Optical Astronomy Concepts Quiz

PH222 - Astrophysical Concepts

• The course is about astronomical concepts.
• The presenter is Aaron Golden from the Centre of Astronomy, School of Natural Sciences at the University of Galway.

Interstellar Medium (ISM)

• Stars create most of the light in a galaxy, but occupy a small fraction of the volume.
• The space between stars is not empty; it's filled with the ISM.
• Stars interact with the ISM through various mechanisms; star formation, stellar winds, radiation, and supernova explosions.
• The ISM exhibits diverse features, such as emission nebulae, dust and gas columns, blast waves from supernovae, and collapsing proto-stellar clouds.
• The proportion of visible baryonic mass in the ISM differs greatly between galaxies: a few percent for elliptical galaxies, approximately 10% for spiral galaxies, and up to 50% for irregular and dwarf galaxies.

Description of the ISM

• Early galaxies were small and irregular, with frequent interactions triggering star formation.
• Star formation depletes the ISM, but massive stars replenish it via supernovae and stellar winds.
• Each generation of stars is more metal-rich than the previous one.
• Metallicity (Z) is defined as the fraction of a star's mass other than hydrogen and helium.
• The Sun, for example, has a Z of 0.02.
• Different stellar populations exist based on their origin time.

Stellar Populations

• Population I stars:
  • Have a flat distribution in the galactic plane.
  • Relatively young, up to about 10⁹ years old.
  • Found in spiral arms.
  • Contain higher metallicity.
  • Often found in clusters (but not always).
• Population II stars:
  • Have a spheroidal distribution (not confined to a single plane).
  • Older than 10¹⁰ years.
  • Located in the galactic bulge and halo.
  • Contain lower metallicity.
  • Tend to be found in bound clusters.

Population I and II stars in the Milky Way

• Population I: Younger stars in the disk, enriched in heavy elements (up to 5%).
• Population II: Older stars in the bulge and halo, low in heavy elements (less than 1%).

Composition of the ISM

• Main physical components: gas, dust, cosmic rays, radiation, and magnetic fields.
• Main chemical components: Hydrogen (relative abundance to H=1.0), Helium (0.1), Carbon (10^-4), Nitrogen (1.2x10^-4), Oxygen (8x10^-4).
• Dust: Small solid particles (1 to 100 nm) which absorb and re-radiate radiation in cold, dense clouds; crucial for reactions.
• Gas: Diverse forms with varying temperatures and densities, like coronal gas (very hot, low density) and H II regions (hot, ionised hydrogen).

The Milky Way Galaxy from above

• Diagram shows distribution of diffuse HI, molecular H2, and hot WIM gas.
• The gas tracks the spiral arm structure.

Collective effects of matter at low densities

• Inter-cloud gas: T ~ few 10³ K; n ~ 3 x 10^⁵ m^-3; neutral H mixed with ionised H.
• Diffuse clouds: T ~ 50 – 100 K; n ~ 3 x 10⁷ m^-3; atomic hydrogen, other elements are mildly ionised.

Measuring the 21cm Hydrogen line

• The 21cm hydrogen line is used to reconstruct the Milky Way's spiral arm structure.

Spectroscopic parallax

• We can deduce a star's absolute magnitude (M) from its spectral class (using an H-R diagram assumption it is on the main sequence)
• Then using apparent magnitude (m) and the formula M = m - 5log(d) + 5, we can determine distance (d) to the star.

Interstellar Extinction

• Extinction is wavelength dependent.
• As wavelength increases, extinction decreases making background stars visible.
• Effects of extinction depend on the density of material along the line of sight.

Inter-stellar extinction (reddening)

• Extinction is wavelength dependent, called reddening.
• A is the magnitude of extinction.
• A ≈ τ (optical depth).

Test your knowledge on key concepts in optical astronomy, including apparent magnitude, extinction, and the interstellar medium. This quiz covers important topics such as baryonic mass and the significance of the 21cm Hydrogen line. Match terms with their descriptions to deepen your understanding of stellar populations and chemical reactions in space.