Transcript Chpt14b

Chapter 14: The Milky Way Galaxy
The scenario for the formation
of galaxies is very reminiscent
of the formation process for
solar systems.
Giant cloud of gas and dust,
shrinks under its own gravity,
rotation produces a flattened
disk.
The halo objects probably
formed first and retained their
spherical distribution and
didn’t get pulled into the disk.
Chapter 14: The Milky Way Galaxy
Spectroscopic radio astronomy
allows astronomers to look
through our galaxy and see the
Doppler velocities of hydrogen
through its 21 cm spin-flip line.
With this information
astronomers can recreate the 3
dimensional structure of our
galaxy including the number of
spiral arms, etc.
This is what we think our galaxy
looks like.
Chapter 14: The Milky Way Galaxy
Astronomers still can’t explain well why we have spiral arms
in galaxies.
The stars, gas, and dust in the galaxy all obey Kepler’s laws
of planetary motion. Thus the inner matterial rotates faster
than the outer matterial. If spiral arms were tied to the
galaxy then after a few 100,000,000 years they would all be
wound up and loose their shape.
Chapter 14: The Milky Way Galaxy
The density wave theory says that a compression front moves
through the disk and compresses the gas and dust to start the star
formation process. In this model new matterial is being
constantly fed into the density regions as old matterial leaves.
Chapter 14: The Milky Way Galaxy
Since matterial orbiting the galaxy must obey Kepler’s laws, if
we could measure the distance and orbital period of matterial
as a function of distance from the center , we could determine
the amount of mass inside that distance.
When we reach the “edge” of the galaxy, we expect the
rotation speed to diminish, but instead it keeps on increasing
as if there is some “invisible” (dark) matter.
Chapter 14: The Milky Way Galaxy
One attempt to detect dark
matter is to look for
gravitational lensing. Even if
the matter is dark it should
disturb the space-time
continuum and produce gravity
like any other matter.
While we have seen several
such events, they could only
account for about 1/2 the dark
matter.