Galaxies• Read Your Textbook: Foundations of Astronomy

– Chapter 16, 17

• Homework Problems Chapter 16– Review Questions: 1, 2, 5-7, 10– Review Problems: 1, 5, 9, 10– Web Inquiries:

• Homework Problems Chapter 17– Review Questions: 2, 4, 7, 8-10– Review Problems: 1, 5, 9, 10– Web Inquiries:

Galaxy Types• Ellipticals (Triaxial ellipsoids)• Lacking significant star formation

E0 through E9

E0 more spherical,

E9 more elliptical (cigar)

Ellipticals

Galaxy Types• Spirals (Disks)• Significant star formation, gas, dust

S0 through S9

S0 spiral arms not well defined

S9 spiral arms very well defined

Spiral Galaxies

Sa, Sb, Sc

Sa tightly wound spiral arms

Sb less so

Sc barely wrapped spiral arms

Spiral Galaxies

• Barred Spirals

Barred Galaxies

Barred GalaxiesSBa, SBb, SBc

(Spiral Arm winding)

Barred Galaxies• Barred Spirals

Galaxy Types• Irregulars (includes those that are interacting)• Lots of star formation, gas and dust

Interacting• Interacting (Irregular)• Mergers

Hubble “Tuning Fork” Classification

Our view from within our own galaxy home.

Milky Way Map

Hydrogen 21-cm Radio Image

Picture (c): Home of a supermassive black hole?

The Galactic Center

You Are Here

Disk and Halo Stars

• Population II Stars (Halo objects): – Older stars

– “First or Second” generation

– Metal poor chemical composition (Anything else but H, He)

– Large space velocities relative to the sun

– High inclination orbits

• Population I Stars (Disk objects):– Younger stars (solar-type)

– “> Second” generation

– Metal rich chemical composition (Formed from enriched ISM)

– Small space velocities relative to the sun

– Low inclination orbits

Population I and II Stars

Population II stars:

formed first

spherically distributed

globular clusters

Population I stars:

formed later

disk distribution

open clusters

Galaxy Formation

Galaxy Schematic

Solar Galactic Orbit

Spiral structure

viewed as the

precession of

elliptical galactic

orbits creating

density waves.

Spiral Arm Structure

Spiral Density Waves

Galaxy RotationInvariably, it is found that the stellar rotational velocities remain

constant, or "flat", with increasing distance away from the

galactic center.

This result is highly counterintuitive since, based on

Newton's law of gravity, the rotational velocity would steadily decrease

for stars further away from the galactic center. Analogously, inner

planets within the Solar System travel more quickly about the Sun than

do the outer planets (e.g. the Earth travels around the sun at about

100,000 km/hr while Saturn, which is further out, travels at only one

third this speed). Kepler’s Third Law: P 2 ~ a3

One way to speed up the outer planets would be to

add more mass to the solar system, between the planets.

Galactic Rotation Curve

Galaxy Rotation Curves

• There is a LOT of non-luminous matter.

• Gravitationally, our observations show that the universe is almost 90% non-luminous matter!

• 90% of the universe is made up of stuff we can not see!

• Solar System Mass to Light Ratio ~ 1– The Sun has 99.85% of the mass and 100% of the light

• This changes to ~100 on galactic and extra-galactic scales– There is a LOT of mass inferred by gravity, but not much light

Dark Matter

• What is this stuff? Normal Stuff (protons, neutrons)– very small faint objects

• brown dwarfs (large “jupiter” planets, star duds)• gas and dust

– stellar remnants • white dwarfs• neutron stars• black holes

– known or exotic yet undiscovered particles with mass• nutrinos

Dark Matter

Galaxy Red Shifts

“Redshift”• Hubble wanted to know the distance to the faint nebulosities

• He took galaxy spectra to obtain their radial velocities.

• Armed with distance determinations to local galaxies utilizing pulsating Cepheid stars, Hubble discovered that galaxies were in general moving away from the Milky Way.

• The recessional velocity of galaxies increases with increasing distance. (Redshift-Distance relation, a.k.a. the Hubble Law)

Hubble’s Redshift-Distance Relation

Hubble Law

Vr = H0D

Vr = radial velocity (a.k.a. redshift,doppler shift)

D = Distance

H0 = Constant (a.k.a. Hubble constant)

Vr

D

Hubble Flow

• Galaxies A, B, and C separated by distance d• After some time (t) this distance has doubled (2d)• Distance between A,B and B,C is now 2d, and A,C is 4d• Recession velocity of B from A is v = d/t,

and B from C from A is v = 2d/t• The farther away you are, the faster you appear to recede!

Local Group Galaxies

Hubble Law

Galaxy Clusters

Hubble Constant

Vr = H0D

H0 = Hubble constant

= 60-80 km/s/Mpc

Units for 1/H0 = seconds

This is an age estimate for our universe.

The true age should be less than this

maximum age estimate because matter has

surely caused some amount of deceleration.

Age Estimation

H0 = 60-80 km/s/Mpc

1/H0 ago, all matter was piled up onto each other

if there has been no acceleration or deceleration.

1/H0 ~ 10-20 Billion years (Gigayears)

H = H(t)Has expansion remained constant throughout time?

Has expansion been slowing down due to the matter in the

universe gravitationally attracting?

OR

Is the universe accelerating?

Therefore, H0 = H(tnow)

The Hubble constant is not a fundamental constant,

it is variable with time.

How Far?