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!!!