Our Galaxy
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Questions and Answers

What best describes the fate of a hyperbolic universe?

  • It will reach a stable size and stop expanding.
  • It will collapse back on itself.
  • It will expand forever at a decreasing rate.
  • It will continuously accelerate outward. (correct)

    • During which era did the strong force separate from the combined forces?

  • Era of Nucleosynthesis
  • Electro-weak Era (correct)
  • Planck Era
  • Grand Unified Field Era

    • What occurs during the Era of Nucleosynthesis?

  • Atoms form and radiation becomes free from matter. (correct)
  • The first galaxies begin to form.
  • Only light elements like hydrogen and helium form.
  • Fundamental forces unify into one force.

    • What phenomenon is necessary to explain observations of the background radiation?

    <p>Rapid expansion during the inflationary epoch</p> Signup and view all the answers

    In a flat universe, what happens to parallel lines?

    <p>They remain parallel indefinitely.</p> Signup and view all the answers

    What is indicated by new observations of the universe?

    <p>The universe is accelerating in expansion.</p> Signup and view all the answers

    What occurs during the Planck Era of the universe?

    <p>Gravity is quantized.</p> Signup and view all the answers

    What is the sum of the angles in a triangle in a hyperbolic universe?

    <p>Less than 180 degrees.</p> Signup and view all the answers

    What misconception did William Herschel have about the location of the Sun in the Milky Way?

    <p>He thought the Sun was at the center of the galaxy.</p> Signup and view all the answers

    How is the distance to a globular cluster calculated using RR Lyrae variables?

    <p>With the formula $M = m - 5 ext{log}(d) + 5$.</p> Signup and view all the answers

    What is the primary reason the Milky Way appears as a flattened disk rather than a spherical shape?

    <p>The outward force from rotation causes the shape to flatten.</p> Signup and view all the answers

    Which statement accurately describes the Local Standard of Rest (LSR)?

    <p>It is the average velocity of all stars, including the Sun, within 100 parsecs.</p> Signup and view all the answers

    What distinguishes Population I stars from Population II stars?

    <p>Population I stars are younger and have higher metal content.</p> Signup and view all the answers

    How is tangential velocity related to proper motion and distance of a star?

    <p>It is calculated as $T = 4.7m d$ where d is in parsecs.</p> Signup and view all the answers

    What constitutes the space velocity of a star?

    <p>The combination of tangential and radial velocities.</p> Signup and view all the answers

    What discovery did Trumpler make regarding interstellar gas and dust?

    <p>It plays a significant role in obscuring our view of the galactic center.</p> Signup and view all the answers

    How do we calculate the mass of the galaxy according to Kepler’s third law?

    <p>M = 4π²a³/GP²</p> Signup and view all the answers

    What does the term 'missing mass problem' refer to in the context of galactic rotation curves?

    <p>The difference between the observed and predicted rotation speeds of galaxies.</p> Signup and view all the answers

    Which type of galaxy exhibits tightly wound arms and a prominent nucleus with gas and dust in its disk?

    <p>Sa</p> Signup and view all the answers

    Which method is considered the most accurate for determining the distance to distant galaxies?

    <p>Observing supernovae</p> Signup and view all the answers

    What do galactic rotation curves suggest about the mass distribution in galaxies?

    <p>There is a significant amount of unseen material involved.</p> Signup and view all the answers

    Which significant astronomical observation did Edwin Hubble make using a 100-inch telescope?

    <p>Measured the first accurate distance to the Andromeda galaxy.</p> Signup and view all the answers

    What is the significance of Hubble’s Law in the context of galaxies?

    <p>It establishes a correlation between recessional velocity and distance of galaxies.</p> Signup and view all the answers

    Which type of galaxy typically contains very little gas and dust?

    <p>Elliptical galaxies</p> Signup and view all the answers

    What was one of the key outcomes of the Great Debate in the 1920s regarding nebulae?

    <p>It identified spiral nebulae as external galaxies.</p> Signup and view all the answers

    What causes the compression of stars, gas, and dust in a spiral galaxy's arms?

    <p>Gravitational density waves.</p> Signup and view all the answers

    What does the Hubble Tuning Fork Diagram classify?

    <p>The types of galaxies and their structural features.</p> Signup and view all the answers

    Which galaxy type exhibits loosely wound arms and a small nucleus?

    <p>Sc</p> Signup and view all the answers

    What is a defining characteristic of irregular galaxies?

    <p>Absence of a clear classification on the Hubble Tuning Fork Diagram.</p> Signup and view all the answers

    Which factor is crucial for determining Hubble’s constant?

    <p>The velocity and distance of a large number of galaxies.</p> Signup and view all the answers

    What did the Big Bang theory predict concerning the universe's expansion?

    <p>The universe started from an infinitely small point and expanded outward.</p> Signup and view all the answers

    Study Notes

    Our Galaxy

    • Galaxy: A collection of stars, gas, and dust gravitationally bound together.
    • Milky Way: The spiral galaxy we inhabit; all visible night sky stars are part of it.
    • Galileo: First to observe the Milky Way as numerous stars.
    • William Herschel (1780s): Incorrectly believed our position was at the Milky Way's center, failing to account for interstellar obscuration.
    • Harlow Shapley: Studied globular clusters, RR Lyrae variables, and determined the location of the galactic center.
    • RR Lyrae variables: Pulsating stars with a known period-luminosity relationship aiding in distance calculations.
    • Interstellar gas and dust (1930s): Trumpler discovered these crucial components that obscure distant star views.
    • Milky Way structure: Flattened disk, with the Sun 25,000 light-years from the center. Globular clusters surround the galactic center. Our location is in the disk, obscured by interstellar gas and dust.

    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 and Galactic Rotation

    • Proper motion (m): Angular change in a star's position due to motion (seconds of arc per second).
    • Tangential velocity (T): Speed of a star across the line of sight, calculated from proper motion and distance [T = 4.7m d (km/sec), d in parsecs].
    • Radial velocity (R): Speed along the line of sight, measured via Doppler shift of spectral lines (R = Dl/l0).
    • Space velocity: The vector sum of 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 (~230 km/sec).

    Mass of the Galaxy

    • Galaxy mass (Mgal): Approximately 9.4 x 1010 solar masses (Msun), calculated via Kepler's third law from orbital period and Sun's distance from the galactic center (25,000 light-years).
    • Galactic rotation curves: Plots of rotational speed versus distance from the galactic center, revealing discrepancies from expected Keplerian orbits, indicating "missing mass".
    • "Missing mass" solution: Small brown dwarf stars in the galactic halo.

    Galactic Center

    • Obscuration: Gas and dust in the galactic plane heavily obscure visible light from the galactic center.
    • Gamma-rays and radio waves: Used to study the galactic center as they penetrate the obscuring material.
    • Expanding gas cloud: Two gas clouds expanding from the galactic center, evidence for an extremely massive object within the central region (possibly a supermassive black hole).

    Universe of Galaxies

    • Immanuel Kant (1755): Speculated about "island universes," now known as external galaxies.
    • Lord Rosse (1845): Observed spiral structure in some nebulae.
    • The Great Debate (1920): Shapley (spiral nebulae in our galaxy) vs. Curtis (spiral nebulae as external galaxies).
    • Edwin Hubble: Used Cepheid variables in the Andromeda spiral nebula to determine its immense distance (2.25 million light-years), proving it was an external galaxy.

    Types of Galaxies

    • Spiral Galaxies: Sa (tight arms, prominent nucleus), Sb (moderate arms, moderate nucleus), Sc (loose arms, small nucleus).
    • Barred Spirals: SBa (tight arms, prominent nucleus), SBb (moderate arms, moderate nucleus), SBc (loose arms, small nucleus).
    • Ellipticals (EO - E7): Little gas and dust.
    • Irregulars: Gas and dust.
    • Hubble Tuning Fork Diagram: Galaxy classification, not an evolutionary sequence.

    Galaxies Distances and Hubble's Law

    • Redshifts of spiral nebulae: Slipher discovered redshifted spectral lines, signifying their recession.
    • Hubble's Law: A direct relationship between a galaxy's distance and its recessional velocity (V = HoD).
    • Hubble constant (Ho): Slope of the graph relating recessional velocity to distance; value was approximately 65 km/sec/Mpc.
    • Methods to determine distances to galaxies: Apparent size and brightness, sizes of HII regions, and Supernovae.

    Big Bang Theory and the Universe's Fate

    • Big Bang theory: Universe began as an infinitesimally small point and expanded, predicting the 3K cosmic microwave background radiation.
    • Einstein's equations: Solutions to equations describing possible universe shapes: spherical, hyperbolic (saddle-shaped), flat.
    • Shape of the universe: New evidence suggests the universe is accelerating outward (hyperbolic) which means it will expand forever .
    • History of the universe: From the Big Bang to the formation of galaxies and stars, including the Planck Era and other epochs.
    • Inflationary epoch: Period of extremely rapid expansion early on in the universe's history, necessary for explaining observations of the background radiation.
    • Future fate of the universe: Ongoing expansion and acceleration.

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    Description

    Explore the fascinating structures and components of our galaxy, the Milky Way. Learn about historical figures like Galileo and Harlow Shapley, and discover key concepts such as RR Lyrae variables and interstellar gas. Perfect for those interested in astronomy and the universe.

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