PHYS 420: Astrophysics & Cosmology

Transcription

1 PHYS 420: Astrophysics & Cosmology

2 Dr Richard H. Cyburt Assistant Professor of Physics My office: 402c in the Science Building My phone: (304) My My webpage: In person or is the best way to get a hold of me.

3 My Office Hours TR 5:30-17:00am W 5:00-5:00pm Meetings may also be arranged at other times, by appointment

4 Midterm 1 Thurs. Feb 9, 4-5:15pm Cosmic Perspective & Scientific Method(s) History & development of Modern Science & Cosmology Information from Light

5 Douglas Adams Hitchhiker s Guide to the Galaxy

6 What is the Universe made of?

7 Information from Light Stars are light bulbs If we understand stars and how bright they are, we can determine their distances Star-light carries information Gross features of the brightness (or intensity) spectrum tells us surface temperature Absorption or emission spectra tell us about the composition of the cosmos

8 What is the Universe made of? 1 Hydrogen Helium Carbon Relative abundance Oxygen Neon Sulfur Nitrogen Magnesium Iron Silicon Boron Lithium Beryllium Atomic number

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11 Discoveries 1800: William Herschel discovers heat rays 1801: Johann Ritter discovers chemical rays Later both renamed infaredand ultraviolet 1895: UV radiation below 200 nm 1895: Wilhelm Ro ntgen discovers X-rays 1897: J.J. Thomson discovers electron 1899: E. Rutherford discovers alpha particle 1900: P. Villard discovers gamma rays 1911: E. Rutherford discovers atomic nucleus

12 Particle Discoveries

13 The Atom Thomson Electrons uniformly distributed through-out atom Like raisons in plum pudding Rutherford Found occasional back-scattering A concentrated nucleus of positive charge

14 Mass Sprectrometer Can determine mass-to-charge ratio of atoms Francis William Aston found that there exists atoms with the same atomic charge, but with different atomic masses. He found that these masses varied by integer amounts and developed the whole number rule. Chadwick s discovery of the neutron explained this observation. A nucleus contains proton and neutrons, both with roughly the same mass, but only protons carrying electric charge.

15 The Atom Nucleus tiny compared to atom Nucleus made of neutrons and protons Cloud of electrons around nucleus Occupying specific orbitals All that positive charge bound in such a tiny space... What s up with that????

16 Periodic Table not adequate for isotopes

17 Chart of Isotopes Each isotope has a specific number of protons and neutrons An element is defined by the number of protons It s atomic number Isotopes all have the same number of protons They are all of the same element They only differ in total mass or number or neutrons Not all isotopes exist in nature! Why???

18 Forces of Nature Gravity: All massive objects are attracted to each and every other massive object Electromagnetism: Charged particles attract/repel each other Strong force: Needed to hold ALL nuclei together, otherwise EM repulsion would explode nuclei Weak force: Needed to explain stable and unstable nuclei & radioactive decay How do we understand these nuclide properties?

19 Total Energy of system Take planets or a simple hydrogen atom: Exam the total energy and ask what it means to be bound Total Energy = Kinetic Energy + Potential Energy What does escape speed tell us?

20 Borrow from Einstein E = mc 2 Energy and mass are the same thing, interchangeable Actual Mass of System = Mass of Constituents + Kinetic Energy + Potential Energy If a system of constituent objects is bound, what does that mean for the mass of the system?

21 Borrow from Einstein E = mc 2 Energy and mass are the same thing, interchangeable Actual Mass of System = Mass of Constituents + Kinetic Energy + Potential Energy If a system of constituent objects is bound, what does that mean for the mass of the system? Actual Mass of System < Mass of Constituents Binding Energy = Mass of Constituents Actual Mass of System

22 Binding Energy of Nuclei

23 Nuclei can be pictured as idealized spheres Volume of Nuclei = Counting Nucleons (neutrons and protons) Volume R 3 A = N+Z Nuclear Radius, R = r 0 A 1/3 Surface Area R 2 A 2/3 Absence of all other forces, neutrons and protons are the same (Nuclei strive towards N=Z) Pay a penalty for being not equal Penalty +,$ ' - Coulomb Energy of Protons $% & $% ' (/*

24 Semi-Empirical Mass Formula Binding Energy = Volume Energy Surface Energy Coulomb Energy Asymmetry Penalty /0 = a ; <=>?@AB ( 1 * ; CD=EDFGH % 1 I * a 1JK889LMK N,H 1 -

25 Chart of Isotopes Each isotope has a specific number of protons and neutrons An element is defined by the number of protons It s atomic number Isotopes all have the same number of protons They are all of the same element They only differ in total mass or number or neutrons Not all isotopes exist in nature! Why???

26 Nuclei enjoy being in lowest energy state Strong force tells us which Nuclide is most stable for a given A Weak force tells us how we decay

27 Nuclear Decays (Here, the axes have flipped) What is happening at the edges? When A is very big? When N is very big? When Z is very big?

28 Binding Energy of Nuclei

29 What is the Universe made of? 1 Hydrogen Helium Carbon Relative abundance Oxygen Neon Sulfur Nitrogen Magnesium Iron Silicon Boron Lithium Beryllium Atomic number

30 Nuclide Properties Cosmic Composition 1 Hydrogen Helium Carbon Relative abundance Oxygen Neon Sulfur Nitrogen Magnesium Iron Silicon Boron Lithium Beryllium Atomic number