1. HUBBLE DEEP FIELD:

2. GALAXY STRUCTURE Disks Bulges
prominent feature in spiral galaxies (like Milky Way)
stars in nearly circular orbits
Analogies: Solar System, accretion disk
Bulges
Coexist with disks in spirals (central bulge, halo)
Ellipticals: whole galaxy is bulge
Stars in random orbits
Analogy: stellar structure (random motions act like pressure)

3. DISK GALAXIES Note bulge “Sombrero” Galaxy M100 (Hubble)
M33, a member of
the Local Group

4. BULGES Amazing fact! Each little dot near M87
is an entire globular cluster like M80!
M80, a globular cluster (~ 106 stars)
within the Milky Way (Hubble)
M87, an enormous
elliptical galaxy

5. GALAXY CONTENTS Dark Matter (~80% of halo, smaller proportion of disk)
Stars
large galaxy : 100 billion stars (range )
“collisionless”, interact by gravitational forces
Gas
>10% of star mass
“dissipative”, tends to form clouds, sink to center
can fall in from outside or escape in wind
Dust
~1% of gas mass, follows gas motion
obscuration

8. PROCESSES VERY UNCERTAIN
GALACTIC NUCLEI
Star concentration increases toward center of galaxy
Near Solar System: stars separated by ~5 light-yr
Near Galactic Center: “ “ “ ~ 0.02 light-yr
Star collisions (mergers?) possible (esp. red giants)
Gas accumulates
formation of new stars?
formation of dense (molecular) clouds, accretion disk
drag on existing stars ...
PROCESSES VERY UNCERTAIN

9. ROUTES TO BLACK HOLE FORMATION in galactic nuclei

10. MILKY WAY - 2.6 million Suns
From Ghez et al. 1998
Infrared image of the
Galaxy’s Center
(Genzel et al.)

11. THE MILKY WAY’S NUCLEUS
0.015 ly
15 yr orbit
Prof. Andrea Ghez,
UCLA