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The extragalactic universe and distance measurements • Discovery of the extragalactic universe • The cosmic distance ladder. X-RAY IR Gamma Ray The Milky Way Credit & Copyright Barney Magrath Credit: Dave Palmer I: THE MILKY WAY AND THE MYSTERIOUS NEBULAE • Galileo – Turned his telescope on the “Milky Way” – a bright band of light stretching across the sky. – Discovered that the Milky Way was made of millions of stars • Thomas Wright (1750) – Suggested that solar system was embedded within an enormous shell of stars – looking through shell gives “band” appearance of Milky Way • Immanuel Kant (1755) – Realized that the Milky Way is a disk of stars containing the solar system – a galaxy – Kant suggested that there were other galaxies like the Milky Way – He supposed that these galaxies cluster in groups of ever increasing scale, filling all of space. – But, where were these other galaxies? • Nebulae – fussy blobs in sky – Most people thought these were patches of glowing gas situated in between the stars • Messier (1780) – Compiled a catalogue of nebulae – Intended as aid to comet hunters (so that they could reject nebulae as uninteresting) Messier actually cared about comets Credit : A. Dimai The Orion Nebula (M42) THE GLOBULAR CLUSTER M13 Credit: Yuugi Kitahara Andromeda “Nebula” (M31) • Herschel (1785) – Extended Messier’s list of nebula – Tried to determine distribution of stars in Milky Way – described Milky Way as “detached nebula”, with Sun near center. – Thought that the nebulae could be similar systems – Turns out that his conclusions were heavily effected by dust in the Milky Way – Milky Way is much bigger and better ordered than he thought. • Lord Rosse (1845) – Observed that some of these nebulae had spiral structure, like Milky Way – Supported Kant’s idea that these spiral nebulae were external galaxies. Modern all sky image A misleading event… • The Andromeda (M31) Nova of 1885 – “New star” appear in M31 – Astronomers knew about similar phenomena in the Milky Way (novae) – They scaled the brightness to get distance of M31 – Found that M31 must be within the Milky Way disk – [They were wrong! The M31 event was actually a supernova and was much more powerful than they assumed.] Slipher & Curtis • Slipher (1912) – Measured velocities of these nebulae (by looking at redshifts) – Found that many of them were moving faster than MW escape velocity (1000-2000 km/s). • Curtis (1917) – Found much fainter novae in other spirals – Discounted Andromeda nova as being strange, and concluded that the spiral nebulae were at great distance. Shapley and the great debate • Shapley – Tried to measure size of the Milky Way from globular cluster distribution – He concluded that Milky Way was huge (100kpc), with Sun near one edge. – Thought that all nebulae were merely satellites of the gigantic Milky Way • 1920 – Issue formally debated at the National Academy of Sciences in DC. – Harlow Shapley argued for “local hypothesis” (idea that nebulae were nearby). – Heber Curtis argued for “island universe” hypothesis. • Needed reliable distance measurements to resolve this issue. MEASURING DISTANCES PARALLAX Stellar parallax • Parallax: – Stars appear to wobble as the Earth moves around Sun. – Can use this to measure distance to stars (since EarthSun distance known well). – If star wobbles with amplitude of 1 arc-second (1/3600th of a degree – 1/2000th diameter of Moon), then it is at distance of 1 parsec (definition of parsec). – 1pc = 3.26 lt-yr – In general, D ( pc) 1 wobble(arcsec ) • Galaxy Size 30 kpc • Until 1990s, could only detect parallax out to 50pc. • Hipparcos satellite – Designed to measure parallax of stars – Can detect wobble out to distance of about 1kpc (1000pc) – Used to map out locations of nearby stars. • GAIA satellite – Due to launch 2010-2012 – Can map out positions and motions of stars across the whole galaxy!! Hipparcos (ESA) Beyond parallax… • Currently (i.e. before GAIA) we can’t detect parallax beyond 1kpc. • Need to use other distance indicators • A basic method uses apparent brightness… – Suppose you know the true luminosity (“power”) of an object – Can then use measurements of its apparent brightness to determine its distance. – In astronomical context, need to find “standard candles” – objects whose luminosity we know. Cepheid variables • Very important type of star for measuring distance. • Luminous variable stars – “Breath” in and out – Periodicities in range 3-30 days – Period and luminosity closely related! – Good standard candles • Have to calibrate the luminosity relation of Cepheids with parallax of nearby examples. From web site of Davison Soper (Univ. of Oregon) Edwin Hubble Hubble found Cepheids in M31 • Edwin Hubble – used 100-inch telescope on Mount Wilson – Found a Cepheid in Andromeda nebula – Proved that Andromeda was a whole galaxy completely separate from the Milky Way. – Firm evidence for the “island universes” hypothesis The Andromeda galaxy (M31) Cepheids in the Virgo galaxy cluster with Hubble Space Telescope (15x106 LY away…) The Virgo cluster z~l or v=Hl If H=100 km/s/Mpc at 3000 km/sec or z=.01 distance 30 Mpc. If H=50 km/sec/Mpc distance 60 Mpc Beyond the Cepheids… • Hubble could only find Cepheids in the closest few galaxies (1-2 Mpc). • Even Hubble Space Telescope cannot find Cepheids beyond the Virgo cluster (16 Mpc) • Beyond 1-2Mpc, Hubble used… – Brightest star method – identify the brightest “star” in the galaxy and assume that it is the same as the brightest star in nearby galaxies. BUT, brightest object may not be a star at all! – Overall galactic apparent brightness method – for distant galaxies, simply use overall brightness of galaxy to gauge distance. Tully-Fisher relation • Tully-Fisher relationship (spiral galaxies) – Correlation between • width of particular emission line of hydrogen(21 cm); width caused by rotation of galxy • Intrinsic luminosity of galaxy – So, you can measure distance by… • Measuring width of line in spectrum • Using TF relationship to work out intrinsic luminosity of galaxy • Compare with observed brightness to determine distance – Works out to about 200Mpc (then hydrogen line becomes too hard to measure)