Our Galaxy and Stellar Populations

Our Galaxy

• Galaxy: A collection of stars, gas, and dust bound by gravity.
• Milky Way: Our spiral galaxy; all night sky stars belong to it.
• Galileo: First to observe the Milky Way's starry composition.
• William Herschel: Incorrectly calculated our position as the center of the Milky Way by failing to account for interstellar obscuration.
• Harlow Shapley: Studied globular clusters, using RR Lyrae variables to determine distances and pinpoint the galactic center's location.
• RR Lyrae variables: Pulsating stars with a period-luminosity relationship, allowing distance calculation using the apparent and absolute magnitudes.
• Interstellar gas & dust (1930s): Discovered by Trumpler; obstructs our view of the galactic center
• Milky Way structure: Flattened disk; Sun located ~25,000 light-years from the center, surrounded by globular clusters.
• Galactic disk: Contains gas and dust, obscuring the galactic center.

Stellar Populations

• Population I: O and B stars, open clusters, high metal content, type I Cepheid variables.
• Population II: Low-mass stars, globular clusters, low metal content, type II Cepheids.

Stellar Motions and Galactic Rotation

• Proper motion (m): Angular change in a star's position due to motion; measured in seconds of arc per second.
• Tangential velocity (T): Speed of a star across the line of sight; related to proper motion and distance via T = 4.7 m d km/sec (d in parsecs)
• Radial velocity (R): Speed along the line of sight; measured using the Doppler shift of spectral lines (R = Dl/l0)
• Space velocity: The vector sum of tangential and radial velocities.
• Local Standard of Rest (LSR): Point in space with average velocity (≈ 230 km/sec) of stars within 100 parsecs of the Sun.

Mass of the Galaxy

• Calculation method: Sun's distance from galactic center (rSun = 25,000 ly ); orbital speed; Kepler's third law. This results in an orbital period T ~ 200,000 years. This is then used in Kepler’s third law to determine a galactic mass of Mgal ~ 9.4x10^10 MSun (solar masses)

• Galactic Rotation Curves: Plot of rotational speed vs. distance from the galactic center.

• Missing Mass Problem: Measured rotation curves don't match expected values; solved by the presence of small, unseen brown dwarf stars in the galactic halo.

Galactic Center

• Obscuration: Gas and dust absorb visible light.
• Gamma-rays & radio waves penetrate obscuration. Evidence suggests a supermassive black hole at the galactic center with at least 2,000,000 solar masses within 2 light-years of the center.

The Universe of Galaxies

• Kant (1755): Speculated about "Island Universes."
• Rosse (1845): Observed spiral structure in nebulae.
• Herschel & son: Catalogued 10,000+ nebulae.
• Great Debate (1920s): Shapley vs. Curtis; Shapley argued spiral nebulae were part of our galaxy; Curtis argued they were separate galaxies.
• Hubble: Used Cepheid variables in the Andromeda spiral nebula to determine Andromeda's distance (~2.25 million light-years), thus confirming it as an external galaxy.

Types of Galaxies

• Spiral Galaxies (Sa, Sb, Sc): Categorized by arm tightness and nuclear prominence.
• Barred Spiral Galaxies (SBa, SBb, SBc): Similar to spirals, but with a bar-shaped structure.
• Elliptical Galaxies (E0-E7): Characterized by little gas and dust.
• Irregular Galaxies: Lack a specific structure.

Hubble Tuning Fork Diagram

• Classification system for galaxies, but not an evolutionary timeline.

Spiral Arms

• Gravitational density waves caused by gravitational instabilities.

Masses and Sizes of Galaxies

• Ranges for Spiral to Irregular galaxies in terms of mass and size
• Galaxy cluster types: Rich (thousands of galaxies) versus poor (few dozens).

Distances to Galaxies and Hubble's Law

• Slipher's work: Observed redshifted spectral lines, indicating galaxies' recession.
• Hubble-Humason's work: Observed redshifted lines for many galaxies: galaxies are moving away.
• Hubble's Law: Direct correlation between distance and recessional velocity of galaxies; V = Ho D. (V is recessional velocity in km/sec, D is distance in Megaparsecs, and Ho is Hubble's constant in km/sec-Mpc).
• Method of determining galaxy distances: size (apparent), brightness (apparent), Size of HII regions, supernovae types (as the most accurate).
• Hubble's constant: Refined measurements from space telescopes, currently around ~65 km/sec/Mpc.

The Big Bang Theory and the History of the Universe

• Big Bang theory: Universe began from an infinitely small point.
• Universe's shapes: Spherical (closes, collapses; hyperbolic (open, expanding forever); flat (expanding forever).
• History of the universe: Planck Era, Grand unified era, electro-weak era, particle era, nucleosynthesis era, atomic era, formation of galaxies and stars, evolution of stars and galaxies.
• Inflationary epoch (10-35 second after Big Bang): Period of extremely rapid expansion.
• Future fate of the universe: Accelerated expansion, leading to a hyperbolic forever expanding universe.
• Age of the universe: ~15 billion years.