Download Integrative Studies 410 Our Place in the Universe

Studying for EXAM III
• On Sun and Stars
• Many chapters in book, which has way too
many details lecture notes
• Focus on the general, repeating features
• Emphasis on concepts and reasoning (Why
are more massive stars hotter, etc.)
• Extends the cosmic
distance ladder out
as far as we can see
Cepheids – about 50
million ly
• In 1920 Hubble used
this technique to
measure the distance
to Andromeda
(about 2 million ly)
• Works best for
periodic variables
Distance Measurements
with variable stars
Cepheids and RR Lyrae: Yard-Sticks
• Normal stars undergoing a
phase of instability
• Cepheids are more massive
and brighter than RR Lyrae
• Note: all RR Lyrae have
the same luminosity
• Apparent brightness thus
tells us the distance to
them!
– Recall: B  L/d2
Galaxies – Island Universes
• A historic tour of the discovery of the
dwindling significance of humans in the
universe:
• From the center of the universe towards the
edge of an average galaxy amongst 100
billion others
How do we know where we are?
• “Obviously” we are living on a flat Earth at
the center of the universe, as a quick look
tells us:
–
–
–
–
The stars, Sun, Moon and planets rotate us
There is no apparent curvature of the ground
The Milky Way is a band that surrounds us
There are no signs for any movement of the
Earth (like wind, or forces throwing us off)
Logic to the Rescue
• How do we avoid these wrong conclusions?
– Sound data
– Flawed interpretation/reasoning
Further observations are necessary to decide!
• Do we have to question everything?
– Yes, in principle.
– The signature of genius is to ask the right
question, not necessarily to answer them.
Exploring our own Island Universe:
The Milky Way
• A galaxy is a
huge
collection of
stars, gas,
dust, neutron
stars, and
black holes,
isolated from
others and
held together
by gravity
Our view of the
Milky Way
• Appears as a milky band of
light across the sky
• A small telescope reveals
that it is composed of many
stars (Galileo again!)
• Our knowledge of the Milky
Way comes from a
combination of observation
and comparison to other
galaxies
How do we know?
Obviously a bogus picture of our milky way!
• Question: How can we say anything
about our Milky Way, if we cannot see
it from outside?
Enter: the Genius
• William Herschel (XVIII century)
• Simple model:
– Assumed all stars have the same
absolute brightness
– Counts stars as a function of
apparent magnitude
– Brighter stars closer to us; fainter
stars further away
– Cut off in brightness corresponds to
a cut off at a certain distance.
• Conclusion: there are no stars
beyond a certain distance
Herschel’s Findings
• Stars thinned out very fast at right angles to Milky Way
• In the plane of the Milky Way the thinning was slower
and depended upon the direction in which he looked
• Flaws:
– Observations made only in visible spectrum
– Did not take into account absorption by interstellar gas and
dust
Discovering other Island Universes
• Data: Lots of nebulous spots known in the
nightsky
• Questions: What are they? All the same?
Different things?
• Need more observations!
 Build bigger telescopes
The first nebula discovered to have
spiral structure: M51
M99 is a
spiral, too!
• Q: do we live in a
spiral?
• Q: Are we in the
center of the
spiral?
• Most probable
answer: No!
Enter: next genius
• Harlow Shapley used variable
stars, e.g. RR Lyrae stars, to
map the distribution of
globular clusters in the galaxy
• Found a spherical distribution
about 30 kpc (30,000 pc)
across
– This is the true size of the
galaxy
• Sun is (naturally!) not at the
center – it’s about 26,000 ly
out
Standing on the shoulders of Giants
• Shapley used methods developed by others
to measure the distance to globulars
• Cepheid variables show luminosity-period
correlations discovered by Henrietta Leavitt
• Shapley single-handedly increase the size of
the universe tenfold!
Structure of the Galaxy
Intra-galactic Dynamics
• Three main parts
of a galaxy:
– Bulge (center of
galaxy)
– Disk (rotating
around center)
– Halo (orbiting
around bulge
with randomly
inclined orbits)
Properties of Bulge, Disk and Halo
Disk
Highly flattened
young and old stars
has Gas and dust
Star formation
White colored,
blue spiral arms
Halo
spherical
Bulge
football-shaped
only old stars
young and old stars
none
lots in center
none since 10 billion yrs
reddish
in inner regions
yellow-white
An up-to-date “Reconstruction”