What does our galaxy look like?

The Milky Way galaxy appears in our sky as a faint band of light

We see our galaxy edge-on

Primary features: disk, bulge, halo, globular clusters

Disk: Ionization nebulae, blue stars star formation

Halo: No ionization nebulae, no blue stars no star formation

If we could view the Milky Way from above the disk, we would see its spiral arms

How do stars orbit in our galaxy?

Stars in the disk all orbit in the same direction with a little up-and-down motion

Orbits of stars in the bulge and halo have random orientations

Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit:

1.0 x 1011 MSun

Much of star formation in disk happens in spiral arms

Whirlpool Galaxy

Ionization NebulaeBlue StarsGas Clouds

Spiral arms are waves of star formation

1.Gas clouds get squeezed as they move into spiral arms

2.Squeezing of clouds triggers star formation

3.Young stars flow out of spiral arms

Saturn’s Rings: Natural Color

15km/s

15km/s

< 1cm/s

Dark Matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence

Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate

Unseen Influences

Mass within Sun’s orbit:

1.0 x 1011 MSun

Total mass:

~1012 MSun

What is the evidence for dark matter in galaxies?

Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter

• “Normal” Matter: ~ 4.4%– Normal Matter inside stars: ~ 0.6%

– Normal Matter outside stars: ~ 3.8%

• Dark Matter: ~ 25%• Dark Energy ~ 71%

Contents of Universe

We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H

Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass

All three methods of measuring cluster mass indicate similar amounts of dark matter

Clusters contain large amounts of X-ray emitting hot gas

Temperature of hot gas (particle motions) tells us cluster mass:

85% dark matter 13% hot gas 2% stars

Our Options

1. Dark matter really exists, and we are observing the effects of its gravitational attraction

2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter

… not as bright as a star.

How dark is it?

• Ordinary Dark Matter (MACHOS)– Massive Compact Halo Objects:

dead or failed stars in halos of galaxies

• Extraordinary Dark Matter (WIMPS)– Weakly Interacting Massive Particles:

mysterious neutrino-like particles

Two Basic Options

Halo stars formed first as gravity caused cloud to contract

Remaining gas settled into spinning disk

Detailed studies: Halo stars formed in clumps that later merged

Our Galaxy

Solar systemGalaxy Center

Earth = 100 nm = virusSun = 10 μm = cellEarth orbit = ¼ cm = pin headSolar system = 20 cm = saucerNearest star = 250 m = lawn

Naked eyestars

EagleNebula

Jewel BoxCluster

CrabNebula

NGC 3310Spiral Galaxy

NGC 5194Spiral Galaxy

NGC 1365Barred Spiral Galaxy

Interactinggalaxies