Galactic Structures and Universe Expansion: Key Insights

Posted on Dec 24, 2024 in Physics

Structure of the Milky Way

Major Components

  • Thin Disk: Contains most of the galaxy’s gas, dust, young stars, and active star formation (e.g., massive young stars in spiral arms).
  • Thick Disk: Older stars, less gas and dust.
  • Bulge: Dense cluster of old stars; includes the galactic center and possibly a bar structure.
  • Halo: Sparse outer region containing ancient Population II stars and globular clusters; dominated by dark matter.

Challenges in Mapping

  • The Sun’s position inside the galaxy obscures our view.
  • Dust in the disk reduces visibility beyond ~6,000 light-years.
  • Measuring distances accurately is difficult due to extinction and brightness calibration issues.

Shapes of Galaxies

Types

  • Spiral: Flat disks with central bulges and spiral arms. Active star formation occurs in arms (e.g., Milky Way).
  • Elliptical: Spherical to elongated shapes with little gas or dust; composed mostly of old stars.
  • Irregular: Chaotic, small galaxies often formed by interactions or mergers.
  • Lenticular: Transition type between spiral and elliptical, with disks but no spiral arms.
  • Peculiar: Result from galaxy mergers; exhibit distorted features like tidal tails.

Mass of Galaxies

Measuring Techniques

  • Rotation Curves: Measure orbital speeds of stars and gas to infer mass distribution.
  • Gravitational Lensing: Deflection of light by massive galaxies reveals mass (including dark matter).

Dark Matter

Galaxies’ rotation curves remain flat far from the center, indicating unseen mass. Dark matter halos likely extend beyond the visible galaxy.

Supermassive Black Holes

  • Found in the centers of most galaxies.
  • Sagittarius A*:
    • Mass: ~3 million solar masses.
    • Measurement: Tracks stellar orbits near the black hole using Kepler’s Laws.

Stellar Populations

  • Population I: Metal-rich, younger stars found in disks and spiral arms.
  • Population II: Metal-poor, older stars found in halos and globular clusters.
  • Population III: Hypothetical, primordial stars with no metals; not yet observed.

Standard Candles

  • Cepheid Variables: Brightness varies predictably with pulsation periods, allowing distance calculations for nearby galaxies.
  • Type Ia Supernovae: Uniform peak luminosity; used for measuring vast cosmic distances. Provide key evidence for dark energy due to their observed dimming in an accelerating universe.

Galaxy Interactions

Outcomes

  • Mergers can result in elliptical galaxies or trigger intense star formation (starbursts).

Indicators

  • Elevated star formation rates.
  • Tidal tails and distorted shapes.
  • Quasars: Form when supermassive black holes accrete large amounts of material during mergers.

The Hubble Constant

  • Describes the rate of expansion of the universe.
  • Current value: ~71 km/s/Mpc.
  • Variability: May have been different in the past due to changes in cosmic density and dark energy effects.

Dark Matter and Dark Energy

  • Dark Matter: Explains flat rotation curves of galaxies. Indirectly observed via gravitational lensing and galaxy clustering.
  • Dark Energy: Causes accelerated cosmic expansion. Detected via supernova observations and cosmic microwave background studies.

Cosmic Microwave Background (CMB)

  • Formation: Occurred ~380,000 years post-Big Bang when the universe cooled enough for atoms to form, making it transparent.
  • Appearance Today: Observed as isotropic 2.73 K radiation stretched by cosmic expansion.

Gravitational Lensing

  • Light from distant objects bends around massive objects.
  • Confirms predictions of general relativity.
  • Used to map dark matter distribution in the universe.

Composition of the Universe

68% dark energy, driving expansion. ~27% dark matter, ~5% baryonic matter (stars, planets, gas, dust).

Active Galactic Nuclei (AGN)

  • Includes quasars, Seyfert galaxies, and blazars.
  • Powered by accretion disks around supermassive black holes.
  • Bright, energetic emissions dominate galaxy centers.

Gravitational Waves

  • Ripples in spacetime caused by massive accelerating objects (e.g., merging black holes or neutron stars).
  • Detected by LIGO and Virgo observatories.

Key Questions and Answers

1. Why is it difficult to determine the structure of the Milky Way?

  • Location: The Sun is embedded within the disk, making it hard to view the entire galaxy.
  • Dust Obscuration: Interstellar dust blocks light, particularly from regions beyond 6,000 light-years.
  • Extinction: Distant objects appear dimmer due to scattering and absorption by dust.
  • Distance Measurement Issues: Determining accurate distances to stars is challenging without precise parallax or brightness calibrations.

2. What are the major parts of the Milky Way?

  • Thin Disk: Contains ~95% of the galaxy’s gas and dust. Active star formation occurs here (e.g., in spiral arms). Hosts Population I stars (young, metal-rich stars).
  • Thick Disk: Older stars with slightly lower metallicities.
  • Bulge: Densely packed region of old stars. Contains the supermassive black hole, Sagittarius A*.
  • Halo: Sparse, extended region with Population II stars (metal-poor, ancient stars). Includes globular clusters and dark matter.
  • Spiral Arms: Regions of star formation. Contain ionized hydrogen regions and massive young stars.

3. How do we determine the mass of the Milky Way?

  • Inside the Sun’s Orbit: Use stellar velocities and apply Newton’s laws. Measure orbital speeds of stars and gas relative to the galactic center.
  • Outside the Sun’s Orbit: Flat rotation curves suggest dark matter dominates beyond visible regions. Stars farther out move faster than expected based on visible mass, indicating a significant dark matter halo.

4. How do we measure the mass of Sagittarius A*?

  • Techniques: Observe orbits of stars near the galactic center using high-resolution telescopes. Apply Kepler’s laws to infer mass from orbital periods and distances.
  • Findings: Sagittarius A* has a mass of ~3 million solar masses, concentrated in a tiny volume.

5. How is Population I different from Population II?

  • Population I: Found in the thin disk and spiral arms. Metal-rich stars with higher fractions of elements heavier than helium. Includes younger stars (e.g., the Sun).
  • Population II: Found in the halo and bulge. Metal-poor stars, older and less massive.
  • Population III (Theoretical): The first stars formed after the Big Bang, composed purely of hydrogen and helium.

6. Which standard candle is best for distant galaxies?

  • Type Ia Supernovae: Extremely bright and observable across vast distances. Known intrinsic brightness due to uniform light curves. Distances calculated by comparing intrinsic luminosity to observed brightness using the distance modulus formula.
  • Cepheid Variables: Useful for nearby galaxies, but not bright enough for extremely distant objects.

7. What is the Hubble constant? Is it really constant?

  • Definition: The Hubble constant measures the rate of universe expansion. Describes the relationship between a galaxy’s distance and its recession velocity.
  • Variability: Not truly constant over time:
    • Early Universe: Rapid inflation during the Big Bang caused a higher expansion rate.
    • Recent Epochs: Expansion has accelerated due to dark energy.

8. What makes astronomers think that dark matter exists?

  • Key Evidence:
    • Galaxy Rotation Curves: Stars in outer regions of galaxies orbit faster than expected from visible mass.
    • Gravitational Lensing: Observed bending of light exceeds predictions based on visible mass alone.
    • Galaxy Clusters: Total mass inferred from motions within clusters greatly exceeds visible matter.
  • Properties: Does not emit, absorb, or reflect light (hence “dark”). Interacts with normal matter through gravity.

9. What makes astronomers think that dark energy exists?

  • Evidence: Observations of Type Ia supernovae show that distant supernovae are dimmer than expected, indicating an accelerating expansion.
  • Cosmic Microwave Background: Imprints suggest dark energy comprises ~68% of the universe.
  • Implications: Opposes gravitational pull, driving galaxies apart faster over time.

10. When and how did the CMB form?

  • Formation: Occurred ~380,000 years after the Big Bang during the recombination era. As the universe cooled, electrons combined with nuclei, allowing photons to travel freely. Marked the universe’s transition from opaque to transparent.
  • Today: Appears as isotropic radiation with a temperature of ~2.73 K. Redshifted from its original 3,000 K due to cosmic expansion.

11. How does galaxy motion provide evidence for the Big Bang?

  • Hubble’s Law: Galaxies move away from us at speeds proportional to their distances, suggesting a uniform expansion.
  • Rewinding Time: If the universe expands in all directions, reversing this motion leads to a single origin point—the Big Bang.
  • Redshift: Light from distant galaxies is stretched (redshifted), confirming their increasing separation.