Overview
This lecture covered the structure, composition, and formation of galaxies, with an emphasis on the Milky Way. Key topics included galaxy mass determination, spiral arms, dark matter, star populations, galaxy formation theories, and academic pathways in physics and astronomy.
Participation & Consultation
- Participation in online classes and watching lecture videos is tracked.
- Limited student engagement may result in removal of lecture materials from the platform.
- Instructor available for in-person or virtual consultations; email is preferred for official communication.
Current Science News & Observations
- El Niño is contributing to heavy rains and flooding.
- Planetary alignment and a solar eclipse observed from Mars were recent astronomical events.
- The Large Hadron Collider has resumed operation, searching for dark matter and energy.
Galactic Structure & Mass
- The Milky Way includes a central bulge, disk, spiral arms, and a halo.
- The Sun orbits the galaxy center at about 225 km/s; its period is ~225 million years.
- The galaxy’s estimated mass is at least 400 billion solar masses, much of which is dark matter.
- Rotation curves suggest mass extends far beyond the visible disk, indicating a galactic "corona" of dark matter.
Spiral Arms & Star Formation
- Spiral arms are traced by young, hot O and B stars, open clusters, and emission nebulae.
- These arms are regions of higher gas density and frequent star formation.
- Spiral Density Wave Theory explains stable spiral patterns and ongoing star formation.
- Self-sustaining star formation via supernovae and stellar winds can create spiral branches and spurs.
Galactic Center & Black Holes
- The Milky Way’s center is obscured by dust but observable in radio and infrared.
- Sagittarius A* is a radio source marking the location of a supermassive black hole (~4 million solar masses).
- Evidence supports a supermassive black hole’s presence, but alternative explanations have been considered.*
Chemical Evolution & Stellar Populations
- Metals in astronomy are all elements heavier than helium and are rare in the universe.
- Population I stars (metal-rich, young, disk component) and Population II stars (metal-poor, old, halo/bulge) differ in age, location, and chemical makeup.
- The Sun is a Population I star.
Formation & Age of the Galaxy
- Oldest open clusters: 9–10 billion years; globular clusters: about 11–13 billion years.
- The disk is younger than the halo; studies of star clusters provide age estimates for galactic components.
- The monolithic collapse (top-down) hypothesis proposes the galaxy formed from a single gas cloud; the bottom-up hypothesis suggests a merger of smaller systems.
Galaxy Classification & Properties
- Galaxies are classified as spiral, barred spiral, elliptical, lenticular, peculiars, and irregulars, based on visual appearance.
- Spiral galaxies are prominent but ellipticals are more numerous, especially among faint galaxies.
- Galaxy properties (size, luminosity, mass) are determined once distances are measured.
Academic Pathways & Research
- Bachelor’s: broad background; Honours: specialization begins; Master’s: research focus; PhD: world expert.
- Choosing a research supervisor and topic is crucial for success in final-year projects.
Key Terms & Definitions
- Dark Matter — Invisible mass in galaxies, detected by gravitational effects.
- Spiral Density Wave Theory — Explains the stable, spiral structure in disk galaxies as regions of high density.
- Population I/II Stars — Metal-rich young stars in the disk (I) vs. metal-poor old stars in the halo/bulge (II).
- Sagittarius A* — Central radio source in the Milky Way indicating a supermassive black hole.
- Monolithic Collapse/Bottom-Up Hypothesis — Competing theories for galaxy formation from a single cloud or merging smaller units.
Action Items / Next Steps
- Attend consultation sessions as needed (in person or online).
- Watch posted lecture videos if not already completed.
- Prepare for next week’s quiz (online, closed book).
- Complete ongoing assignments (e.g., moon project with original observations).
- Review classification types and properties of galaxies for upcoming lectures/exams.