Lecture 12 (1)

Introduction to Stellar & Galactic Astrophysics

  • Course: PHYC 2452, Intro to Stellar & Galactic Astrophysics

  • Lecture: 12, Structure of the Milky Way & Galactic Distance Scale

  • Date: 27 Feb 2025

  • Note: Dalhousie University is on the ancestral territory of the Mi’kmaq People.

Components of the Milky Way

  • The Milky Way consists of several key populations:

    • Disk: Contains stars and interstellar medium.

    • Halo: Encompasses older stars and globular clusters.

    • Bulge: Central region with a dense star population.

    • Galactic Centre: Point of interest containing a black hole.

    • Globular Clusters: Dense clusters of old stars.

Galactic Centre

  • Sagittarius A (Sgr A)*: The central radio source at the Galactic Centre.

    • Contains a densely packed cluster of stars orbiting about Sgr A*.

    • Stars in the central 0.04 parsecs have orbital velocities ~1000 km/s.

    • Implies Sgr A* is a massive, compact object; evidence of a black hole.

Discovery and Characteristics of Sgr A*

  • Discovered by Karl Jansky in 1931.

  • Represents a supermassive black hole of approximately 4 million solar masses.

  • Studied through the motion of nearby stars (the S-stars), particularly star S2:

    • S2 orbits Sgr A* every 15.2 years at a velocity of 1500 km/s.

    • Density and proximity indicate compactness required for black hole.

    • Sgr A* is the first confirmed supermassive black hole.

The Galactic Bulge

  • Definition and Composition: The bulge is an extension of the disk, comprising old stars with higher metallicity.[Fe/H] ~ 0 to +0.5.

  • Stars in the bulge are believed to be enriched by nearby gas from the Galactic Centre.

  • Observational challenges due to dust clouds obstructing views.

Observing the Bulge

  • Baade’s Window: A region allowing visibility of the bulge by avoiding dust clouds.

  • Named after Walter Baade, who worked to define stellar populations.

    • Distinctions: Population I (younger, metal-rich) and Population II (older, metal-poor).

Structure of the Milky Way Disk

  • The disk consists of stars, gas, and dust; critical for understanding structure and composition.

  • Observations conducted in far infrared and radio wavelengths due to visibility restrictions in optical light.

Key Observational Diagnostics

  • H I 21 cm line: Probes warm neutral hydrogen gas.

  • CO 1-0 line: Probes cool molecular gas, important for star formation areas.

  • Far infrared emission: Indicates presence of interstellar dust.

Doppler Shift Phenomenon

  • Spectral lines shift due to the motion of stars and gas relative to Earth:

    • Redshift: Light moving away from us, longer wavelengths.

    • Blueshift: Light moving towards us, shorter wavelengths.

  • Relevant equation: Δλ/λ = v/c, where v is the line of sight velocity, and c is the speed of light.

Structure and Properties of Globular Clusters

  • Globular clusters are dense, old collections of stars, mostly of Population II, and were formed early in the galaxy's history.

  • Approximately 150 globular clusters orbit the Milky Way, with ω Centauri being the most luminous.

  • Cluster stars provide insights into stellar evolution due to uniformity in age and composition.

    • Blue stragglers observed in clusters are formed from mergers of lower-mass stars.

Examples of Notable Globular Clusters

  • ω Centauri: Contains ~10 million stars; very massive (~4 million solar masses).

  • 47 Tucanae: The second brightest globular cluster with an age of 13 Gyr, showing evolved stars across various stages of stellar evolution.

Conclusion

  • The Milky Way consists of intricate structures and facilitates various astrophysical studies.

  • Understanding the Milky Way aids in grasping galaxy formation, evolution, and dynamics.