Our Galaxy and Stellar Populations

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Questions and Answers

What characterizes a hyperbolic universe?

The sum of angles in a triangle is greater than 180 degrees.

Parallel lines diverge and angles in a triangle sum to less than 180 degrees. (correct)

The universe is static and neither expands nor contracts.

Parallel lines eventually intersect.

What occurred during the era of nucleosynthesis?

Only protons and neutrons formed.

Elementary particles formed in a high-temperature environment.

The strong force split from the weak and electromagnetic forces.

Atoms formed and radiation became free from matter. (correct)

What was the state of the universe at T = 0?

The universe was already formed but not yet evolved.

Space, time, matter, and energy were all contained in a singularity. (correct)

The universe was infinite and spacious.

It was filled with matter and energy at a steady rate.

Which statement is true regarding the fate of the universe?

The universe will expand forever in a hyperbolic manner. Signup and view all the answers

What significant event occurred during the Inflationary Epoch?

The Higgs field decayed and caused rapid expansion. Signup and view all the answers

What defines a flat universe?

The universe will expand forever without accelerating. Signup and view all the answers

Which era describes the formation of elementary particles?

Particle Era Signup and view all the answers

During which time frame did the strong force split off from the other forces?

10-35 to 10-10 sec Signup and view all the answers

What is the primary reason William Herschel's conclusion about the Milky Way's structure was incorrect?

He did not account for the gas and dust blocking distant stars. Signup and view all the answers

Which type of stars are included in Population I of the Milky Way?

Type I Cepheid variables and O and B stars Signup and view all the answers

How did Harlow Shapley determine distances to globular clusters?

Using the period-luminosity relationship of RR Lyrae variables Signup and view all the answers

What does the formula $M = m - 5 log(d) + 5$ help to determine?

The distance to a star in parsecs Signup and view all the answers

What is the Local Standard of Rest (LSR) defined in relation to?

The average velocity of stars within 100 parsecs from the Sun Signup and view all the answers

Which statement accurately describes the structure of the Milky Way Galaxy?

It consists of a flattened disk with globular clusters surrounding the center. Signup and view all the answers

What does proper motion measure in stars?

Angular change in position due to stellar motion Signup and view all the answers

What is the characteristic of RR Lyrae variables that makes them useful for distance measurement?

They have a consistent period-luminosity relationship. Signup and view all the answers

What is the estimated mass of the Galaxy?

$9.4 \times 10^{10} M_{\text{Sun}}$ Signup and view all the answers

Which type of galaxy has tightly wound arms and a prominent nucleus?

Spiral Sa Signup and view all the answers

What causes the gravitational density waves that lead to the formation of spiral arms?

Gravitational instabilities Signup and view all the answers

What phenomenon allows astronomers to study the highly obscured galactic center?

Gamma rays and radio waves Signup and view all the answers

Which law is used to calculate the mass of the Galaxy?

Kepler's Third Law Signup and view all the answers

What is the relationship described by Hubble's Law?

Distance and recessional velocity of galaxies Signup and view all the answers

What method provides the most accurate distance measurement to galaxies?

Supernova observations Signup and view all the answers

What is a common characteristic of elliptical galaxies?

Very little gas and dust Signup and view all the answers

What issue arises from the measured rotation curves of galaxies?

Models based on visible matter do not match up Signup and view all the answers

How was it determined that the Andromeda Nebula is an external galaxy?

By observing Cepheid variables Signup and view all the answers

What is the approximate current value of Hubble's constant?

65 km/sec/Mpc Signup and view all the answers

What distinguishes irregular galaxies from other types?

They have no defined shape Signup and view all the answers

What was the 'Great Debate' in 1920 about?

The nature of Nebulae Signup and view all the answers

What is true regarding the size of spiral galaxies?

They vary from 20,000 to 50,000 light years in size Signup and view all the answers

Study Notes

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.

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Description

Explore the fascinating structure and components of our galaxy, the Milky Way, along with the significant contributions of astronomers like Galileo and William Herschel. Learn about stellar populations, including Population I stars and their characteristics, as well as the role of interstellar gas and dust. This quiz delves into the science behind our cosmic neighborhood.