PH222 - Astrophysical Concepts PDF

PH222 - Astrophysical Concepts Aaron Golden Centre of Astronomy, School of Natural Sciences University of Galway 1 Gravity IV 2 Thus far... stars (and giant planets) 3 Time to expand our horizons......

PH222 - Astrophysical Concepts Aaron Golden Centre of Astronomy, School of Natural Sciences University of Galway 1 Gravity IV 2 Thus far... stars (and giant planets) 3 Time to expand our horizons... 4 M31 - the Andromeda Galaxy 5 M31 - to scale... 6 Galaxies before 1845... Charles Messier (1730-1817) started a catalog of 'fuzzy objects' which ensured people wouldn't mistake them for comets. This is a sketch of the 51st object - M51 - by John Herschel, obtained with a (home made) 0.46 m reflecting telescope. Looks like a point source (star) with some sort of abberation... 7 The HST/JWST of it's time... 8 Largest telescope in the world up until 1917 The breakthrough 'image'... This is the sketch of M51 made by William Parsons in 1850 using the Leviathen of Parsonstown (Birr). The resolution of the telescope and it's exceptional optics allowed Parsons to resolve fine structure in the overall morphology of M51 In other observations Parsons could clearly identify individual stars in the `spiral arms' of these structures The next question to ask was - how far away were these exotic `nebulae'...? 9 10 Galaxies... 11 12 Interlude 1 Cepheid Variables & Standard Candles Of the many type of variable stars known, the Cepheids are particularly important. These are large (4-20 M ) F-K class stars which undergo radial pulsation due to changes in the opacity of He in their photospheres. Helum in outer layers heats up ionizes. More ionised Helium, greater opacity outer layers expand Outer layers cools, Helium ionisation drops less opaque radiatively cools gravity pulls layers back Periods range from 1 - 100 days... 13 The Period-Luminosity Law Henrietta Swan Leavitt discovered a direct link between the luminosity and the period of Cepheid variables following an analysis of all such stars in the Large Magellenic Cloud. This was an epochal discovery as it allowed astronomers to derive distances to Cepheids - and their host galaxies.. The first standard candle to derive distances... 14 Distances, distances... 15 Anatomy of a Galaxy 16 Galactic Halos M31 is 770 , so = = 2.2 17 Interlude 2 Globular Clusters... Stable, tightly bound clusters of tens of thousands to millions of stars Intense gravitational attraction between the closely packed (from 10 to 300 lyrs in diameter) stars gives globular clusters their regular, spherical shape Extremely long-lived, surviving to be billions of years old Stars all formed at approximately the same time and location, and had similar initial composition - deep freeze for the early Universe 18 Back to galaxies... 19 20 Interlude 3 Galaxies, Galaxies everywhere... 21 How to make sense of them all?? Interlude 3 - Galaxy Phylogenetics Apply ideas from evolutionary and a common origin to reconstruct a phylogenetic tree for galaxy evolution... 22 Back to Rosse's observations that the 'nebulae' were made of stars Are they near or far? Huge implications based on this simple question... 23 The Great Debate Debate. 26 April 1920 Are the spiral nebula independent galaxies or small nebulae in the Milky Way halo? Heber Curtis (Allegheny Observatory) Harlow Shapely (Harvard College) 24 The Successor to the Leviathan in Birr Hooker 100" telescope at the Mount Wilson Observatory Completed in 1917 Used continuously up until 1997! Using the Hooker telescope, Edwin Hubble demonstrated the presence of Cepheid variables in these spiral nebula (1920) Using Leavitt's Law the spiral nebula are millions of pcs away from our own galaxy... 25 Hubble's study of galaxy redshifts... Hubble took spectra for 24 galaxies for which he had identified Cepheids (and so knew their distances) By studying the 'offset' in dominant absorption lines he identified a clear correlation between the Doppler shift velocity necessary to produce the 26 offset against the Cepheid derived distance. Expansion speed distance? 27 Caveat alert on... 28 Hubble's Law 29 Determining is still huge area of research... We'll see the relevance of this later 30 Whiteboard time... 31 Determine the distance to 3C273 in Mpc (z = 0.158) The first quasar - discovered in 1963 We're going to use two formula = and = The relativistic formula is 1+ = (1 - ), where = 32 Determine the distance to 3C273 in Mpc (z = 0.158) Start with = = = (0.158)(3.0 10 m/s) = 4.74 10 m/s = 4.74 10 km/s ( < 0.1 so ok to use non-relativistic case) But = = = = 6.77 10 Mpc = 677 Mpc 33 Wrap Up History of astronomy - this time the nature of the `nebula'... saw how new instrumentation new discoveries The idea of standard candles Cepheids & the Leavitt Law Galaxies 101 Hubble's Law & the expansion of the galaxies 34