HUBBLE DEEP FIELD:

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)

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

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

GALAXY CONTENTS Dark Matter (~80% of halo, smaller proportion of disk) Stars large galaxy : 100 billion stars (range 108-1012) “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

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

ROUTES TO BLACK HOLE FORMATION in galactic nuclei

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

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