Galaxies

Chapter 16

Topics

• Types of galaxies

• Dark Matter

• Distances to galaxies

• Speed of galaxies

• Expansion of the universe and Hubble’s law

You’re one in a million(or billion, or trillion)!

• Our Sun is part of a group of stars (galaxy) called the Milky Way.

• Are we the only galaxy?• As of the 1920s, the answer was “yes”• Now, the answer is an emphatic “no”• A Hubble Deep Field image of only one 30

millionth of the sky showed thousands of visible galaxies (that means that an image of the entire sky, if possible, would shown hundreds of billions of galaxies).

Types of Galaxieselliptical, spiral (normal and barred), lenticular, irregular

Elliptical

• little dust and gas• composed mainly of

older stars• little rotation

Spiral• bulge in the middle

• halo, disc, arms

• old stars dominate the bulge

• young stars dominate the arms

Barred Spiralarms unwind from a bar of stars

Lenticular

• disk (like a spiral)• no arms (like an

elliptical)• little gas and dust

(like an elliptical)• old stars

Irregular• Probably a result

of gravitational interaction of a galaxy with another galaxy

• Small and Large Magellanic Clouds orbit the Milky Way

Colliding Galaxies

Clusters and Superclusters

• Galaxies are found in binaries or larger groups

• Milky Way has two companions, the LMC and SMC

• Andromeda has two satellite galaxies

• Virgo cluster contains about 1,000 galaxies

Virgo Cluster

A few of the galaxies in the Virgo Cluster

Where’s the mass?• calculations of mass in galaxies do

not match the mass of observed stars and gas

• there is unobserved mass--dark matter

• what could it be?– planets?– brown dwarfs?– faint white dwarfs?– neutron stars?– neutrinos?– undiscovered particles?

• one of the great mysteries of astronomy

Measuring the distance to galaxies

• Cepheid stars have variable brightness– Period depends on luminosity; the larger the period, the

greater the luminosity– Use luminosity and measured brightness to determine

distance– Edwin Hubble, in 1924, determined that these spiral nebulae

were indeed distant galaxies

• Peak brightness of Type Ia supernovae– assume that peak luminosity is the same for all Type Ia

supernovae– use supernovae in galaxies of known distance to calculate

peak luminosity

The universe is expanding!

• Measure distance to galaxies• Measure recessional speed of galaxies

– by analyzing the doppler shift of emission spectra from the galaxies– spectra are redshifted - galaxies are moving away from us!

• Galaxies at greater distances are moving faster– Hubble law

• By determining the Hubble constant, we can calculate approximate distances to the most distant galaxies.

• Determining the Hubble constant is the hard part due to uncertainty in distance data– between 50 and 80 km/s/Mpc