Transcript The extragalactic universe and distance measurements

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)