非常棒-中子星,黑洞及其聯(lián)系

1、Low Mass (M 8 M) Stellar Evolution Main Sequence (core hydrogen fusion) Red Giant Star (core contraction, shell hydrogen fusion) Helium Burner (helium fusion in core) 2nd Giant Branch (core contraction, shell hydrogen and helium fusion, mass loss) Due to mass loss, the star is now less than 1.4 M (t

2、he Chandrasekhar limit) Planetary Nebula (ionization of mass lost as a giant star) White Dwarf star (inert carbon/oxygen core)Planetary NebulaeThe Endpoint A White DwarfNote: Electron Degeneracy only works if the star is less than 1.4 M. This is the Chandrasekhar Limit. If the star is more massive t

3、han 1.4 M, something else must happen. In stars with final masses over the Chandrasekhar limit, the gravity becomes so great that even carbon and oxygen can fuse. The result is a host of products, including neon, sodium, magnesium.The Death of a High Mass StarSince 24Mg weighs less than two 12C atom

4、s, the result is energy!The Death of a High Mass StarThe products of fusion are getting heavier! Carbon-burning (temporarily) supplies energy to core. The core expands, shell-burning stops, and the star contracts. It doesnt take long to burn all the carbon/oxygen. When the C/O is gone, the core agai

5、n contracts, and C/O fusing is forced into a shell around the core. The Death of a High Mass StarEventually, magnesium, etc., will begin to fuse. When it does, the result is The Death of a High Mass Stars Aluminum, Silicon, Phosphorus, Sulfur , and Energy! Magnesium-burning (temporarily) supplies en

6、ergy to core. The core expands, shell-burning stops, and the star contracts. The magnesium, etc., fuses very quickly, and when its gone, the core again collapses, and shell burning begins. The Death of a High Mass StarSoon, the core fuses silicon. When it does, the main products areThe Death of a Hi

7、gh Mass Star Iron, Cobalt, Nickel, and Energy! This time silicon-burning (temporarily) supplies the energy. The core expands, shell-burning stops, and the star contracts. Silicon fuses extremely quickly, and when its gone, the core again collapses, and shell burning begins. The Death of a High Mass

8、StarWhen the stars core turns to iron, it again collapses. The increased pressure and temperature then causes iron to fuse. HoweverThe products of iron fusion weigh more than the initial iron nucleus. According to E = m c2, this means that iron fusion does not make energy, it absorbs energy.The Deat

9、h of a High Mass StarUp to iron, the products are lighter than the ingredients: + m c2After iron, the products are heavier than the ingredients: - m c2For heavy elements, you make energy by fission.Fission and FusionWhen the stars core turns to iron, it again collapses. The increased pressure and te

10、mperature then causes iron to fuse. HoweverThe products of iron fusion weigh more than the initial iron nucleus. According to E = m c2, this means that iron fusion does not make energy, it absorbs energy.The Death of a High Mass StarThe more iron that fuses, the more energy is taken out of the core.

11、 The temperature decreases, the gas pressure decreases, the core collapses faster, more iron fuses and The star explodes! In that explosion, every element heavier than iron is created. This is the only way these heavier elements (such as silver, gold, etc.) can be created in a supernova explosion.Su

12、pernovaThe Products of SupernovaeIn a supernova, all the elements previously made in a star are thrown out into space. In addition, every element heavier than iron is made and ejected as well.The SupernovaeFor about a month, a supernova will outshine an entire galaxy of 100,000,000,000 stars!Many of

13、 the elements made in a supernova explosion are radioactive, i.e., they make energy by nuclear fission. This is keeps the material bright for some time.Supernova RemnantsGalactic SupernovaeIn a galaxy such as the Milky Way, a supernova should occur once every 50 to 100 years. The last few wereCrab S

14、upernova (1054 A.D.)Tychos Supernova (1572 A.D.)Keplers Supernova (1604 A.D.)Casseopia A (1680 A.D.?) SN 1006 (1006 A.D.)Neutron StarsIn addition to ejecting a large amount of (nuclear processed) matter into space, a supernova explosions will leave behind a stellar remnant. In the remnant, the elect

15、rons of atoms are crushed into their nucleus. The star becomes one gigantic atomic nucleus made up only of neutrons a neutron star.Neutron StarsNeutron stars have masses that are similar to that of the Sun, but they are extremely small only a few miles across!And because neutron stars are so small,

16、they spin very rapidly, due to conservation of angular momentum. Neutron stars rotate about once a second!PulsarsNeutron stars are extremely small, so, by L = 4 R2 T4 , their blackbody emission is minimal. However, they can beam light out from their magnetic poles via synchrotron emission.If the “se

17、archlight” points towards earth, we see a pulsar.Q ui ckTi m e?and aCi nepak decom pressorare needed to see thi s pi cture.PulsarsPulsar light comes out at all wavelengths, but is especially bright in the radio and the x-ray. The Crab pulsar is detectable in the optical. (When first detected, these

18、objects were dubbed “LGMs” for Little Green Men)Q ui ckTi m e?and aCi nepak decom pressorare needed to see thi s pi cture.What Supports a Star Against Gravity?Type of StarWhat Holds it up?LimitationNormal StarsGas Pressure Must continually generate energyWhite Dwarfs Electron DegeneracyMass must be

19、less than 1.4 MNeutron StarsNeutron DegeneracyMass must be less than 3 MWhat if a neutron star is greater than 3 M? The neutrons will get crushed! There is nothing left to hold up the star. You get a The Speed of LightImagine yourself in a river. The time it takes for you to swim upstream is longer

20、than it takes for you to swim downstream.The equivalent should be true for light. The time it takes for light to move upstream (against the motion of the Earth) should be longer than the time it takes to go downstream. But it isnt! The speed of light is always the same!Special RelativityPremise: con

21、stant velocity motion is relative (i.e., are you moving, or is the entire world moving past you?)Since the speed of light is always the same, this has some weird implications.Implication: A Speed LimitImagine holding a flashlight. You turn the flashlight on, and the light illuminates your path ahead

22、.Now perform the same experiment while running, i.e., while racing a beam of light. Can you win? ANSWER: NO! For you are not running you are standing still, and the whole world is running past you. And the speed of light as you measure it is always the same!Wacky Addition of VelocitiesImagine running at the speed of light in one direction, while another person runs at the speed of light in the other direction.0.75 c0.75 c0.94 cYou do not observe the other