuclei of small atoms combine to form a new, more massive nucleus; the process releases energy • Nuclear fusion occurs inside star. • Nuclei of hydrogen atoms are the primary fuel for the sun’s/ star’s fusion. • Nuclear fusion produces most of the suns’ energy and consists of three steps.

9. Nuclear Fusion • Two hydrogen nuclei, or protons, collide and fuse. • In this step, the positive charge of one of the protons is neutralized as that proton emits a particle called a positron. • As a result, the proton becomes a neutron and changes the original two protons into a proton-neutron pair. Step 1

10. Nuclear Fusion • Another proton combines with this proton-neutron pair to produce a nucleus made up of two protons and one neutron. • In the third step, two nuclei made up of two protons and one neutron collide and fuse. • As this fusion happens, two protons are released. • The remaining two protons and two neutrons are fused together and form a helium nucleus. • At each step, energy is released. Step 2 and 3

11. Nuclear Fusion Final Product The Final Product • One of the final products of the fusion of hydrogen in the sun, for example, is always a helium nucleus. • The helium nucleus has about 0.7% less mass than the hydrogen nuclei that combined to form it do. • The lost mass is converted into energy during the series of fusion reactions that forms helium. • The energy released during the three steps of nuclear fusion causes the sun/star to shine and gives the sun/ star its high temperature.

12. Star Formation A Delicate Balancing Act • As gravity increases the pressure on the matter within the star, the rate of fusion increase. • In turn, the energy radiated from fusion reactions heats the gas inside the star. • The outward pressures of the radiation and the hot gas resist the inward pull of gravity. • This equilibrium makes the star stable in size.

13. The Main Sequence= 2nd Stage • The second and longest stage in the life of a star is the main-sequence stage. • During this stage, energy continues to be generated in the core of the star as hydrogen fuses into helium. • A star that has a mass about the same as the sun’s mass stays on the main sequence for about 10 billion years. • Scientists estimate that over a period of almost 5 billion years, the sun has converted only 5% of its original hydrogen nuclei into helium nuclei.

14. Leaving the Main Sequence= 3rd Stage Giant Stars Giant a very large and bright star whose hot core has used most of its hydrogen. • A star enters its third stage when almost all of the hydrogen atoms within its core have fused into helium atoms. Billion years. • A star’s shell of gases grows cooler as it expands. As the gases in the outer shell become cooler, they begin to glow with a reddish color. • These stars are known as giants.

15. Leaving the Main Sequence Supergiants- • Main-sequence stars that are more massive than the sun will become larger than giants in their third stage. • These highly luminous stars are called supergiants. • These stars appear along the top of the H-R diagram.

16. Final Stages of Stars= 4th Stage Planetary Nebulas • As the star’s outer gases drift away, the remaining core heats these expanding gases. • The gases appear as a planetary nebula, a cloud of gas that forms around a sunlike star that is dying.

17. Final Stages of Stars= 5th Stage White Dwarfs • As a planetary nebula disperses, gravity causes the remaining matter in the star to collapse inward. • The matter collapses until it cannot be pressed further together. • A hot, extremely dense core of matter - a white dwarf - is left. • White dwarfs shine for billions of years before they cool completely. • The gases appear as a planetary nebula, a cloud of gas that forms around a sunlike star that is dying.

18. Final Stages of Stars Novas and Supernovas Nova a star that suddenly becomes brighter • Some white dwarfs revolve around red giants. • When this happeneds, the gravity of the white dwarf may capture gases from the red giant. • As these gases accumulate on the surface of the white dwarf, pressure begins to build up. • This pressure may cause large explosions. • These explosions are called Novas.

19. Final Stages of Stars Novas and Supernovas • A white dwarf may also become a supernova, which is a star that has such a tremedous explosion that it blows itself apart. • The explosions of supernovas completely destroy the white dwarf star and may destroy much of the red giant.

20. Chapter 30 The Final Stages of Massive Stars

21. Final Stages of Massive Stars Supernovas in Massive Stars • Massive stars become supernovas as part of their life cycle. • After the supergiant stage, the star collapses, producing such high temperatures that nuclear fusion begins again. • When nuclear fusion stops, the star’s core begins to collapse under its own gravity. • This causes the outer layers to explode outward with tremendous force.

22. Final Stages of Massive Stars Neutron Stars Neutron star a star that has collapsed under gravity to the point that the electrons and protons have smashed together to form neutrons • Stars more massive than the sun do not become white dwarfs. • After a star explodes as a supernova, the core may contract into a neutron star.

23. Final Stages of Massive Stars Pulsars Pulsar a rapidly spinning neutron star that emits pulses of radio and optical energy • Some neutron stars emit a beam of radio waves that sweeps across space and are detectable here on Earth. • These stars are called pulsars. For each pulse detected on Earth, we know that the star has rotated within that period.

24. Final Stages of Massive Stars Black Holes Black hole an object so massive and dense that even light cannot escape its gravity • Some massive stars produce leftovers too massive to become a stable neutron star. • These stars contract, and the force of the contraction leaves a black hole.