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Announcements

Announcements. No office hours with Prof. Steinmetz. Lecture 9 Background: Measuring Distances in Astronomy II. Standard Rulers: Size = Distance x q (angle on sky) Need to know the real size of the object. Standard Candles: L apparent = L absolute / D 2

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Announcements

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  1. Announcements • No office hours with Prof. Steinmetz

  2. Lecture 9Background: Measuring Distances in Astronomy II

  3. Standard Rulers: Size = Distance x q(angle on sky) Need to know the real size of the object Standard Candles: Lapparent = Labsolute / D2 Need to know the true luminosity of an object Three Types of Distance Measurement Direct Measurements: Measuring the physical distance to an object directly

  4. Direct Measurements Important • Light Travel Time: Measure the time taken for a radar pulse to bounce off of an object or a signal to arrive from a spacecraft • Parallax: Stars appear to wobble when observed from different directions; the nearer the star, the larger the motion. Good to ~1 kpc

  5. Standard Rulers • Expanding Photosphere Method (EPS or Baade-Wesselink) • Type II supernova explosions • Measure speed of expansion of debris and time since explosion Þ real size of nebula • Useful to distances of 10-100 Mpc

  6. Standard Rulers • Water Masers: Measure the proper motions and accelerations of water masers in the accretion disks of AGN to get actual orbital radius of masers and mass of central object. Only one measurement so far. • Gravitational lensing: Time delay of fluctuations in lensed object gives info on geometry. Depends on mass of lens and theoretical lensing model. Good to ~ 1 Gpc

  7. Standard Candles Important • Main Sequence fitting: Calibrate the luminosity of main sequence stars with nearby clusters with parallax distances and fit clusters farther out. Good to 10-100 kpc

  8. Standard Candles • Luminosity functions • Choose a type of object with a charcteristic distribution of absolute luminosities • Measure distribution of apparent luminosities in a distant galaxy • Scale to match true luminosities, get distance • Globular clusters and planetary nebulae good to ~50-100 Mpc

  9. Standard Candles Important • Cepheid and RR Lyrae variables • Pulsating stars which change in brightness with a characteristic period • Period is proportional to absolute luminosity • Common and bright (esp. Cepheids), thus visible in nearby galaxies • Good to ~20 Mpc

  10. Standard Candles • Surface brightness fluctuations • Distant objects appear smaller • More stars per pixel in a galaxy far, far away • Smoother light distrubution, less variation from pixel to pixel • Amplitude of fluctuations proportional to distance • Good to ~100 Mpc, z~0.01

  11. Courtesy John Tonry

  12. Standard Candles Important • Galaxy kinematics • Tully-Fisher relation: rotation speed of spiral galaxies proportional to mass of glaxy proportional to total luminosity • Dn-s, Fundamental Plane, Faber-Jackson relations: velocity dispersion and size of elliptical galaxies proportional to total luminosity • Good to ~500 Mpc, z~0.1

  13. Standard Candles Important • Type Ia supernovae • Exploding white dwarf star • Shape of light curve and dimming timescale give absolute luminosity • Extermely luminous so they can be observed at great distances • Good to ~1 Gpc, z~1

  14. Other Methods • Novae as standard candles: not very standardized, good only to ~20 Mpc • Sunyaev-Zel'dovich effect: good to ~1 Gpc, z~1 but model dependent, not well calibrated yet. Measure density of x-ray emitting gas in clusters with CMB, measure temperature independently, gives absolute x-ray luminosity.

  15. The Distance Ladder • Different techniques useful at different distances: use nearby standards to calibrate more distant ones where they overlap • Cepheids are a key step: many in the Milky Way and LMC, so distances are directly measurable by parallax or only a step away, yet bright enough to overlap many secondary distance indicators • Cepheids  luminosity functions, SBF, galaxy kinematics, SNIa

  16. The Distance Ladder • Gravitational lensing, Sunyaev-Zel'dovich effect, Expanding Photosphere Method provide independent checks of Cepheid based distance scale • Lensing and SZ effect potentially useful out to very large distances

  17. Announcements • No office hours with Prof. Steinmetz

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