Transcript chap_11

Chapter 11
Galaxies and the Foundation of
Modern Cosmology
The Milky Way
A band of light
making a circle
around the celestial
sphere.
What is it?
Our view into the
plane of our galaxy.
If we could view the Milky Way from above the
disk, we would see its spiral arms.
We see our galaxy edge-on.
Primary features: disk, bulge, halo, globular clusters
We observe the star–gas–star cycle operating in Milky
Way’s disk using many different wavelengths of light.
How is gas recycled in our galaxy?
Insert TCP 6e Figure 19.6
X rays from
hot gas in
supernova
remnants
reveal newly
made heavy
elements.
Where do stars tend to form in our
galaxy?
Much of the star
formation in the disk
happens in the spiral
arms.
Ionization nebulae
Blue stars
Gas clouds
Whirlpool Galaxy
Molecular clouds
in Orion
Composition:
• Mostly H2
• About 28% He
• About 1% CO
• Many other
molecules
Orion Nebula Proplyds
Gravity forms
stars out of
the gas in
molecular
clouds,
completing
the star–gas–
star cycle.
Radiation
from newly
formed stars
is eroding
these starforming
clouds.
How did our galaxy form?
Our galaxy formed from a cloud of intergalactic gas.
Halo stars formed first as gravity caused gas to contract.
Remaining gas settled into a spinning disk.
Stars continuously form in disk as galaxy grows older.
Insert TCP 6e Figure 19.19
Detailed studies show that halo stars formed in clumps
that later merged.
Insert TCP 6e Figure 20.4
Barred spiral galaxy: has a bar of stars across the bulge
Lenticular
galaxy:
has a disk like
a spiral galaxy
but much less
dusty gas
(intermediate
between
spiral and
elliptical)
Elliptical
galaxy:
all spheroidal
component,
virtually no disk
component
Red-yellow
color indicates
older star
population.
Irregular galaxy
Blue-white color indicates
ongoing star formation.
Spheroid
dominates
Hubble’s galaxy classes
Disk
dominates
The collisions we observe nearby trigger bursts of
star formation.
Modeling such collisions on a computer shows that two
spiral galaxies can merge to make an elliptical.