Great Ideas in Science:Lecture 8 – Stars & Galaxies
Professor Robert HazenProfessor Robert HazenUNIV 301UNIV 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 IdeasStars 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 StarsObserving Life Cycles of Stars Measure many different stars and look for patterns,
especially in brightness vs. temperature
The Birth of StarsThe 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 Main Sequence and the Death of Starsthe 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 Main Sequence and the Death of Starsthe Death of Stars
Stars about the mass of the sun1. Hydrogen burning at
faster rate2. Red giant (Move off
main sequence)3. Helium burning4. Begin collapse5. White dwarf
The Main Sequence and The Main Sequence and the Death of Starsthe Death of Stars
Very Large Stars– Successive collapses
and burnings– Iron core– Catastrophic collapse
into a supernova
SupernovaSupernova
Neutron Stars and PulsarsNeutron Stars and Pulsars
Neutron Star– Dense and small– High rotation rate– Little light
Pulsar– Special neutron star– Electromagnetic radiation– End state of supernova
Black HolesBlack 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 StarsSummary: 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
CosmologyCosmology
Great Idea: The universe began billions of years ago in the big
bang and it has been expanding ever since.
The Nebula DebateThe 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 Edwin Hubble and the Discovery of GalaxiesDiscovery 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)Galaxies (Andromeda)
Kinds of GalaxiesKinds of Galaxies• Spiral
• Elliptical
• Irregular & Dwarf
• Quasars
TYPES OF GALAXIESTYPES OF GALAXIES
DEEP FIELD IMAGEDEEP FIELD IMAGE
The Large-Scale The Large-Scale Structure of the UniverseStructure of the Universe
• The Local Group– Milky way, Andromeda galaxy, and ~50 others
• Groups, clusters, superclusters
• Voids
The Astronomical Distance ScaleThe Astronomical Distance ScaleHow Far Away Are Galaxies?How Far Away Are Galaxies?
• Parallax
• Standard candles– Cepheid variable stars– Large galaxies– Type 1 supernovae
The Big BangThe Big BangDistant galaxies are moving away from Distant galaxies are moving away from
us – the farther away they are, the us – the farther away they are, the faster theyfaster they’’re moving.re moving.
The early universe was hotter and denser The early universe was hotter and denser than today.than today.
These studies also hint at how the These studies also hint at how the universe will end.universe will end.
Evidence for the Big BangEvidence 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 HubbleThe Redshift and Hubble’’s Laws Law
• Galactic redshift
The Redshift and HubbleThe Redshift and Hubble’’s Laws Law
• Galactic redshift
• Hubble’s Law– The farther a galaxy,
the faster it recedes– V = H x d
Some Useful AnalogiesSome Useful Analogies Raisin-Bread
Dough AnalogyExpanding Balloon
Analogy
Some General Characteristics Some General Characteristics of an Expanding Universeof an Expanding Universe
• All matter heats when compressed and cools when it expands.
• Hence, universal “freezings”
1010-43-43 Second: The Freezing Second: The Freezing of All Forcesof All Forces
• Two fundamental forces– Gravity– Strong-electroweak force
1010-35-35 Second: The Freezing of the Second: The Freezing of the Electroweak and Strong ForcesElectroweak and Strong Forces
• Three fundamental forces• The “Large Hadron Collider (LHC)
will probe this period.
1010-10-10 Second: The Freezing of the Second: The Freezing of the Weak and Electromagnetic ForcesWeak and Electromagnetic Forces
• Four forces become separate
• Older particle accelerators– Reproduce from here forward– Provide experimental evidence
for evolution of universe
1010-5-5 Second: The Freezing of Second: The Freezing of Elementary ParticlesElementary Particles
• Elementary particles are formed
• Prior: Quarks and leptons
• After: Electrons, protons and neutrons
Three Minutes: Three Minutes: The Freezing of NucleiThe Freezing of Nuclei
• Nuclei become stable
• Only nuclei of H, He and Li
• Plasma
Before One Million Years:Before One Million Years:The Freezing of AtomsThe Freezing of Atoms
• Formation of Atoms
• Radiation released
• Cosmic microwave background
What We DonWhat We Don’’t Know:t Know:Dark Matter and Ripples Dark Matter and Ripples at the Beginning of Timeat the Beginning of Time
• Dark Matter
• Ripples at the beginning of time
The End of the UniverseThe 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!!!