Download 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.