1. The Evolution of High-mass Stars

2. Central Temperature >1,000,000,000 K H fusion C,O fusion He fusion If the core’s mass is >1.4 M  (i.e., initial star mass >8 M  ), gravity is strong enough to overcome electron degeneracy, and the core contracts to the point that it’s hot enough for carbon fusion.

3. The Death of a High Mass Star The fusion of C and O halts the collapse of the core for a short time. But after the C and O is fully converted to a heavier element (Mg), contraction resumes.

4. The Death of a High Mass Star Over time, progressively heavier elements are fused within the core of the star and in shells surrounding it.

5. The Death of a High Mass Star When the star’s core fuses into iron, it resumes its collapse once again. However, iron is unable to fuse into heavier elements, which spells disaster for the star…

6. The Death of a High Mass Star As the iron core collapses, the pressure increases, which leads to increasing temperature. But iron is unable to fuse into heavier elements, so the core continues to contract and become hotter. Eventually, the core is so hot that the photons (gamma rays) produced by it have enough energy to split the iron nuclei into lighter nuclei. This is the reverse of the fusion that has occurred previously in the core, and hence consumes energy rather than producing it.

7. The Death of a High Mass Star Because energy is consumed by the splitting of the iron nuclei, the temperature drops, and the gas pressure decreases, causing the collapse of the core to accelerate rather than halt. The core implodes in a split second. The matter at the center of the core is compressed into a neutron star or a black hole. The outer part of the imploding core rebounds off of the matter at the center, sending a shock wave outward that blows the star apart. This is a Type II supernova explosion.

8. Type II Supernova Explosion

9. For about a month, a supernova will shine brighter than an entire galaxy of 100 billion stars. Type II Supernova Explosion

10. Evolution of High-mass Stars after the Main Sequence supernovaered giants neutron stars black holes After the red giant stage, a high- mass star (>8 M  ) undergoes a Type II supernova explosion, leaving behind a neutron star or a black hole. Among stars at >8 M , the most massive ones are more likely to produce black holes. Neutron stars and black holes are not plotted on an H-R diagram because they produce little or no light.

11. Only the lightest 3 elements were present when the universe was born. The Origin of Heavy Elements

12. All heavier elements were made in the centers of stars, either during fusion or during Type II supernova explosions. All elements heavier than iron are created in supernovae.

13. Recent Supernovae our Galaxy In our galaxy, a Type II supernova occurs once every couple hundred years. The last few were: Crab Supernova (1054 A.D.) Tycho’s Supernova (1572 A.D.) Kepler’s Supernova (1604 A.D.) Casseopia A (1680 A.D.?) SN 1006 (1006 A.D.)

15. If a supernova occurs within 100 light years of Earth, the radiation would destroy part of the ozone layer in Earth’s atmosphere, reducing protection against the Sun’s UV radiation. Effects of a Nearby Supernova on Earth A supernova occurs within 100 light years roughly every 100 million years. Supernovae may have played a role in some of Earth’s mass extinctions. One of the closest stars capable of a supernova is Betelgeuse, which is 600 light years away and should explode in about 100,000 years. It will be brighter than the Moon in our sky, and visible during daytime.

16. Neutron Stars A Type II supernova explosion leaves behind either a neutron star or a black hole. In a neutron star, the electrons have been crushed into the atomic nuclei, so the star is one gigantic atomic nucleus made up only of neutrons.

17. Neutron stars have masses between 1.4 and 3 M , but have radii of only 10 km, so they are extremely dense. A teaspoon of matter from a neutron star has a mass 1 billion metric tons! Because neutron stars are so small, they spin very rapidly, due to conservation of angular momentum. Neutron stars rotate once in only a second or less. Neutron Stars

18. Because they are so compact and dense, neutron stars have extremely strong gravity. The surface gravity on a neutron star is 100 billion times stronger than on Earth. If you set foot on a neutron star, you would immediately be flattened into a layer of matter a few atoms thick! Neutron Stars

19. Pulsars Neutron stars are extremely small, so according to L= R 2 T 4, their luminosities are tiny. However, they can beam light out from their magnetic poles. As the neutron star rotates, this beam of light can sweep across the Earth periodically like a searchlight. This variety of neutron star is called a pulsar.

20. Planets around a Pulsar The timing of a pulsar’s light is normally extremely regular and precise. But if a planet orbits the pulsar, the pulsar orbits as well, which causes the time between pulses to change. Pulsar timing demo In 1994, Penn State Prof. Alex Wolzczan discovered the first planets outside the solar system when he noticed that the timing of a pulsar was irregular. These planets probably formed after the supernova explosion from the debris that was left behind.

21. What Supports a Star Against Gravity? Type of StarWhat Holds it up?Limitation Normal StarsGas Pressure Must continually generate energy White DwarfsElectron Degeneracy Mass must be less than 1.4 M  Neutron StarsNeutron Degeneracy Mass must be less than ~ 3 M  Black Hole. What if a neutron star is greater than ~ 3 M  ? Gravity is strong enough to crush the neutrons into each other, and nothing can hold up the star. This is a Black Hole.

22. If 2 stars in a binary system are close enough to each other, one star can engulf the other when it expands to become red giant.

23. If one star doesn’t engulf the other, it may still expand enough so that material from its surface is pulled onto the other star. The matter spiraling onto the second star is called an accretion disk. Accretion Disks

24. X-rays from Accretion Disks According to Kepler’s laws, matter closer to a star orbits faster than matter farther away. Because the atoms in the inner part of an accretion disk are orbiting very fast, there’s a lot of friction among those atoms which leads to a very high temperature. The disk around a white dwarf, neutron star, or black hole can be hot enough to produce lots of X-rays.

25. Looking for Accretion Disks in X-rays Because accretion disks can be much hotter than stars, they can be detected by X-ray telescopes. Optical PictureX-ray Picture

26. Novae If a white dwarf is in orbit with another star, and the two are close enough, matter can be pulled from the 2nd star onto the white dwarf. When enough matter accumulates on the white dwarf, it undergoes a hydrogen fusion explosion, like a hydrogen bomb. This explosion appears as a sudden burst of light (500,000 L  ) and is called a nova. Only the layer of hydrogen on the white dwarf’s surface explodes; the star itself is unharmed. Many novae can occur over millions of years as layers of hydrogen repeatedly accumulate and detonate.

27. Type Ia Supernovae Recall that white dwarfs are held up by electron degeneracy. If accretion of matter from another star increases a white dwarf’s mass above 1.4 M , it has enough gravity to overcome electron degeneracy, and it begins to collapse. The collapse increases the temperature of the white dwarf enough so that fusion of C and O is ignited, and it explodes. This is a Type Ia supernova. A Type Ia supernova is just as bright as Type II supernova, but it doesn’t leave behind a neutron star or black hole. The white dwarf is completely destroyed.

28. Two Types of Supernovae

29. Time Mass

30. Slides beyond this point contain extra material that you might find interesting but is not covered on homeworks and exams

31. Accretion onto Neutron Stars: Millisecond Pulsars When a star explodes as a supernova, the neutron star that is left behind rotates about once a second. However, if a star accretes onto this neutron star, it can cause it to spin 1000 times faster!

32. Evaporated Stars Accretion disks around neutron stars (or black holes) emit large numbers of very energetic x-ray photons. These x-rays can strike the companion star’s atmosphere, and heat it up so much that the star literally evaporates. All that remains may be some rubble around a bare millisecond pulsar.

33. In the 1960’s, the U.S. used satellites to monitor for gamma rays produced by tests of nuclear weapons in space. The satellites occasionally detected flashes of gamma rays of unknown origin from random directions in space. Gamma Ray Bursts

34. In the late 1990’s, astronomers measured the first distances for gamma ray bursts, and found that they were billions of light years away. Because they could be detected at such large distances, the bursts must be incredibly luminous. In fact, they are the brightest and most powerful explosions known in the universe. Gamma Ray Bursts

35. Over the last decade, the Swift satellite has greatly improved our understanding of gamma ray bursts. It was funded by NASA, and Penn State is the lead partner in building its instruments and operating it. Gamma Ray Bursts

36. Astronomers now believe that gamma ray bursts are produced through 2 types of explosions. In each case, much of the radiation is focused in a narrow beam. We witness a gamma ray burst when the beam happens to be directed at Earth. Gamma Ray Bursts Unusually powerful supernovaeMerging neutron stars