Transcript Document
Cosmology and extragalactic astronomy Mat Page Mullard Space Science Lab, UCL 5. The cosmic distance ladder Slide 2 4. The cosmic distance ladder • This lecture: • Measuring distances to – things within the solar system – things within the solar neighbourhood – things within the galaxy – nearby galaxies – distant galaxies Slide 3 The problem • How big is the galaxy? – and the rest of the Universe? • How far away are galaxies – how luminous are they? • Fundamental problem Slide 4 Distance indicators • Measuring distances on Earth is easy. On galactic scales it’s a bit harder... • Hubble’s law relates distance to redshift, but how do you measure Hubbles constant? • There is no simple way to measure the distance to anywhere. Different indicators must be used for different distances • The ‘distance ladder' Slide 5 Short range measurements • The Moon – – – – Laser ranging. Used to be from Haleakala on Maui. Apollo landers left reflectors on surface! +-10cm several years ago. • The planets – Radar ranging (usually to Venus) – Use orbital periods and Kepler’s laws to infer distances to other planets. • The size of the Solar system is fundamental because... Slide 6 Parallax • Original method for measuring distances to moon and planets – two observers on opposite side of Earth. • For nearby stars, use opposite points of Earth’s orbit around the sun. – – – – Baseline of 2 AU Good to 30pc limited by atmospheric distortions Bessel, 1830s, 61 Cyg 0.3” Slide 7 Hipparcos • HIgh Precision PARallax COllecting Satellite • 1989-1993 • Good out to 500pc – When the catalogue was released, everything moved • New Satellite GAIA will do far more, out to the other side of the Galaxy • Gaia does other things besides Masers Slide 8 • Interstellar gas clouds emit intense microwaves at specific frequencies. • Doppler shift gives speed • True speed plus proper motion gives distance • Maybe 10s of Mpc but new technique Slide 9 Standard Candle Methods • Many methods in this class • Know absolute luminosity somehow – Apparent magnitude gives distance by inverse square law • RR Lyrae variables used like this to find our place in the Galaxy. • Distances that can be reached depend on the intrinsic brightness of the standard candle – and the limiting magnitude of your telescope Main Sequence Stars • For main sequence stars luminosity and colour are related • If you know the spectral type of a main sequence star, the HR diagram gives the luminosity • Use inverse square law to get distance • Good to 10s of kpc • Also known (confusingly) as spectroscopic parallax Slide 10 Slide 11 Cepheids • The most famous, canonical standard candle – Period luminosity relationship – know period know luminosity – Good to 30 Mpc • RR Lyrae similar but lower luminosity – (around L= 100 Lo) – good to 100 kpc Slide 12 Novae • Novae increase in brightness by several magnitudes rapidly (~1 day) and decay slowly. • Time to dim by 2 magnitudes is related to absolute magnitude. – Standard candle • Good to 300 Mpc Supernovae Slide 13 • Type 1a supernova has maximum absolute magnitude Mmax = -19.6 • Supernovae formed by white dwarfs accreting too much gas • Can go several Gpc Tulley-Fisher relation Slide 14 • In a spiral galaxy, rotational velocity tends to constant vmax at large radius • spread in velocities causes doppler broadening of spectral lines Dl/l = 2 vmax/c • Tulley, Fisher: vmax a L1/4 • For ellipticals motions are random, but spread in velocities s a L1/4 • Called the Faber-Jackson relation Slide 15 Brightest Galaxies: • Brightest ellipticals in rich clusters seem to have L = 1012 Lo • Bright so visible to 10 Gpc (pretty distant!) • But problems at large distances – Observational bias – Long time ago – age dependence? Slide 16 Overview of the Distance Ladder: Slide 17 Key points • Lots of ways to measure distance, but only really 3 classes: – direct (e.g. radar or laser ranging) – geometric - parallax – standard candles • Short distance measurements used to calibrate the long distance ones – Interdependency – Errors in calibration mount up. • Best indicators are those which are good over a wide range of distances.