The extragalactic universe and distance measurements

• Discovery of the extragalactic universe

• The cosmic distance ladder.

X-RAY

Gamma Ray

IR

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 DISTANCESPARALLAX

Stellar parallax

• Parallax:– Stars appear to

wobble as the Earth moves around Sun.

– Can use this to measure distance to stars (since Earth-Sun 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,

)arcsec(

1)pc(

wobble

D

• 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=.01distance 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)