1 Astrophysics & cosmology1

2 Cosmic onion A structural hierarchy:quarks & leptons, nuclei, atoms, molecules, macromolecules …. … biomes, Earth, solar system, Milky Way galaxy, Local Group of galaxies, Local Supercluster; filaments, walls & voids studying the very large & very small: ‘look but don’t touch’ Structures known to science exist as every length scale. Can you put them into order, from smallest (quarks & leptons) to largest (universe)?

3 Widening horizons spacelocal .. global .. solar system .. galaxy .. expanding universe time personal .. Historical .. Geological .. cosmic a sense of scale

4 A brief history 134 BC - Hipparcus catalogues 850 starsCopernicus describes a heliocentric system Newton’s law of universal gravitation Herschel: nebulae (blurry, light patches in night sky) Mitchell proposes ‘black hole’ (light particles feel gravity) Olber’s paradox 1910/11 - observing & cataloguing many stars leads to the H-R diagram Einstein: general theory of relativity Eddington: hydrogen fusion in stars Hubble: ‘island universes’ (galaxies) Hubble’s ‘law’: red shift and cosmic expansion Bethe & von Weizsäcker: proton–proton fusion chain in low-mass stars, carbon–nitrogen–oxygen cycle in high-mass stars Zwicky: galaxy clusters act as gravitational lenses =>‘dark matter’

5 brief history continues …Fred Hoyle (‘steady state’ theorist) derisively coins the term ‘Big Bang’ Margaret & Geoffrey Burbidge, Fowler & Hoyle: stellar nucleosynthesis can explain the observed abundances of all heavy elements from lithium up to iron. Penzias & Wilson discover cosmic microwave background (CMB) 1970s - Vera Rubin: galaxies rotate faster than visible matter can account for. ‘dark matter’ 1980 Alan Guth proposes early ‘inflationary’ period to account for ‘flatness problem’ & smoothness of CMB 1995 Hubble Ultra Deep Field shows the early Universe. Perlmutter: 1A supernovae red-shift less than expected. expansion previously slower, i.e. accelerating - ‘dark energy’ WMAP image of cosmic microwave background radiation (CMBR) => age of the universe is ± 1% billion yr 2013 Planck, CMBR: age of the Universe is ± billion yr

6 Astronomical wonders A universe 13.82 billion years old… with ~100 billion galaxies … each galaxy having ~100 billion stars “… big and old and, as a result, rare events happen all the time.” ‘as many stars as there are grains of sand on all the beaches of the world’. Show IOP DVD Teaching astronomy and space video ‘How Big is the Universe?’ (6 min). Mention ‘Powers of Ten’ Video -

7 Key ideas A few key ideas explain the nature of the universe and our place in it. What are they?

8 Key ideas gravity creates the large-scale structure of the universe – stars, galaxies, superclusters, filaments & voids local physics is everywhere the same the universe (spacetime & matter) began in a very hot & dense state – the ‘Big Bang’ we are made of stardust – all elements except H He, Li, Be, B were forged in stars All cosmological models embody the general theory of relativity: ‘Matter tells space-time how to curve, and curved space tells matter how to move’ (John Wheeler).

9 Made of stardust What elements are humans made from? Can you put them in order, from the most common element to least common?

10 Key evidence Modern astronomy gathers radiation across whole em spectrum. Starlight reveals a star’s temperature, luminosity, composition, mass, distance and motion. Looking out is looking at history. Moon as it was s ago. Sun as it was 8min 19s ago. Alpha Centauri 4.27 yr ago. Most distant galaxies 13 billion years ago. Telescopes are, in a sense, time machines. star and galaxy processes: populations with a range of ages suggest their evolution supercomputers enable dynamic modelling from basic physics principles (e.g. gravity, plasma physics, nuclear fusion) big bang: observed abundance of light elements, cosmic background radiation and the current observed expansion of the universe particle experiments can recreate conditions approaching those of the big bang Hubble telescope can see back 7 billion light-years. Radio telescopes see back to ‘last scattering surface’ i.e. 13 billion light-years.

11 Analysing starlight A great discovery: Planets and stars are made from the same kind of stuff as we find and study here on Earth.

12 Measuring distance to a starA ladder of distances parallax luminosity & inverse square law (colour gives luminosity) Cepheid variables (periodic time gives luminosity) Units of distance: light-year, parsec, Mpc ‘intrinsic brightness’ = luminosity; ‘observed brightness’= intensity at telescope. [Astronomers use ‘absolute magnitude’ and ‘apparent magnitude’] ??issue a data table with apparent magnitude of selection of celestial objects Hipparcos (early 1990’s): 118,218 stars charted with the highest precision (to 1 milliarcsec). The Tycho 2 Catalogue, completed in 2000 has 2.5m stars, and includes 99% of all stars down to magnitude 11, almost times fainter than the brightest star, Sirius. Hipparcos (ESA) website has downloadable ‘star globe’ for students to make.

13 Hertzsprung-Russell diagramstellar populations: stars catalogued by 1924 Guide Star Catalog, used by Hubble telescope: ~1 billion stars ESA animation of the H-R diagram 90% of stars are main sequence each star has a life story show IOP DVD Teaching astronomy and space, video ‘Life cycle of stars’ (5 min), then weblink to ESA animation

14 eso.org/public/archives/images/screen/eso0728c.jpg

15 Protostar to main sequenceHow a protostar reaches the main sequence depends on its mass. relate star mass to H-R descriptors luminosity and temperature

16 After the main sequenceoutreach.atnf.csiro.au/education/senior/astrophysics/images/stellarevolution/hrpostmainsuntrack.jpg

17 Astrophysics gravity nuclear fusion radiation pressure fusion:small nuclei merge into bigger ones. energy released is kinetic energy of fusion products. energetically favourable (exothermic) up to Fe-56.

18 On this H-R diagram, familiar stars are named. daviddarlingOn this H-R diagram, familiar stars are named. daviddarling.info/images/Hertzsprung-Russell_diagram.jpg

19 This kind of diagram is common in school-level textbooksThis kind of diagram is common in school-level textbooks. Explanations missing.

20 Redshift Spectra of all galaxies, except Local Group, are shifted towards red. Change in wavelengths indicates speed of recession. show IOP DVD Teaching astronomy and space, video of classroom demonstration ‘Redshift’ (2 min). z = (wavelength observed – wavelength emitted)/wavelength emitted

21 Hubble law gradient of this graph is the ‘Hubble constant’, HCLEA software enables students to plot this graph from data supplied: gradient of this graph is the ‘Hubble constant’, H

22 Expanding universe If the rate of expansionis constant, then age of the Universe can be calculated from distance = velocity x time Show this using physical model such as balloon or physio exercise band. Also show that, from the viewpoint of any galaxy, the more distant galaxies always recede faster. This can be done either using graph paper overlaid by a transparency, or using the Wolfram simulation ‘TheExpandingUniverse’. The concept of an expanding universe requires the existence of a ‘standard ruler’ of constant length.

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24 From plasma to gas ~ years after Big Bang

25 Last scattering surface

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27 Universe at large scalesimulation observation what’s this?

28 Universe: mass - energyWMAP: March 2013

29 Still unsolved how did galaxies form?stellar fusion of 12C (key element, both for life and for larger nuclei) not fully understood unseen matter needed to account for rate of rotation of galaxies, galaxies in collision, gravitational lensing - what is this ‘dark matter’? not yet observed in any laboratory what drives the accelerating expansion of the Universe, countering gravity? (‘dark energy’)

30 Support, references talkphysics.org National schools observatoryIoP DVD Teaching Astronomy and Space National schools observatory David Sang (ed, 2011) Teaching secondary physics ASE / Hodder &

31 Black hole a region of space from which nothing, not even light, can escape. Rockets need to achieve an ‘escape velocity’ to break away from the Earth’s field and travel to other planets ~11 km s-1. escape v for Moon ~ 2 kms-1 escape v for Sun ~ 620 kms-1 John Mitchell, in 1783: Like Newton, regarded light as a particle. Speed of light is ~500 times faster than Sun’s escape v. Therefore light would not be able to escape star with same density and mass > 500 MS. Today: Initial mass determines a star’s luminosity, size, life story (evolution, lifespan, and eventual fate). White dwarf can evolve into a black hole. Nuclei of many galaxies (including ours) contain a black hole.

32 Olber’s paradox How can the night sky be dark if the Universe is infinite and eternally static? animation