Download Galaxies

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
Ellipticals
E0 through E9
E0 more spherical,
E9 more elliptical (cigar)
Galaxy Types
• Spirals (Disks)
• Significant star formation, gas, dust
Spiral Galaxies
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
Barred Galaxies
• Barred Spirals
Barred Galaxies
SBa, 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
Milky Way Map
Our view from within our own galaxy home.
Hydrogen 21-cm Radio Image
The Galactic Center
Picture (c): Home of a supermassive black hole?
You Are Here
Disk and Halo Stars
Population I and II Stars
• Population II Stars (Halo objects):
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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):
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Younger stars (solar-type)
“> Second” generation
Metal rich chemical composition (Formed from enriched ISM)
Small space velocities relative to the sun
Low inclination orbits
Galaxy Formation
Population II stars:
formed first
spherically distributed
globular clusters
Population I stars:
formed later
disk distribution
open clusters
Galaxy Schematic
Solar Galactic Orbit
Spiral Arm Structure
Spiral structure
viewed as the
precession of
elliptical galactic
orbits creating
density waves.
Spiral Density Waves
Galaxy Rotation
Invariably, 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: P2 ~ 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
Dark Matter
• 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
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
V r = H0D
Vr = radial velocity (a.k.a. redshift,doppler shift)
D = Distance
H0 = Constant (a.k.a. Hubble constant)
Vr
D
Hubble Flow
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Galaxies A, B, and C separated by distance d
After some time (Dt) 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/Dt,
and B from C from A is v = 2d/Dt
• The farther away you are, the faster you appear to recede!
Local Group Galaxies
Hubble Law
Galaxy Clusters
Hubble Constant
V r = 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?