Astronomy Chapter 23: Our Galaxy

Our Galaxy (Chapter 23)

• Galaxy: A gravitationally bound collection of stars, gas, and dust.
• Milky Way: Our spiral galaxy, containing all visible night sky stars.
• Galileo: First to observe the Milky Way as countless individual stars.
• William Herschel: Incorrectly thought we were at the Milky Way's center by counting stars in 683 regions, neglecting obscuring gas & dust.
• Harlow Shapley: Studied globular clusters and RR Lyrae variables to determine their distances, establishing the Milky Way's overall structure and our location away from the center.
• RR Lyrae variables: Pulsating stars with a period-luminosity relationship used to measure distance.
• Interstellar gas and dust: Discovered in the 1930s, these materials obscure our view of the galactic center.
• Milky Way structure: A flattened disk with the Sun approximately 25,000 light-years from the galactic center. Globular clusters are concentrated around the center. We reside in the disk, where gas and dust are located.
• Stellar Populations:
• Population I: O and B stars, open clusters, high metal content, type 1 Cepheid variables.
• Population II: Low-mass stars, globular clusters, low metal content, type II Cepheids.
• Stellar Motions:
• Proper motion: Angular change in stellar position due to motion.
• Tangential velocity: Stellar speed across the line of sight, calculated using proper motion and distance (T = 4.7md km/sec, where d is in parsecs).
• Radial velocity: Stellar speed along the line of sight, measured using Doppler shifts of spectral lines.
• Space velocity: The combined tangential and radial velocities.
• Local Standard of Rest (LSR): A point in space with an average velocity of stars within 100 parsecs of the Sun (approximately 230 km/sec).
• Galaxy Mass: Estimated using the Sun's distance from the galactic center, orbital speed, and Kepler's third law. Approximately 9.4 x 10¹⁰ solar masses (M_sun).
• Galactic Rotation Curves: Measured using the 21-cm hydrogen line, showing disagreement with models based on visible matter.
• Missing Mass: Galaxies rotate faster than expected, indicating unseen mass (likely dark matter).
• Galactic Center: Highly obscured by gas and dust, studied using gamma-rays and radio waves. Evidence suggests a supermassive black hole.

The Universe of Galaxies (Chapter 24)

• Kant (1755): Speculated about "island universes" (separate galaxies).
• Lord Rosse (1845): Observed spiral structure in nebulae.
• Herschel & Son: Cataloged thousands of nebulae.
• The Great Debate (1920): Shapley vs. Curtis – Shapley argued spiral nebulae were in our galaxy; Curtis said they were external galaxies.
• Edwin Hubble: Determined Andromeda was an external galaxy (2.25 million light years away) using Cepheid variables.
• Galaxy Types:
• Spiral: Sa (tight arms, prominent nuclei), Sb (moderate arms, moderate nuclei), Sc (loose arms, small nuclei).
• Barred Spiral: SBa (tight arms, prominent nuclei), SBb (moderate arms), SBc (loose arms).
• Elliptical: E0 to E7 (little gas & dust)
• Irregulars: No distinct shape (with gas/dust).
• Hubble Tuning Fork Diagram: A classification system, not an evolutionary sequence.
• Spiral Arms: Gravitational density waves that compress gas/dust, triggering star formation.
• Galaxy rotational curves: Rotation speed doesn't decrease with distance from the center as expected, suggesting missing mass (dark matter).
• Galaxy Masses & Sizes: Vary greatly depending on type.
• Galaxy Clusters: Groups of galaxies held together by gravity, ranging in size from rich clusters (thousands of galaxies) to poor clusters (dozens).
• Local Group: The cluster containing our Milky Way Galaxy.
• Hubble's Law: Recessional velocity (speed of galaxies moving away) is directly proportional to distance.

Distances to Galaxies and Hubble's Law

• Slipher: Discovered redshifted spectra in spiral nebulae, showing they are moving away.
• Hubble & Humason: Observed galaxies had redshifted lines; all but the closest are moving away.
• Hubble's Law: V = HoD (Velocity = Hubble constant x Distance).
• Hubble's Constant (Ho): Determining Ho required measuring distances and velocities of galaxies, and the slope of the velocity-distance plot.
• Methods of determining galaxy distances: Based on apparent size, brightness, HII regions, and most accurately, supernovae.
• Space Telescopes and Hubble Constant: Improved distance measurements leading to a more accurate value for Hubble's constant (~ 65 km/sec/Mpc).

The Big Bang Theory (Chapter 26)

• Big Bang Theory: The universe originated from an infinitely small point.
• Age of the universe: Approximately 15 billion years.
• Shape of the universe: Spherical, flat, or hyperbolic, depending on the density of matter and energy.
• History of the universe: Describes how the universe evolved from the Big Bang to the present.
• Planck Era: Early moments, requires theories beyond current physics.
• Grand Unified Field Era: All fundamental forces unified.
• Electro-weak Era: Strong force separates from electro-weak.
• Particle Era: Elementary particles form.
• Era of Nucleosynthesis: Formation of light elements (H, He).
• Era of Atoms: Formation of neutral atoms and decoupling of radiation.
• Galaxy Formation: Early galaxies form.
• Continued Evolution: Continued formation of stars & galaxies and the evolution of life.
• Inflationary Epoch: A period of incredibly rapid expansion very early.
• Future Fate: Observations indicate an accelerating expansion, implying a hyperbolic universe, expansion will continue indefinitely.

Explore Chapter 23 on our galaxy, the Milky Way. Learn about its structure, key astronomers like Galileo and Harlow Shapley, and the significance of RR Lyrae variables in measuring distances. Delve into the fascinating components that define our cosmic home.