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1IASS - 2006/7

1IASS - 2006/7. Class Meets:- Tuesdays 12.00 in LTB Thursdays 10.00 – 12.00 in TB13. Lecturer:- W.Gelletly Office 13BB03 e-mail:- W.Gelletly@surrey.ac.uk. All diagrams and pictures on slides

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1IASS - 2006/7

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  1. 1IASS - 2006/7 Class Meets:- Tuesdays 12.00 in LTB Thursdays 10.00 – 12.00 in TB13. Lecturer:- W.Gelletly Office 13BB03 e-mail:- W.Gelletly@surrey.ac.uk All diagrams and pictures on slides All notes and pictures etc on Physics Intranet – 1IASS-08 Books:- An Introduction to Modern Astrophysics by Carroll and Ostlie Second edition-this has SI units. Universe by Freedman and Kaufmann – Sixth edition. Assessment:- 70% written examination in summer 30% multiple choice test on Thursday 21st,February at 11.00

  2. Milky Way-over Alps with lunar eclipse

  3. M31-Andromeda Galaxy-2.2Mly from Earth, Part of our Local cluster of galaxies. It will collie with the Milky Way in about 6 billion years or so.

  4. Photo-mosaic picture of the Sombrero Galaxy taken with the Hubble Space Telescope over several orbits.Glowing central bulge of stars surrounded by pancake shaped disc.

  5. M74 Gemini – Spiral galaxy in Pisces - It is about 30Mly away and has  1011 stars Taken with Gemini North Telescope – Mauna Keau

  6. Helix Nebula-The glowing gas consists of N,O and H ejected by a Solar Mass star in its death throes. Radiation from the remaining central star causes the gas to glow.This star will become a White Dwarf. Dwarf. Photograph by D. Malin

  7. Cometary knots in the Helix Nebula. Gaseous objects seen with the HST in the Helix Nebula. The head is about twice the size of the solar system and the tail is of length 1011 km. They may be formed when hot, low density gas emitted by the dying star collides with cool, higher density gas ejected some 104 years earlier. Picture taken Aug 1994 with wide- Field planetary camera 2 - HST

  8. Crab-UV

  9. Crab-vis

  10. CRAB-IR

  11. Crab-X-ray

  12. Supernova seen by Chinese in 1054 A.D. Bright clumps moving outwards at v ~ 0.5c Filaments have lower mass and higher velocity than models would predict. Crab Nebula at X-ray wavelengths There is a pulsar ( = neutron star) at the centre which rotates at about 30 times per second. It is about 7 kly away in Taurus

  13. Crab Nebula(7kly away in Taurus)-several exposures with HST. Clear changes in central regions-wisp like structures moving outwards with v=0.5c and halo which is stationary but brightens and weakens.

  14. Coma Cluster - 20Mly across containing thousands of galaxies(300 seen here). It is about 270 Mly away.Two supergiant galaxies seen in centre.

  15. Hubble Deep Field-A very narrow sample of the sky looking as far back as 10 10 years in some cases.Data taken over 10 consecutive days.

  16. Globular Cluster M10 – 16,000 ly away in ORPHIUCUM Diameter is  70 ly. Mainly post-Main sequence stars

  17. Aurora seen over Edmonton on 4/11/2003. We see the Clover Bar Power station on the Saskatchewan river. The auroral light is seen reflected in a small pond.

  18. Isabel Terra (category 5) hurricane passing east of the Bahamas

  19. NASA LONDON

  20. Ngc6240 merging and infrared bright galaxy in Orpiuchus

  21. SN1987A- Supernova in large Magellanic cloud recorded in October 1987. We see the region before and after the supernova.

  22. Supernova 1998bw- scales are different on right (before) and left (after)

  23. Eagle Nebula- 7000ly away Pillars of H and dust. In pillars gas is contracting to form new stars. Radiation boils away low density material at ends of pillars.

  24. Lagoon nebula  5000ly away, 100 ly across. It is in Sagittarius

  25. Slides 1-12 1.The milky Way seen over the Alps during a lunar eclipse. 2.&3.the Andromeda Galaxy-M31.It lies 2.5Mly away and is orbitted by two small elliptical galaxies M32 and M110.It will collide with the Milky Way in about 6 billion years. 4.Sombrero Galaxy about 50 Mly away in Virgo.We see it edge on.We can see the large central bulge and the distinct dust lanes in the edge.This obscures stars behind them but is a region of star formation and there are many bright stars.In the bulge there are many globular clusters. 5.M74-a photogenic spiral galaxy in Pisces.It is about 30 Mly away and has about 1011 stars. This is more or less how the Milky Way must look from outside.Picture taken with the Gemini North telescope on Mauna Keau. 6.Helix Nebula-the end of a Sun-like star.The It is a planetary nebula with a central White Dwarf Star.Radiation from this star is fluorescing the clouds of H,O and N thrown off in the final unstable stages of the star’s life. Slide which follows shows some detail. 7-11.Crab Nebula-In 1054 AD the chinese recorded a bright new star seen in daylight for quite a long time.It was a Supernova and we see the remnants in these pictures.The first four show it as seen in four different regions of the electromagnetic spectrum[UV,visible,IR,X-ray].The final picture shows detail.The bright clumps of light are moving outwards at v = 0.5c.The filaments have lower mass and higher velocity than models would predict.There is a pulsar = neutron star at the centre,the remnant of the original star.It rotates at 30 times per sec. 12.Coma cluster-One of the most dense galactic clusters known with about 10,000 galaxies. Each of these galaxies contains more than 1 billion stars.This is a regular cluster about 270 Mly awayand 20 Mly across.Most of the galaxies are elliptical.Two giant ellipticals dominate in the centre of picture and we see a star local to us top right.

  26. Slides 13-17 • 13.Hubble Deep Field-a narrow region of the sky viewed by the HST 4 days.We see more and more galaxies no matter how far we go back in time. 14.M10 globular cluster with a few hundred thousand stars.It is 16,000ly away in Ophiucum.Diameter is 70 ly.Stars are highly evolved and are mainly Red and Blue Giants.These are post-Main Sequence stars. 15.Aurora seen over Edmonton on 4/11/2003.Clover Bar Power station photographed by auroral light from North Saskatchewan River.Small pond reflects green auroral light. 16.Isabel Terra-a hurricane,almost category five,passing east of the Bahamas.Huge swirling storms,called typhoons in East,get their energy from warm evaporated ocean water.As water vapour vapour cools and condenses it heats air,lowers pressure and causes cooler air to rush in.Winds can be up to 250 km per hour. 17.London at night from orbit.The M25,Heathrow and Gatwick are clearly visible.

  27. Slides 18-22 • 18.NGC6240-I optical and X-ray regions. 19.SN1987A-A supernova in the Large Magellanic Cloud in October 1987.We see before and after.This marks the death of a massive star. 20.Similar picture of another ,much more distant supernova. 21.Eagle Nebula.This region of star formation is 7000ly away. Evaporating gaseous globules emerging from pillars of H and dust. In interior of pillars gas is contracting to form new stars.At the ends of the pillars intense radiation from young stars boils away low density material. 22.Lagoon Nebula[M8].5000ly away,100ly across.It can be seen in Sagittarius with the naked eye.In the detail shown with HST we see two funnel shaped regions where stars are forming.The vast walls of dust hide other hot young stars.

  28. The nature of Astronomy • Astronomy relies almost entirely on observation. • We rely on radiation and particles emitted by astronomical objects. • We can measure radiation intensities, spectra etc - We deduce compositions of stars temperatures of their surfaces total luminosities atmospheres of planets etc. - We can also obtain information on material between us and the emitting object (gas and dust) from absorption and scattering.  How does Astronomy work? Observations Models of processes Predictions Test by further observations  To do this we must make a series of assumptions—next slide

  29. Assumptions  The Laws of Physics apply. Even although the conditions in the astrophysical objects may be well beyond anything we can explore on Earth.  In particular we assume the Laws are invariant w.r.t. place and time. Caveats • The evidence to support these assumptions is very limited. • It is also of quite different quality for each of the four forces we know. • There may also be forces or other Laws of which we are unaware. Then our models would be quite inadequate. Astrophysics is an application of Physics - we must understand Physics.

  30. Electromagnetic Wave-Schematic picture • Here we see a plane wave propagating in vacuum in the z-direction with velocity v = c where c = 3 x 10 8 m/sec. • The wave is linearly polarised and the E field oscillates along the y-axis. • The magnetic field(B) is along the x-axis and is in phase with the E field • E and B are always in the same proportion • , and c depend on mode of production and medium • In medium it can be scattered, reflected,refracted and slowed down.

  31. Consequences of the Finite Velocity of Light • Velocity c =  with c = 3 x 108 ms-1in vacuum. Typically visible = c/vis = 3 x 108 / 500 x 10-9 = 6 x 1014 Hz Velocity is reduced by  1% in gas and tens of percent in solids • Finite velocity means time delay. Thus Time(Sun-Earth) = 1.496 x 1011/ 3 x 108 = 0.5 x 103  8 mins. Time(Moon-Earth) = 4 x 108 / 3 x 108  1.3 s • From Alpha Centauri  4 years From nearby galaxies  105 - 106 years Across a “typical” galaxy  105 years •This leads to the definition of the LIGHT YEAR (ly) as the distance travelled by light in vacuum in 1 year. 1 ly = 3 x 108 x 3.15 x 107 m = 9.45 x 1012 km  1013 km = 9.461 x 1012 km

  32. The age of the Earth is  5 x 109 years So if we observe a galaxy 1010 ly away the light was emitted before the Earth was formed. ANALOGY (commonly quoted in textbooks) If the Universe began at midnight. Earth formed in mid-afternoon. Plants began to produce oxygen in early evening. Humans began 2 mins. from Midnight Magellan circumnavigated globe 0.003 secs. from Midnight Each of us lives for < 0.001 secs.

  33. Electromagnetic Spectrum-Radio waves to gamma rays. Shown as a function of wavelength() and frequency(). All such waves have velocity c = .

  34. The Story so far The Nature of Astronomy - based on Observation not experiment - very different from Physics generally - relies on assumptions, particularly that the laws of Physics are invariant in space and time and can be applied in the very different conditions which may prevail in the astronomical objects we observe. To proceed we need to be reminded of some of the simple physics we will use to explain what we see.

  35. The Electromagnetic Spectrum • The figure shows the electromagnetic spectrum over all wavelengths. • EM radiation exists at all wavelengths and has the same basic properties.In particular they always propagate with velocity v = c in vacuum and can be refracted, reflected, scattered etc • The various parts of the spectrum are named by the method of production and not by energy or wavelength. For example gamma rays arise from transitions between levels in atomic nuclei.

  36. Blackbody Radiation • General question:-What is the spectrum of EM radiation emitted by an object of arbitrary temperature T in thermal equilibrum. We assume that this “blackbody” reflects no radiation at any  •Max Planck showed that the spectrum is given by ud  = 8hc -5.d [exp(hc/kT) - 1] where ud  is the energy density =energy/unit volume • Although no perfect blackbody exists solids and stars follow Planck’s Law very closely.Note that picture is on log-log scale.

  37. Blackbody Radiation Sun-Yellow Red • Spectrum of Blackbody Radiation as a function of wavelength. • Energy emitted by four blackbodies with equal surface areas. • Note that they are plotted on log-log scale. • Area is proportional to total power per unit surface area (PA) • Stefan-Boltzmann Law- PA = .T4 • PA is in Wm-2 and  = 5.67 x 10 -8Wm-2 K-4

  38. Wien’s Displacement Law • Doubling T increases P by 16 since PA = .T4 Luminosity = Surface area x PA • Note that maximum wavelength max shifts with .This can be quantified in Wien’s Displacement Law. max.T = const. = 2.9 x 10-3 mK • This quantifies the observation that an object changes colour with Temperature e.g.At room temp. spectrum peaks in infra-red. • Very important since it allows us to obtain a measure of the SURFACE TEMPERATURE of a star from max.For the Sun max is in blue with but a lot of radiation in red so it looks yellow.For stars with T = 3000k max is in infrared but significant amount in red.Red Giants are at this T.

  39. Betelgeuse Bellatrix Orion nebula Alnitak, Alnilam and Mintaka Rigel Saiph

  40. Photons • So far everything I have said assumes that EM Radiation is a wave. • Planck’s Law was deduced assuming it is emitted by oscillators with discrete energies. Einstein introduced the idea that it consists of particles called PHOTONS each with E = h = hc/ where h = 6.6 x 10-34 Js is Planck’s constant h is a very small number so number of visible photons needed for us to see is very large.

  41. Peak of blackbody spectrum gives surface temperature only. Photons emitted in the centre are scattered and absorbed before they go very far. They heat the layers outside them. The surface is heated by conduction and convection of the gas. Sun appears to be a blackbody at 5850 K. In centre it is  5 x 107 K

  42. Stellar Spectra and Kirchhoff’s Laws • Late 18th/early 19th C William Wollaston and others saw dark lines imposed on Sun’s blackbody spectrum. Light was absorbed at these s. • 1814-Fraunhofer had catalogued 500 lines and noted Na line. • Kirchhoff showed that the dark lines are due to absorption of light at that  by atoms of a particular element.His results are summarised in Kirchhoff’s Laws:- 1)Hot, dense gas or solid emits a continuous spectrum. 2)A hot,diffuse gas produces bright spectral lines. 3)A cool,diffuse gas in front of a source with a cts. Spectrum produces dark spectral lines. • Full explanation had to wait for the Bohr-Rutherford theory of the atom.

  43. Summary of Kirchhoff’s Laws in pictorial form

  44. Stellar Spectra – A digression • Main features of the Bohr-Rutherford Model - Central atomic nucleus containing Z protons and N neutrons - A = N + Z - Neutral atom has Z electrons - Electrons are held in place by electrostatic force • - Two main assumptions namely 1) The only orbits are those where the electron’s angular momentum is an integral multiple of h/2 (h = Planck’s constant ) = n h/2 2) Electrons emit no radiation so long as they remain in an allowed orbit. Radiation is emitted or absorbed when an electron makes a transition from a higher(lower) to a lower(higher) state. - States are characterised by Quantum numbers n,l where n is the Principal quantum number and l is the orbital ang. Mom. Quantum no.

  45. Stellar Spectra - 2 • Main results from Bohr-Rutherford Model -The radii of the orbits are given by R = 0.h2.n2,where me is the .me.Ze2 is the electron mass and 0 is the electrical permittivity of free space. -The energies of the orbits are given by En = - me.e4.Z2 80.h2.n2 • In this simple model n dictates the level energies hence Principal Q.N. -For H(Z = 1) we find that En = - 13.6/ n2 eV -Note that the zero of energy is at infinity. • Note:-the elements in increasing Z are Helium(He),Lithium(Li), Beryillium(Be),Boron(B),Carbon(C),Nitrogen(N),Oxygen(O), Fluorine(F),Neon(Ne),Sodium(Na),Magnesium(Mg),Aluminium(Al), Silicon(Si),Phosphorus(P),Sulphur(S),Chlorine(Cl),Argon(Ar), Potassium(K),Calcium(Ca),Scandium(Sc),------------------------------

  46. Stellar Spectra-3 • Pauli Principle:-No two electrons can have the same quantum numbers. As a result each level can hold two electrons.The Periodic Table is built up by placing successive electrons in levels two at a time.Noble gases represent atoms with last electron particularly tightly bound. • If we think of light in terms of photons then E = h.Photons emitted or absorbed in atoms have discrete energies given by h = me.e4.Z2[1/n12 - 1/n22] 802.h2.c from our expression En = - me.e4.Z2for the level energies. 80.h2.n2 • In general we see series of spectral lines with 1/ = R[1/m2 - 1/n2],where R is a constant and m,n are integers. In H, Balmer series has m = 2,n = 3,4,5---- Paschen series has m = 3,n = 4,5,6,------

  47. Levels and Transitions in Hydrogen The figure shows the levels in the hydrogen atom. The zero of energy is at infinity. The levels are labelled by the Principal Quantum Number n and by the energy[on the left] The series of spectral lines that were found empirically by various experimenters are shown. Each series ends on a particular level.

  48. Stellar spectra-4 Here we see the atomic spectra for white light,sunlight and a series of elements. Note that the last spectra are for Na in emission and absorption. These spectra provide clear fingerprints for the chemical elements.

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