Astrophysics & cosmology

1. Astrophysics & cosmology

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’

3. Widening horizons space local .. global .. solar system .. galaxy .. expanding universe time personal .. Historical .. Geological .. cosmic a sense of scale http://www.numbersleuth.org/universe/

4. A brief history 134 BC - Hipparcus catalogues 850 stars 1534 - Copernicus describes a heliocentric system 1687 - Newton’s law of universal gravitation 1786 - Herschel: nebulae (blurry, light patches in night sky) 1783 - Mitchell proposes ‘black hole’ (light particles feel gravity) 1823 - Olber’s paradox 1910/11 - observing & cataloguing many stars leads to the H-R diagram 1915 - Einstein: general theory of relativity 1920 - Eddington: hydrogen fusion in stars 1924 - Hubble: ‘island universes’ (galaxies) 1929 - Hubble’s ‘law’: red shift and cosmic expansion 1935-39 - Bethe & von Weizsäcker: proton–proton fusion chain in low-mass stars, carbon–nitrogen–oxygen cycle in high-mass stars 1937 - Zwicky: galaxy clusters act as gravitational lenses =>‘dark matter’

5. brief history continues … 1950 - Fred Hoyle (‘steady state’ theorist) derisively coins the term ‘Big Bang’ 1957 - Margaret & Geoffrey Burbidge, Fowler & Hoyle: stellar nucleosynthesis can explain the observed abundances of all heavy elements from lithium up to iron. 1964 - 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. 1998 - Perlmutter: 1A supernovae red-shift less than expected. expansion previously slower, i.e. accelerating - ‘dark energy’ 2003 - WMAP image of cosmic microwave background radiation (CMBR) => age of the universe is 13.73 ± 1% billion yr 2013 Planck, CMBR: age of the Universe is 13.798 ± 0.037 billion yr

6. Astronomical wonders http://htwins.net/scale2/ 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.” http://www.atlasoftheuniverse.com/index.html

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

9. Made of stardust

10. Key evidence • Modern astronomy gathers radiation across whole em spectrum. Starlightreveals a star’s temperature, luminosity, composition, mass, distance and motion. • Looking out is looking at history. Moon as it was 1.1991888s 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 thebig bang

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 star A ladder of distances • parallax • luminosity & inverse square law (colour gives luminosity) • Cepheid variables (periodic time gives luminosity) Units of distance: light-year, parsec, Mpc

13. Hertzsprung-Russell diagram stellar populations: • 200 000 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

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

15. Protostar to main sequence How a protostar reaches the main sequence depends on its mass.

16. After the main sequence

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.

20. Redshift Spectra of all galaxies, except Local Group, are shifted towards red. Change inwavelengths indicates speed of recession.

21. Hubble law gradient of this graph is the ‘Hubble constant’, H

22. Expanding universe If the rate of expansion is constant, then age of the Universe can be calculated from distance = velocity x time

24. From plasma to gas ~400 000 years after Big Bang

25. Last scattering surface

27. Universe at large scale simulation observation what’s this?

28. Universe: mass - energy WMAP: 2003-2012 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 IoP DVD Teaching Astronomy and Space National schools observatory David Sang (ed, 2011) Teaching secondary physics ASE / Hodder www.zooniverse.org & www.zooteach.org http://www.phdcomics.com/tv/#020

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