The story so far

1
The story so far

• We have considered
• the origin of the Universe
• the formation and evolution of galaxies
• the birth, life and death of stars
• How does this relate to the existence of the
  solar system and of life on Earth?

2
Observation and theory

• Astronomy is an observational science
• theories need support from observations
• cosmology
• expansion of universe from Doppler shift
• primordial abundances from old stars, ancient
  hydrogen clouds, etc.
• properties of microwave background
• large-scale structure
• microwave background
• galaxy surveys

• galaxies
• observation of distant galaxies
• detailed study of nearby galaxies and Milky Way
• stars
• detailed study of Sun
• studies of stellar clusters
• spectroscopic observations
• evidence for nucleo-synthesis, age, etc.
• Probably some of the material in this course is
  wrong
• we just dont know what

3
In the beginning.

• Our universe began about 15 billion years ago
• brief period of exponential expansion 10-35 s
  after initial big bang resulted in nearly flat,
  very uniform visible universe today
• inflation
• after this universeexpands much moreslowly

WMAP
4
In the beginning.

• Continuing expansion causes universe to cool
• in a few minutes cools enough to form the light
  isotopes 2H, 3He, 4He, 7Li
• observations of these isotopes test theory
• after 300000 years cools enough for neutral atoms
  to form
• microwave background radiation emitted
• its properties probe conditions in early universe

Ken Nollett, Caltech
5
The formation of structure

• Tiny irregularities in the early Universe are
  seen in the micro-wave background
• denser patches would have had slightly stronger
  gravity, attracting material towards them
• seeds of structure formation
• produce small protogalaxies which interact and
  merge to form modern galaxies

Simulation by the Los Alamos theoretical
astrophysics group
6
The galaxies

• The Milky Way is a large spiral galaxy
• it has a bulge, disc and halo
• most of its mass is apparently dark matter
• it is located in a small group on the fringes of
  the large Virgo cluster
• Other galaxies have had more turbulent histories
• collisions and mergers play important roles in
  galaxy evolution

7
The stars

• Stars form when cool, dense clouds of
  interstellar gas collapse under gravity
• they spend most of their lives on the main
  sequence fusing hydrogen to helium
• eventually they expel their outer regions as
  planetary nebulae or supernovae
• thus disseminating heavy elements into the
  interstellar gas

8
The elements
dissemination
production

• The top ten elements

Note that the most common elements in your body
all occur in the top ten, formed by a variety of
mechanisms (most obvious absentees are calcium
and phosphorus)
9
A hospitable universe?

• The universe is gt10 billion years old
• did not expand so fast stars never formed
• did not recollapse in rapid Big Crunch
• There is an asymmetry in the interactions of
  matter and antimatter
• they do not entirely cancel each other out
• universe has significant matter content

• We have friendly nuclear physics
• 2H is stable, but 2He isnt
• stars work!
• 12C has an energy level at just the right place
• carbon is a common element
• supernovae explode
• heavy elements are incorporated into new stars
  and planets

10
Observations and observers

• No obvious reasons for lucky values of many
  parameters
• new physics?
• reasons are there, but we dont yet know them
• selection bias
• if values were not lucky, universe would not
  permit intelligent life to evolve
• we exist
• therefore, parameters must have lucky values
• ...weak anthropic principle

• Why look for physics reasons?
• anthropic principle arguments less fruitful
• useful scientific theories explain existing data
  and make testable predictions
• leads to deeper understanding of nature
• more elegant/satisfying

11
A planet-building universe

• Massive stars produce heavy elements
• and disseminate them into interstellar medium via
  planetary nebulae and supernovae
• Heavy elements in cool gas tend to clump together
  to form small dust grains
• reason for opacity of gas clouds in Milky Way

• Theory and observation (cratering record) suggest
  planets of solar system formed by accretion
• dust grains collide and stick to form
  successively larger bodies
• probably fairly easy process if stars form from
  dust-rich material

12
Planetary systems

• Many young stars are surrounded by dusty discs
• theory suggests these should develop into
  planetary systems
• Over 200 planets have now been observed around
  other stars
• planetary systems are relatively common
• at least around stars with high heavy element
  content
• little evidence about howtypical our system is
• current detection methodsbiased against
  systemslike ours
• next lecture!

Mike Ressler, NASA/JPL