Chapter 14 Our Galaxy
14.1 The Milky Way Revealed
The Milky Way galaxy appears in our sky as a faint band of
light
Dusty gas clouds obscure our view because they absorb
visible light.
This is the interstellar medium that makes new star systems.
We see our galaxy edge-on Primary features: disk, bulge,
halo, globular clusters.
If we could view the Milky Way from above the disk, we would
see its spiral arms.
Stars in the disk all orbit in the same direction with a
little up-and-down motion.
Orbits of stars in the bulge and halo have random orientations.
Sun’s orbital motion (radius and velocity) tells us mass
within Sun’s orbit:1.0 x 1011 MSun
14.2 Galactic Recycling
Star-gas-star cycle Recycles gas from old stars into new
star systems.
High-mass stars have strong stellar winds that blow bubbles
of hot gas
Lower mass stars return gas to interstellar space through
stellar winds and planetary nebulae.
X-rays from hot gas in supernova remnants reveal newly made heavy
elements.
Supernova remnant cools and begins to emit visible light as
it expands New elements made by supernova mix into interstellar medium.
Multiple supernovae create huge hot bubbles that can blow
out of disk
Gas clouds cooling in the halo can rain back down on disk.
Atomic hydrogen gas forms as hot gas cools, allowing electrons
to join with protons
Molecular clouds form next, after gas cools enough to allow
to atoms to combine into molecules
Radiation from newly formed stars is eroding these star forming
clouds.
We observe star-gas-star cycle operating in Milky Way’s disk
using many different wavelengths of light.
Infrared light reveals stars whose visible light is blocked
by gas clouds
Radio waves from carbon monoxide (CO) show locations of
molecular clouds.
Long-wavelength infrared emission shows where young stars
are heating dust grains
Gamma rays show where cosmic rays from supernovae collide
with atomic nuclei in gas clouds
Ionization nebulae are found around short-lived high-mass
stars, signifying active star formation
14.3 The History of the Milky Way
Halo stars formed first as gravity caused cloud to contract.
Remaining gas settled into spinning disk
Stars continuously form in disk as galaxy grows older.
Detailed studies: Halo stars formed in clumps that later
merged
14.4 The Mysterious Galactic Center
Stars appear to be orbiting something massive but invisible
… a black hole? Orbits of stars indicate a mass of about 4 million MSun.
Posts
No comments:
Post a Comment