Great Ideas in Science: Lecture 8 – Stars & Galaxies Professor Robert Hazen PROV 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.

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 System (a few light minutes to hours) 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) Position in sky Wavelength (radio to gamma ray) Intensity (brightness) Variation of 1-3 with time 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

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

Telescopes are Photon Collectors GMU Observatory Telescopes are Photon Collectors GMU Observatory! Monday observing nights

Orbiting Observatories Great Observatories Program Hubble Space Telescope Spitzer Infrared Telescope Chandra X-Ray Observatory

Orbiting Observatories James Webb Space Telescope

The Structure of the Sun Stellar core Convection zone Photosphere Corona

The Structure of the Sun Solar Wind Stream of particles Northern lights

The Sun’s Energy Source: Fusion 3-steps of hydrogen burning P + P  D + e+ + neutrino + energy D + P  3He + photon + energy 3He + 3He 4He + 2 protons + photon + energy

The Variety of Stars Differences among stars Life Cycle; depends on: Color (= temperature) Apparent Brightness ( a distance effect) Absolute brightness (Based on total energy output) Life Cycle; depends on: Total mass Age

Observing Life Cycles of Stars Measure many different stars and look for patterns, especially in brightness vs. temperature

A Hierarchy of Scientific Ideas Fact (a confirmed observation) Hypothesis (an educated guess) Law (a predictive mathematical description of nature) Theory (a well established explanation of nature)

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 Hydrogen burning at faster rate Red giant (Move off main sequence) Helium burning Begin collapse White dwarf

The Main Sequence and the Death of Stars Very Large Stars Go through successive collapses and burnings Ultimately an iron core forms The star then collapses catastrophically into a supernova

Supernova

Neutron Stars and Pulsars Very dense and small Has a high rotation rate Almost no visible light Pulsar One kind of neutron star Produces intense radio electromagnetic radiation This is the end state of some supernovas

Black Holes Result of the collapse of the largest stars Nothing can escape from its surface We cannot see them directly, but: We see gravitational impact on other stars We can detect x-ray and gamma rays

Summary: Fates of Stars 1/10th Sun  Brown dwarf Sun  Main sequence  Red giant  white dwarf 8 suns  Supernova  Neutron Star (Fe) They last 100 million years Heavy elements are made in supernovas 20 suns  Supernova  Black Holes Black holes are points of pure 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? Before 1920s no instruments could answer this question

Edwin Hubble and the Discovery of Galaxies Edwin Hubble in 1919 New Mount Wilson 100” telescope He used cepheid variable stars to measure distance to nebula Galaxies Hubble discovered the universe is made of billions of galaxies. These findings opened up the field of Cosmology

Galaxies (Andromeda)

Kinds of Galaxies Spiral Elliptical Irregular & Dwarf

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 Large-Scale Structure of the Universe

The Astronomical Distance Scale How Far Away Are Galaxies? For close stars use “parallax” For distant stars use “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 Universal expansion Abundance of light elements, especially D/H 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 away a galaxy, the faster it recedes V = H x d

The Redshift and Hubble’s Law Galactic redshift Hubble’s Law The farther a galaxy, the faster it recedes V = H x d

The Redshift and Hubble’s Law Galactic redshift Hubble’s Law The farther a galaxy, the faster it recedes V = H x d

The Redshift and Hubble’s Law Galactic redshift Hubble’s Law The farther a galaxy, the faster it recedes V = H x d

The Redshift and Hubble’s Law Galactic redshift Hubble’s Law The farther a galaxy, the faster it recedes V = H x d

Expanding Balloon Analogy 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”

What We Don’t Know: Dark Matter and Ripples at the Beginning of Time

What We Don’t Know: Dark Matter and Ripples at the Beginning of Time

The End of the Universe Is the universe open, closed, or flat? Current data now suggests: The total mass suggests an open universe Type Ia supernovas reveal the expansion rate And the universal expansion is speeding up!!! Ascribed to “Dark energy” 70% of the universe’s mass is is something mysterious!!!