What does our galaxy look like?
Jan 18, 2016
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