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  3 He + photon + energy 3) 3 He + 3 He  4 He + 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 = H x d

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”

Second: The Freezing of All Forces Two fundamental forces –Gravity –Strong-electroweak force

Second: The Freezing of the Electroweak and Strong Forces Three fundamental forces The “Large Hadron Collider (LHC) will probe this period.

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!!!