Astronomy Finals Study Guide Pt.2 (copy)

The Cosmic Perspective: Star Stuff(Chapter 17)

1. Stellar Mass and Fusion

• Core Pressure and Temperature

  • The mass of a main-sequence star determines its core pressure and temperature.

• Higher Mass Stars:

  • Have higher core temperatures and more rapid fusion rates, resulting in:

  • Greater luminosities

  • Shorter lifetimes

• Lower Mass Stars:

  • Have cooler cores and slower fusion rates, resulting in:

  • Smaller luminosities

  • Longer lifetimes

  • Have deeper convective zones.

2. Life Tracks of Stars

• Star clusters, containing stars of different masses born around the same time, provide insights into stellar life stories.

• The comparison of mathematical models of stars with observational data aids in understanding their life tracks.

3. Life Stages of a Low-Mass Star

• Remains on the main sequence as long as it can fuse hydrogen into helium in its core.

• Upon exhausting hydrogen fusion:

  • The core cools and shrinks, eventually leading to:

  • Expansion into a red giant.

• After the red giant stage, helium fusion begins by creating carbon in its core, resulting in significant changes in the star’s luminosity and temperature.

4. Helium Fusion and the Helium Flash

• Helium fusion initiates at elevated temperatures, necessary due to increased electrostatic repulsion compared to hydrogen fusion.

• The Helium Flash occurs when the core temperature of a low-mass red giant rises enough for helium to fuse, leading to a sharp increase in the helium fusion rate.

5. Life Cycle After Helium Flash

• After helium fusion begins, a star becomes more stable, exhibiting a horizontal branch on the Hertzsprung-Russell (H-R) diagram.

• Eventually, once the core exhausts helium:

  • Helium fuses in a shell around the core, while hydrogen fuses into helium in an outer shell.

  • This stage culminates in a planetary nebula, ejecting outer layers, leaving behind a white dwarf.

6. Life Stages of a High-Mass Star

• Life stages include:

  • Main sequence (hydrogen fuses in core): High-core temperatures allow for CNO cycle (carbon, nitrogen, oxygen catalyzed fusion).

  • Supergiant phase (hydrogen shell fusion): Expansion occurs as fusion continues in a shell around the core.

  • Late life involves multiple shell fusion processes, ultimately creating an iron core.

7. Supernova and Element Formation

• The supernova explosion results when the iron core collapses under gravity, generating high-energy environments where heavier elements are formed.

• Core collapse leads to the expulsion of outer layers, leaving behind either a neutron star or black hole, depending on the mass.

8. The Role of Mass in Stellar Life

• A star's mass fundamentally governs its life story:

  • Determines max core temperature, rate of fusion, and element creation potential.

• High-mass stars (e.g., >8 solar masses) end in supernovae, while low-mass stars (e.g., <2 solar masses) typically evolve into white dwarfs.

9. Companions and Mass Exchange

• Binary stars, such as Algol, exhibit unique life stories due to mass exchange:

  • More massive stars lose material to their companions, altering their evolutionary paths.

The Cosmic Perspective: Our Galaxy(Chapter 19)

1. Structure of the Milky Way (19.1)

• Major Components:

  • Disk: Contains stars, gas, and dust; site of active star formation.

  • Bulge: Central region, dense with stars.

  • Halo: Spherical region with older, metal-poor stars.

  • Globular Clusters: Dense star clusters in the halo; among the galaxy's oldest members.

• Viewed Edge-On:

  • Appears as a thin, flattened disk with a bulge at the center.

  • Spiral arms evident from an overhead perspective.

2. Stellar Orbits in the Galaxy

• Disk Stars:

  • Orbit in nearly circular paths within the disk.

  • Move in the same direction with vertical bobbing due to gravitational tugs.

• Bulge and Halo Stars:

  • Exhibit random, elliptical orbits.

  • Lack uniform direction and stay out of the disk.

• Stellar Collisions:

  • Extremely rare due to the vast distances between stars, despite high star density.

3. Measuring Galactic Mass

• Orbital Velocity Law:

  • Mass enclosed within a star’s orbit can be calculated using orbital speed and radius.

  • The Sun’s orbit shows significant unseen mass within its orbit (~100 billion solar masses).

4. Galactic Recycling (19.2)

• Star–Gas–Star Cycle:

  • Evolved stars return material to the interstellar medium (ISM), which forms new stars.

• High-Mass Stars:

  • Emit strong stellar winds and explode as supernovae, creating hot gas bubbles.

• Low-Mass Stars:

  • Return gas gently via planetary nebulae.

• Supernova Remnants:

  • X-ray observations show mixing of heavy elements into the ISM.

• Superbubbles & Galactic Fountains:

  • Multiple supernovae combine to blow gas into the halo.

  • Gas cools, falls back into the disk, continuing the cycle.

5. Star Formation Process

• Molecular Cloud Formation:

  • Hot gas cools → atomic hydrogen → molecular gas (e.g., CO).

  • Cold, dense clouds collapse under gravity to form stars.

• Sites of Star Formation:

  • Ionization Nebulae: Surround hot, young stars; emit light from ionized gas.

  • Reflection Nebulae: Appear blue; reflect starlight off dust.

  • Spiral Arms: Contain compression waves that trigger star formation.

• Long-Term Outlook:

  • In ~1 trillion years, most gas will be locked in stellar remnants (white dwarfs, low-mass stars).

6. Observing the Milky Way Across the Spectrum

• Radio: Reveals atomic/molecular gas (H & CO lines).

• Infrared: Detects heat from dust around young stars.

• X-ray/Gamma-ray: Trace supernova remnants, black holes, and cosmic ray activity.

7. Formation and Evolution of the Galaxy (19.3)

• Halo Stars:

  • Old and metal-poor; formed early in galactic history.

  • Streams of stars suggest past mergers with dwarf galaxies.

• Formation Model:

  • Begins with a collapsing intergalactic gas cloud.

  • Halo stars form early in scattered clumps.

  • Remaining gas settles into a rotating disk.

  • Ongoing star formation continues in the disk.

8. The Galactic Center (19.4)

• Supermassive Black Hole Evidence:

  • Observations of stellar orbits indicate a massive, invisible object.

  • Estimated mass: ~4 million solar masses.

• High-Energy Activity:

  • X-ray flares suggest matter being torn apart and consumed.

  • EHT image (2022): Visual confirmation of a black hole consistent with models.

The Cosmic Perspective: Galaxies and the Foundation of Modern Cosmology(Chapter 20)

1. Galaxies and the Universe

Cosmology: The study of the structure and evolution of the universe.

• Galaxies are "island universes" — each contains billions of stars.

• Deep images like the Hubble Ultra Deep Field show a wide variety of distant galaxies.

• Studying galaxy distances helps determine the age of the universe.

2. Types of Galaxies

Spiral Galaxies

• Flat, rotating disk with spiral arms, bulge, halo.

• Contain blue, young stars (active star formation).

• Subtypes: Normal spirals and barred spirals.

• Example: Milky Way, Andromeda.

Elliptical Galaxies

• Round or oval, no disk or arms.

• Red-yellow color from old stars.

• Range from dwarf ellipticals to giant ellipticals.

• Example: M87.

Irregular Galaxies

• No regular shape.

• Often rich in gas and dust, active star formation.

• Example: Large and Small Magellanic Clouds.

Lenticular Galaxies

• Disk-like but without spiral arms.

• Intermediate between elliptical and spiral.

3. Galaxy Groupings

• Groups: Small collections (dozens), mostly spirals, e.g., Local Group.

• Clusters: Hundreds to thousands of galaxies, mostly ellipticals.

• Superclusters: Larger structures made of clusters.

4. Galaxy Color and Activity

• Blue cloud: Spiral and irregular galaxies with ongoing star formation.

• Red sequence: Ellipticals with old, inactive stars.

5. Measuring Distances

Standard Candles

• Objects with known luminosity (e.g., Cepheids, Type Ia supernovae).

• Distance found by comparing luminosity to observed brightness.

Cepheid Variables

• Discovered by Henrietta Leavitt.

• Leavitt’s Law - Longer periods = greater luminosity

• Brightness related to pulsation period.

• Key to determining distances to nearby galaxies.

• Very luminous.

White-Dwarf Supernovae

• Explode with consistent peak brightness.

• Used to measure distances up to 10 billion light-years.

6. Hubble’s Discovery

• Edwin Hubble used Cepheids to measure distance to Andromeda.

• Proved that it lies far outside the Milky Way — a separate galaxy.

• Solved the "spiral nebulae" debate.

7. Hubble–Lemaître Law

• Galaxies move away from us (redshift).

• Velocity = H₀ × Distance, where H₀ is the Hubble constant.

• More distant galaxies move faster — shows expansion of the universe.

8. Age of the Universe

• Universe is expanding at a measurable rate.

• Age estimated by time = distance ÷ speed → ~14 billion years.

9. Nature of Expansion

• Space itself is expanding, not galaxies moving through space.

• No center or edge to the universe.

• Balloon analogy: Surface expands uniformly.

10. Cosmological Principle

• Universe is homogeneous and isotropic on large scales.

• Same in all directions, and every location is typical.

11. Lookback Time and Redshift

Lookback Time

• Time light has traveled to reach us = distance in years.

Cosmological Redshift

• Light waves stretched as the universe expands.

• Higher redshift = greater distance and earlier time in universe’s history.

12. Observable Universe

• We can only see as far as light has had time to travel.

• Beyond the cosmological horizon, we can’t observe — light hasn’t reached us yet.

The Cosmic Perspective: Galaxy Evolution(Chapter 21)

1. Looking Back Through Time

• Distant light = Ancient view:

  • Light from faraway galaxies shows them as they were when the light left.

  • Deep observations = glimpses into galaxy formation and youth.

• Galaxy formation model:

  • Matter originally spread almost uniformly.

  • Slightly denser regions collapsed via gravity into protogalactic clouds.

  • Gas (mostly H and He) formed first stars.

2. Why Galaxies Differ

• Protogalactic cloud conditions:

  • Spin:

    • Low → less disk → elliptical galaxy.

    • High → more disk → spiral galaxy.

  • Density:

    • High → quick star formation → elliptical.

    • Low → slow formation → disk settles → spiral.

• Galaxy interactions:

  • Collisions were more frequent in early universe.

  • Mergers can transform spirals into ellipticals.

  • Shells/stars in ellipticals = signs of past collisions.

  • Giant ellipticals in clusters likely formed via mergers.

3. Gas Cycling in Galaxies

• Star-forming galaxies:

  • Blue color = active star formation.

  • Red ellipticals = older stars, less gas.

• Starburst galaxies:

  • Rapid star formation (up to 100× Milky Way).

  • Supernovae drive galactic winds — can blow gas out.

  • Especially strong in small galaxies — may remove most gas.

4. Supermassive Black Holes (SMBHs)

• Active Galactic Nuclei (AGN):

  • Bright central regions powered by SMBHs.

  • Quasars: Brightest AGNs; very distant → early universe.

  • Rapid brightness changes → tiny region of origin (smaller than solar system).

• Evidence for SMBHs:

  • Stellar orbits near centers indicate massive, compact objects.

  • E.g., M87’s black hole = ~6 billion solar masses.

• Energy source:

  • Infalling matter → kinetic → heat → radiation.

  • Accretion disk: can convert 10–40% of mass into energy.

5. SMBHs and Galaxy Evolution

• Co-evolution:

  • Black hole mass correlates with galaxy bulge mass.

  • Suggests feedback loop between SMBH growth and galaxy evolution.

• Dormant AGNs:

  • Many galaxies likely had active phases in the past.

6. Radio Galaxies

• Jets and Lobes:

  • Some AGNs shoot out jets of plasma → radio waves.

  • Lobes may span hundreds of thousands of light-years.

  • Jets interact with galactic and intergalactic gas.

7. Gas Beyond the Stars

• Intergalactic gas:

  • Detected via absorption lines in quasar spectra.

  • Light from quasars passes through multiple gas clouds.

  • Spectral complexity reveals the presence of protogalactic and intergalactic clouds.

The Cosmic Perspective: The Birth of the Universe(Chapter 22)

1. The Big Bang Theory

Early Universe Conditions

• Extremely hot and dense.

• Photons → particle–antiparticle pairs due to high temperatures.

• Governed by four fundamental forces:

  • Gravity, Electromagnetism, Strong, and Weak.

Forces and Unification

• At high energy, forces may unify.

• Physics seeks to explain this with Grand Unified Theories (GUT) and quantum gravity.

Eras of the Early Universe

1. Planck Era:

   • <10⁻⁴³ s, all forces unified, unknown physics.

2. GUT Era:

   • Gravity separates; ends when strong force separates.

3. Electroweak Era:

   • Weak and electromagnetic forces unify.

4. Particle Era:

   • Protons, neutrons form slightly more than antimatter.

5. Nucleosynthesis:

   • Fusion creates helium, lithium (3–4 min).

6. Nuclei Era:

   • ~3–20 min; plasma of nuclei and electrons.

7. Atom Era:

   • ~380,000 years; atoms form, radiation escapes → CMB.

8. Galaxy Era:

   • Galaxies form ~1 billion years after Big Bang.

2. Evidence for the Big Bang

A. Cosmic Microwave Background (CMB)

• Leftover radiation from ~380,000 years after Big Bang.

• Detected by Penzias & Wilson (1965).

• Perfect thermal spectrum at 2.73 K, now redshifted to microwaves.

• Shows slight fluctuations (structure "seeds").

B. Elemental Abundances

• Big Bang Nucleosynthesis predicted:

  • ~75% H, ~25% He, trace deuterium/lithium.

• Matches observed light element abundances in primordial gas.

3. Inflation

Solves Three Mysteries:

1. Structure Origin:

   • Tiny quantum fluctuations stretched to large scales.

2. Uniformity:

   • Opposite sides of the universe once close → explains similar CMB temperature.

3. Geometry:

   • Inflation flattens spacetime → total density ≈ critical density (flat universe).

Support for Inflation

• CMB data (e.g., WMAP, Planck):

  • Confirms predicted fluctuations ("seeds").

  • Universe:

    • ~5% ordinary matter

    • ~27% dark matter

    • ~68% dark energy

    • Age: ~13.8 billion years

4. Olbers’ Paradox and the Night Sky

• Paradox: If the universe were infinite, static, and eternal, the night sky would be as bright as the Sun.

• Resolution:

  • The universe has a finite age.

  • There hasn’t been enough time for light from all stars to reach us.

  • Night sky darkness supports a dynamic, evolving universe.