Chapter 15 galaxies and the
foundation of modern cosmology
*****15.1 islands of stars*****
Our deepest images of the universe
shows a great variety of galaxies, some of them billions of light years away.
A galaxy's age it's distance and
the age of the universe are all closely related
The study of the galaxies is thus
intimately connected with cosmology the study of the structure and evolution of
the universe.
Disk component stars of all ages
many gas clouds
Spheroidal component bulge and halo
old stars few gas clouds
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 spirals and
elliptical.
Irregular galaxy neither spiral nor
elliptical
Irregular galaxy neither spiral nor
elliptical blue whit colors
Spiral galaxies are often found in
groups of galaxies up to a few dozen galaxies per group.
Elliptical galaxies are much more
common in huge clusters of galaxies hundred to thousands of galaxies.
*****15.2 distance of galaxies*****
Brightness alone does not provide
enough information to measure the distance.
Step 1 determine size of solar
system using radar.
Step 2 determine distance of stars
out to a few hundred light years using parallax.
Cepheid variable stars are very
luminous
The light curve of the cepheid
variable star shows that its brightness alternately rise and falls over a fifty
day period.
Cepheid variable stars with longer
periods have greater luminosities
White dwarfs supernovae can also be
used as standard candles.
Hubble wetted the debate by
measuring the distance to the andromeda galaxy using cepheid variables as
standard candles.
Hubble also knew that the spectral
features of virtually all galaxies are redshifted they're all moving away from
us.
Distances of the farthest galaxies
are measured from red shifts
One example of something that
expands but has no center or edge is the surface of a balloon.
Distance between faraway galaxies
change while light travels
Astronomers think in terms of look
back time rater than distance.
Expansion stretches photon
wavelengths causing a cosmological redshift directly related to look back time
*****15.3 galaxy evolution*****
Denser regions contracted forming
protogalactic clouds
H and He gases in these clouds
formed the first star.
Supernovae explosion from the first
star kept much of the gas from forming stars
Leftover gas settled into a
spinning disk
Observations of some distant red
elliptical galaxies support the idea that most of theory stars formed very
early on the history of the universe.
We must consider the effects of
collisions.
Collisions were much more likely in
early time
The collisions we observe nearly
trigger burst of star formation.
Modeling such collision on a
computer show that two spiral galaxies can merge to make an elliptical.
Collision may explain why
elliptical galaxies tend to be found where galaxies are closer together.
Giant elliptical galaxies t the
center of clusters seem to have consumed a number of smaller galaxies.
Starburst galaxies are forming
stars so quickly that they will use up all their gas in less than a billion
years.
The intensity of supernova
explosions in starburst galaxies can drive galactic winds.
The intensity of supernova
explosions in starburst galaxies can drive galactic winds
*****15.4 quasars and other active
galactic nuclei*****
Radio galaxies contain active
nuclei shooting out vast jets of plasma which emit radio waves
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