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 (D)</p> Signup and view all the answers

In a flat universe, what happens to parallel lines?

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

What is indicated by new observations of the universe?

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

What occurs during the Planck Era of the universe?

<p>Gravity is quantized. (D)</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. (A)</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. (C)</p> Signup and view all the answers

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

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

What constitutes the space velocity of a star?

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

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

<p>Observing supernovae (B)</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. (A)</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. (D)</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. (D)</p> Signup and view all the answers

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

<p>Elliptical galaxies (D)</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. (A)</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. (C)</p> Signup and view all the answers

What does the Hubble Tuning Fork Diagram classify?

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

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

<p>Sc (C)</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. (C)</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. (C)</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. (B)</p> Signup and view all the answers

Flashcards

Galaxy

A massive collection of stars, gas, and dust bound together by gravity.

Milky Way

The spiral galaxy we inhabit. All the stars we see at night are part of it.

Globular Clusters

Tightly packed groups of stars, often found in the halo of galaxies, with a high concentration of older, low-mass stars.

RR Lyrae Variables

Pulsating stars whose pulsation period directly correlates with their absolute magnitude (brightness).

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Proper Motion

The apparent change in a star's position in the sky over time due to its actual motion.

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Tangential Velocity

The speed of a star perpendicular to our line of sight. It's related to proper motion and distance.

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Radial Velocity

The speed of a star towards or away from us, measured by the Doppler shift of its spectral lines.

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Local Standard of Rest (LSR)

A reference point in space with a velocity averaging the motion of nearby stars, including our Sun.

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What is the Planck Era?

The first stage of the universe, from 0 to 10^-43 seconds after the Big Bang. This era is characterized by incredibly high temperatures and energies, where all four fundamental forces were unified.

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What is the Grand Unified Field Era?

This era, lasting from 10^-43 to 10^-35 seconds after the Big Bang, saw no normal atoms but involved the strong, weak, and electromagnetic forces becoming unified.

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What is the Electro-weak Era?

From 10^-35 to 10^-10 seconds after the Big Bang, the strong force separated from the other forces, leaving three fundamental forces: gravity, strong, and electro-weak. Elementary particles began to form.

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What is the Era of Nucleosynthesis?

This era, spanning from 0.001 seconds to 3 minutes after the Big Bang, saw protons and neutrons combine to form the first atoms, primarily hydrogen and helium.

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What is the Inflationary Epoch?

A period very early in the universe's history (around 10^-35 seconds) when the universe expanded faster than the speed of light due to the decay of an energy field known as the Higgs field.

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What is the difference between a flat and a hyperbolic universe?

A flat universe will expand forever but at a constant rate. A hyperbolic universe will expand forever and accelerate outward.

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How does the shape of the universe affect our understanding of parallel lines?

In a flat universe, parallel lines remain parallel. In a hyperbolic universe, parallel lines eventually diverge.

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What is the significance of the Cosmic Microwave Background Radiation?

It's a faint glow of radiation from the early universe, offering evidence for the Big Bang and providing insights into the universe's shape and expansion.

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Galactic Rotation Curve

A plot of the rotational speed of objects around the galactic center at different distances from the center.

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Missing Mass Problem

The discrepancy between the observed rotational speed of galaxies and predictions based on visible matter (stars, gas, and dust).

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21-cm Line of Hydrogen

A specific radio wavelength emitted by neutral hydrogen atoms, used to map the distribution of hydrogen in galaxies.

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Kepler's Third Law (Galaxy)

Used to estimate the mass of the galaxy by relating the orbital period of the Sun around the galactic center to the distance and gravitational force.

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Galactic Center

The densest and most active region of a galaxy, often containing a supermassive black hole.

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Spiral Galaxy (Sa, Sb, Sc)

Disk-shaped galaxies with spiral arms, classified by the tightness of the arms and the size of the central bulge.

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Barred Spiral Galaxy (SBa, SBb, SBc)

Spiral galaxies with a central, bar-shaped structure instead of a bulge, also classified based on arm tightness.

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Elliptical Galaxy (E0-E7)

Smooth, oval-shaped galaxies with little gas and dust, classified by their degree of ellipticity.

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Irregular Galaxy

Galaxies with no defined shape or structure, often with gas and dust.

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Hubble Tuning Fork Diagram

A classification scheme for galaxies, arranging them based on their morphology (shape and structure), but NOT evolutionary sequence.

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Spiral Arms (Gravitational Density Waves)

Regions of compressed gas and stars in spiral galaxies, formed by gravitational instabilities. These regions promote enhanced star formation.

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Hubble's Law (Galaxies)

The observation that galaxies are moving away from us, with speed proportional to their distance. This supports the idea of an expanding universe.

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Recessional Velocity (Galaxies)

The speed at which galaxies are moving away from us, measured using the redshift of their light.

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Hubble Constant (Ho)

A constant of proportionality in Hubble's Law, representing the rate of expansion of the universe.

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Supernova (Distance Measurement)

Exploding stars used as standard candles to determine distances to galaxies, as their intrinsic brightness is known.

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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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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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