1. Units to cover 66, 67,68

2. Homework 9 Unit 64, problems 4, 5, 9 Unit 65, problems 4, 10 Unit 66, problem 6

3. Formation of Planetary Nebula As a red giant expands, it cools –Outer layers cool enough for carbon flakes to form –Flakes are pushed outward by radiation pressure –Flakes drag stellar gas outward with them –This drag creates a high- speed stellar wind! –Flakes and gas form a planetary nebula

4. The Hourglass Nebula

5. White Dwarf Stars At the center of the planetary nebula lies the core of the star, a white dwarf –Degenerate material –Incredibly dense Initially the surface temperature is around 25,000 K Cools slowly, until it fades from sight.

6. Figure 64.05e

7. Our Sun will end its life by becoming A. a molecular cloud B. a pulsar C. a white dwarf D. a black hole

8. Mass Transfer and Novae A Roche lobe can be seen as a sphere of gravitational influence around a star Red Giant stars can fill their Roche lobes In a binary star system, the Roche lobes of the two stars can touch, and mass can pass between them. If a white dwarf is in orbit around a red giant companion star, it can pull material off the companion and into an accretion disk around itself Material in the accretion disk eventually falls to the surface of the white dwarf

9. Novae If enough material accumulates on the white dwarf ’ s surface, fusion can be triggered, causing a massive explosion This explosion is called a nova If this process happens repeatedly, we have a recurrent nova.

10. A Post-nova expansion

11. The Chandrasekhar Limit and Supernovae If mass is added to a white dwarf, its gravity increases If the white dwarf mass exceeds 1.4 solar masses (the Chandrasekhar Limit), the end of the white dwarf is near. The additional gravity squeezes the degenerate material in the white dwarf, causing it to compress by a small amount This compression causes the temperature to soar, and this allows carbon and oxygen to begin to fuse into silicon The energy released by this fusion blows the star apart in a Type 1a supernova

12. Type 1a Supernova – Another standard candle! The light output from a Type 1a supernova follows a very predictable curve –Initial brightness increase followed by a slowly decaying “ tail ” All Type 1a supernova have similar peak luminosities, and so can be used to measure the distance to the clusters or galaxies that contain them!

13. Formation of Heavy Elements Hydrogen and a little helium were formed shortly after the Big Bang All other elements were formed inside stars! Low-mass stars create carbon and oxygen in their cores at the end of their lifespan, thanks to the higher temperatures and pressures present in a red giant star High-mass stars produce heavier elements like silicon, magnesium, etc., by nuclear fusion in their cores –Temperatures are much higher –Pressures are much greater Highest-mass elements (heavier than iron) must be created in supernovae, the death of high-mass stars

14. The Lifespan of a Massive Star

15. Layers of Fusion Reactions As a massive star burns its hydrogen, helium is left behind, like ashes in a fireplace Eventually the temperature climbs enough so that the helium begins to burn, fusing into Carbon. Hydrogen continues to burn in a shell around the helium core Carbon is left behind until it too starts to fuse into heavier elements. A nested shell-like structure forms. Once iron forms in the core, the end is near…

16. Core Collapse of Massive Stars Iron cannot be fused into any heavier element, so it collects at the center of the star Gravity pulls the core of the star to a size smaller than the Earth ’ s diameter! The core compresses so much that protons and electrons merge into neutrons, taking energy away from the core The core collapses, and the layers above fall rapidly toward the center, where they collide with the core material and “ bounce ” The “ bounced material collides with the remaining infalling gas, raising temperatures high enough to set off a massive fusion reaction. The star then explodes. This is a supernova!

17. Before and After – a Supernova

18. Light Curve for a Supernova The luminosity spikes at the moment of the explosion, and gradually fades, leaving behind a…

19. The Crab Nebula

20. A Surprise Discovery Pulsating radio sources were discovered, and were named “pulsars” All pulsars are extremely periodic, like the ticking of a clock. –But in some cases, much, much faster…

21. An Explanation An idea was proposed that solved the mystery A neutron star spins very rapidly about its axis, thanks to the conservation of angular momentum If the neutron star has a magnetic field, this field can form jets of electromagnetic radiation escaping from the star If these jets are pointed at Earth, we can detect them using radio telescopes. As the neutron star spins, the jets can sweep past earth, creating a signal that looks like a pulse. Neutron stars can spin very rapidly, so these pulses can be quite close together in time!

22. Slowing Down? Over time, the spin rate of a pulsar can decrease at a small but measurable rate Sometimes the pulsar’s diameter shrinks slightly, causing a momentary increase in the pulsar’s rotation These “glitches” are short lived, and the spin rate begins to decrease again.

23. Interior Structure of a Neutron Star

24. Black Holes It takes for a test particle infinite time to fall onto a black hole. How can black holes grow in mass?

25. Viewing a black hole You may be asking, “If light cannot escape a black hole, how can we see one?” If a black hole is in orbit around a companion star, the black hole can pull material away from it. This material forms an accretion disk outside of the event horizon and heats to high temperatures As the gas spirals into the black hole, it emits X-rays, which we can detect!

26. Light curves from a black hole binary system

27. General Relativity Einstein predicted that not only space would be warped, but time would be affected as well The presence of mass slows down the passage of time, so clocks near a black hole will run noticeably slower than clocks more distant The warping of space has been demonstrated many times, including by observations of the orbit of Mercury The slowing of clocks has been demonstrated as well!

28. Gravitational Redshift Photons traveling away from a massive object will experience a gravitational redshift. –Their frequency will be shifted toward the red end of the spectrum

29. Star Clusters Stars form in large groups out of a single interstellar cloud of gas and dust These groups are called star clusters Open clusters have a low density of stars – there is lots of space between the cluster’s members They can contain up to a few thousand stars in a volume 14 to 40 light years across The Pleiades is a very familiar open cluster

30. Globular Clusters Some clusters are much more densely packed than open clusters. These globular clusters can have as many as several million stars, in a volume 80 to 320 light years across!

31. A snapshot of stellar evolution Because all stars in a given cluster formed at the same time out of the same cloud of material, we can learn a lot about stellar evolution by examining a cluster’s stars We can locate each star in a cluster on an HR diagram and look for the “turnoff point”, the point on the main sequence above which the stars in the cluster have run out of fuel and become red giants We can deduce the age of a cluster by finding this turnoff point.

32. Finding a Cluster’s Age