Lecture 23 Stellar Evolution & Death (High Mass) November 21, 2018 1
2 High Mass Stars (M > 5 M ) Section 13.3 Bennett, The Essential Cosmic Perspective, 7 th ed. High mass stars have: More mass Greater gravity Higher temperatures and pressures in the core. Fusion reactions do not stop with Helium burning in the core as they do in smaller stars.
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4 Star becomes giant similar to small mass star. Helium burning ends in core. Core contracts. Temp and pressure in core increase. He shell burning begins. Core continues collapse. Then carbon fusion begins in the core. Carbon fuses into higher-mass elements. Process continues as core runs out of fuel.
5 Fusion of different elements continues through neon, oxygen, silicon and finally iron.
6 Star expands to become a Supergiant. Star moves back and forth on the HR diagram with each type of fusion.
7 Each stage of fusion lasts for a shorter period of time Fusion Temp Duration (million K) H 40 7 mill. yrs He 200 500000 yrs C 600 600 yrs Ne 1200 1 yr O 1500 6 mo. Si 2700 1 day
8 Death of High Mass Star Iron builds up in the core. Iron cannot be fused and produce more energy. What keeps iron core from collapsing? First: electron degeneracy
9 Death of a High Mass Star After core has a mass greater than 1.4 M (Chandrasekhar limit) the electron degeneracy is not strong enough. Electrons are forced to combine with the protons to create neutrons. Core collapses until pressure from physical force of neutrons bouncing against each other stops it. Core rebounds and runs into outer material that is still falling inward.
10 Supernova Collision produces huge shock wave pushing all material outward in an immense explosion called a supernova. Explosion can be as bright as an entire galaxy (billions of stars) for a few days Some of the energy creates elements heavier than iron. These elements are distributed to the rest of the galaxy. Interactive Fig 13.17 core detail Interactive Fig 13.15 track on HR diagram
11 Supernova 1987a
12 Eta Carinae (100-150 Solar Masses) Last outburst in 1841
13 Supernova leave a large shell of slowly expanding material around a central core (supernova remnant).
14 Stars like the Sun probably do not form iron cores during their evolution because A. all the iron is ejected when they become planetary nebulas. B. their cores never get hot enough for them to make iron by nucleosynthesis. C. the iron they make by nucleosynthesis is all fused into uranium. D. their strong magnetic fields keep their iron in their atmospheres. E. they live such a short time that it is impossible for iron to form in their cores.
15 Neutron Stars Section 14.2 Bennett, The Essential Cosmic Perspective, 7 th ed. Supernova remnant Tightly packed neutron core. Size ~ 20 km (small asteroid or city) Mass ~ 1.4 to 3 M Density very high 1 tsp. > 100,000,000 tons on Earth. Some stars rotate many times per second Conservation of angular momentum
16 Strong magnetic field When star collapses, magnetic field is concentrated.
17 Neutron Star -- HST
18 Where would a neutron star be found on an H-R diagram? A. Region A B. Region B C. Region C D. Region D E. Region E F. Region F G. Region G H. Region H
19 Where would a neutron star be found on an H-R diagram? A. Region A B. Region B C. Region C D. Region D E. Region E F. Region F G. Region G H. Region H Neutron stars are hot and very tiny so they d be found near region F on an H-R diagram.
20 1967 Jocelyn Bell Pulsars Observed object emitting pulses of radio waves. Pulses repeated every 1.34 seconds
21 Pulsars Hundreds more have been found. Some pulse in optical, X-rays, or gamma rays. Periods typically range from 0.03 to 0.30 sec. Periods gradually increase with pulsar s age Angular momentum is not fully conserved Earth slows due to tidal friction Pulsars slow due to radiated energy Some pulsars are associated with supernova remnants.
22 Hewitt proposed it is a rapidly rotating neutron star beaming radiation. Magnetic pole and rotational axis not quite lined up. Strong magnetic field. Charged particles at poles of magnetic fields and emit large amounts of energy. Pulsars Lighthouse Model
23 Not all neutron stars are seen to pulse Beam may not be pointed at the Earth Animation (Arny & Schneider, Explorations, 5 th ed., Figure 14-9) Unknown if all neutron stars are pulsars Earth never sees beam of energy Earth Earth sees beam of energy Earth
24 Crab Nebula
25 Crab Pulsar The pulsar must be young because it is seen at visible and X-ray wavelengths. Old pulsars emit at lower energy radio wavelengths. Comins & Kaufmann, Discovering the Universe 7 th ed., Fig. 13-18.
26 As time progresses, the pulse rate for most solitary pulsars is A. decreasing, because rotational energy is being used to generate the pulses. B. remaining constant due to conservation of angular momentum. C. varying periodically as the neutron star expands and contracts D. increasing, because the neutron star slowly contracts.
27 The Age of Star Clusters Section 12.3 Bennett, The Essential Cosmic Perspective, 7 th ed. Open Clusters --loose clusters of 10-100 stars Globular Clusters -- Old, tightly bound group of 100 s or 1000 s of stars All stars in a cluster are formed at the same time. Age of a cluster can be determined by looking at what point the stars turn off of the main sequence turn-off point. Age of Cluster = Lifetime of star at turn-off point.
28 Figure 20.17, Chaisson and McMillan, 5 th ed. Astronomy Today, 2005 Pearson Prentice Hall Interactive Figure 12.17
29 Illustrative movie
30 Young Cluster -- Hyades cluster Around 600 million years old Figure 20.19, Chaisson and McMillan, 5 th ed. Astronomy Today, 2005 Pearson Prentice Hall
31 Old Cluster -- 47 Tucanae One of the oldest clusters, about 12 to 14 billion years old Figure 20.20, Chaisson and McMillan, 5 th ed. Astronomy Today, 2005 Pearson Prentice Hall
32 The Pleiades is a very young cluster. What would you expect its overall color to be when observed from the Earth? A. Blue B. Yellow/White C. Red D. None of the above
33 End of material on Exam 3 Exam 3 Information Bring a #2 pencil! Bring a calculator. No cell phones or tablets allowed! Contents: Worked-out problems (2 questions, 10 points) True/False (10 questions, 20 points) Multiple Choice (35 questions, 70 points. None of these require a calculation.)