1616

GalaxiesGalaxies

Island Universes

1616

Hubble Tuning Fork

1616

Ellipticals

1616

Ellipticals

• Huge• No gas.• No dust.• No young

stars.• Nothing but

old stars.– Random

orbits.

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Spirals

• Like Milky Way.• Disks and

bulge.• Young stars

and old.• Gas and dust.• Stars forming.• Stars dying. M81 and M82 – Copyright R. Gendler

1616M63 Copyright – S. Miyazaki, Suburu

1616NGC1365 Copyright – VLT

1616M33 – Copyright NOAO

1616Sombrero Galaxy – Copyright P. Barthel VLT

M10 – copyright Credner and Kohle

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Irregulars

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Distance to Near Galaxies

• Need a standard candle.• If there is something in a galaxy which

has a known luminosity we can determine a distance.

• Several candidates:– Variable stars– Supernovae– “Brightest” stars

1616

Nearby Galaxies

• Cepheids• Period• Luminosity

Mv

• Know mv

• Get Distance

10pc

distancelog5 10Mm

1616

• For Cepheid in M100• P = 20 days.• From P-L: L = 10000 x

Sun

• Msun = 5, so MCep = -5

• m = 20• m – M = 25• So 25/5 = 5 = log(d/10pc)• How log works:

– What is 100 = 10x?– Same as saying 2 = log(100)

• So 5 = log(d/10pc)• d/10pc = 100000• D = 1,000,000 pc

10pc

distancelog5 10Mm

How it works

1616

The Local Group

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Groups

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The Virgo Cluster

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Clusters

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Distant Galaxies

• Can’t see individual stars.• Supernovae rare.• Can use nearby galaxies to get

distances to further galaxies.• Distance ladder:

– Parallax nearby stars– Nearby stars H-R diagram– H-R diagram distant stars (variables)– Variable stars nearby galaxies– Nearby galaxies Hubble’s Law

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21cm Radiation

• Neutral hydrogen (HI) gives off light, = 21cm.

Milky Way HI emission – Copyright J. Dickey

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Extragalactic HI

• Observe HI in other galaxies.• Measure wavelength of 21 cm radiation.• Doppler Shift: Get velocity away from us.

1616

Hubble’s Law

• Measure the velocity of every galaxy.

• Nearly all are redshifted.

• Use Cepheids to measure distances to nearby galaxies.

• Result: The faster it’s moving, the farther away it is.DH v

distance velocity

o

Ho = 71 km/s/Mpc

1616

Map the Universe• v = HoD

• If you know Ho: 71 km/s/Mpc

• Measure v• Get D• Find:

VoidsWallsClusters

1616

140 Mpc

70 Mpc

1616

Concept Test

• Imagine that Cepheid variables were more luminous than previously thought. As a result, Hubble’s constant would be:a. Smaller than previously thought.b. Larger than previously thought.c. Unchanged since we aren’t changing

either the velocity or position of the galaxy.

d. None of the above.

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Limits to Hubble’s Law

• Negative velocity?• Galaxy pairs?• Clusters?• Orbits?

1616

Homework #16Homework #16

• For Friday Read: Bennett Ch 22.1 – 22.4

• Do Ch22:– Problems: 9, 12