Download Stars - Robert M. Hazen

Great Ideas in Science:
Lecture 8 – Stars & Galaxies
Professor Robert Hazen
UNIV 301
Great Idea:
The Sun and other stars use nuclear
fusion reactions to convert mass into
energy. Eventually, when a star’s nuclear
fuel is depleted, the star must burn out.
Key Ideas
Stars have a history – a beginning and an end
1. Stars (and planets) begin as clouds of dust
and gas, called nebulae.
2. Stars radiate heat and light, which come from
the energy of nuclear fusion reactions.
3. Planets form like stars, but they are too small
to begin nuclear fusion reactions.
Definitions
• Astronomy is the study of photons
arriving from space.
• Astrophysics is the study of the
origin, evolution, and fate of stars
and clusters of stars.
• Cosmology is the study of the origin
evolution and fate of large-scale
structures of the universe.
What do we see from Earth?
Very close (a few light seconds)
• Moon
• Meteors
• Satellites
What do we see from Earth?
The Solar Sytem (a few light days)
• Planets
• Asteroids
• Comets
• Other objects
What Do We See From Earth?
Milky Way Galaxy (to about 200,000 ly)
Other stars
Nebulae
Hydrogen halo
Central dust concentration
What Do We See From Earth?
Beyond our galaxy (more than 1,000,000 ly)
• Other galaxies
• Clusters of galaxies
• Quasars
Almost all astronomical data
come from Photons
(Electromagnetic Waves)
1. Position in sky
2. Wavelength (radio to gamma ray)
3. Intensity (brightness)
4. Variation of 1-3 with time
5. Polarization
Observing Stars:
What do we want to know?
• Distance:
– parallax (to 300 ly)
– standard candles
Observing Stars:
What do we want to know?
• Distance (parallax; standard candles)
• Composition (from line spectra)
Observing Stars:
What do we want to know?
• Distance (parallax;
standard candles)
• Composition (from
line spectra)
• Motion
– absolute motion
– red shift
Observing Stars:
What do we want to know?
• Distance (parallax; standard candles)
• Composition (from line spectra)
• Motion (absolute motion; red shift)
• Temperature (from color)
• Brightness (apparent vs. absolute)
• Mass (from dynamics and theory)
Telescopes are Photon Collectors
• Earth-based or satellite
• Various detectors
– Eye
– Film
– Electronic
Telescopes are Photon Collectors
Orbiting Observatories
• Great Observatories Program
– Hubble Space Telescope
– Spitzer Infrared Telescope
– Chandra X-Ray Observatory
The Structure of the Sun
– Stellar core
– Convection zone
– Photosphere
– Chromosphere
– Corona
The Structure of the Sun
• Solar Wind
– Stream of particles
– Northern lights
The Sun’s Energy Source: Fusion
3-steps of hydrogen burning
1) P + P  D + e+ + neutrino + energy
2) D + P  3He + photon + energy
3) 3He + 3He 4He + 2 protons +
photon + energy
The Variety of Stars
• Differences
– Color (= temperature)
– Apparent Brightness
• Distance effect
• Absolute brightness
– Energy output
– = Luminosity
• Life Cycle
– Total mass
– Age
Observing Life Cycles of Stars
Measure many different stars and look for patterns,
especially in brightness vs. temperature
The Birth of Stars
The Nebular Hypothesis
Terrestrial (Inner) Planets
• Mercury, Venus, Earth, Mars
– Rocky and relatively small
– Mercury and Venus too hot for life
– Mars may have had life long ago
Gas Giant (Outer) Planets
Jupiter, Saturn, Uranus, Neptune
Gas Giant (Outer) Planets
Jupiter, Saturn,
Uranus, Neptune
– Layered structure
– No solid surface
The Main Sequence and
the Death of Stars
• Stars much less
massive than the sun
– Brown dwarf
– Glows 100 billion years
• No change in size,
temperature, energy
output
The Main Sequence and
the Death of Stars
Stars about the mass
of the sun
1. Hydrogen burning at
faster rate
2. Red giant (Move off
main sequence)
3. Helium burning
4. Begin collapse
5. White dwarf
The Main Sequence and
the Death of Stars
Very Large Stars
– Successive collapses
and burnings
– Iron core
– Catastrophic collapse
into a supernova
Supernova
Neutron Stars and Pulsars
Neutron Star
– Dense and small
– High rotation rate
– Little light
Pulsar
– Special neutron star
– Electromagnetic radiation
– End state of supernova
Black Holes
– Result of collapse large star
– Nothing escapes from surface
– Cannot see them
• See impact on other stars
• Detect x-rays, gamma rays
Summary: Fates of Stars
• 8 suns  Supernova  Neutron Star (Fe)
• 100 million years
• Heavy elements made in supernova
• 20 suns  Supernova  Black Holes
• Points of mass
Cosmology
Great Idea: The universe began billions
of years ago in the big bang and it has
been expanding ever since.
The Nebula Debate
• Nebulae are cloud-like objects
– Are they clouds of dust and gas?
– Or huge collections of stars?
• Harlow Shapley vs. Heber Curtis
– Debate over distance of nebulae
• Before 1920s no instruments
could answer this question
Edwin Hubble and the
Discovery of Galaxies
• Edwin Hubble in 1919
– Mount Wilson 100” telescope
– Used cepheid variable stars to
measure distance to nebula
– 3 days/800x; 30 days/10,000x
• Galaxies
– Hubble discovered universe is
made of billions of galaxies
• Cosmology
Galaxies (Andromeda)
Kinds of Galaxies
•
•
•
•
Spiral
Elliptical
Irregular & Dwarf
Quasars
TYPES OF GALAXIES
DEEP FIELD IMAGE
The Large-Scale
Structure of the Universe
• The Local Group
– Milky way, Andromeda galaxy, and ~50 others
• Groups, clusters, superclusters
• Voids
The Astronomical Distance Scale
How Far Away Are Galaxies?
• Parallax
• Standard candles
– Cepheid variable stars
– Large galaxies
– Type 1 supernovae
The Big Bang
Distant galaxies are moving away from
us – the farther away they are, the
faster they’re moving.
The early universe was hotter and denser
than today.
These studies also hint at how the
universe will end.
Evidence for the Big Bang
1. Universal expansion
2. Abundance of light elements,
especially D/H
3. Cosmic microwave background
radiation at ~ 2.7 Kelvin
The Redshift and Hubble’s Law
• Galactic redshift
The Redshift and Hubble’s Law
• Galactic redshift
• Hubble’s Law
– The farther a galaxy,
the faster it recedes
–V=Hxd
Some Useful Analogies
Raisin-Bread
Dough Analogy
Expanding Balloon
Analogy
Some General Characteristics
of an Expanding Universe
• All matter heats when compressed
and cools when it expands.
• Hence, universal “freezings”
10-43 Second: The Freezing
of All Forces
• Two fundamental forces
– Gravity
– Strong-electroweak force
10-35 Second: The Freezing of the
Electroweak and Strong Forces
• Three fundamental forces
• The “Large Hadron Collider (LHC)
will probe this period.
10-10 Second: The Freezing of the
Weak and Electromagnetic Forces
• Four forces become separate
• Older particle accelerators
– Reproduce from here forward
– Provide experimental evidence
for evolution of universe
10-5 Second: The Freezing of
Elementary Particles
• Elementary particles are formed
• Prior: Quarks and leptons
• After: Electrons, protons and neutrons
Three Minutes:
The Freezing of Nuclei
• Nuclei become stable
• Only nuclei of H, He and Li
• Plasma
Before One Million Years:
The Freezing of Atoms
• Formation of Atoms
• Radiation released
• Cosmic microwave background
What We Don’t Know:
Dark Matter and Ripples
at the Beginning of Time
• Dark Matter
• Ripples at the
beginning of time
The End of the Universe
• Open, closed or flat universe
• Current data
– Total mass suggests open universe
– Type Ia supernova reveal expansion
• The universal expansion is
speeding up!!!
– “Dark energy”
– 70% of universe’s mass!!!