PHYS117: General Astronomy - Nov 7

Components of the Milky Way:
- Bulge:
- Contains older stars.
- Extends approximately 2 kpc across.
- Light obscured by dust in visible spectrum; observed in x-ray, infrared, and radio energies.
- Contains numerous stars, with visible structures such as loops, wisps, and threads.
- Disk:
- Sun's location in the galaxy situated approximately 28,000 light-years from the galactic center.
- Comprises various star populations and is the site of active star formation.
- Halo:
- Encompasses globular clusters and old dim stars extending up to 100,000 light-years.
- Contains dark matter, which is critical for understanding the galaxy's total mass and gravitational influence.
- Globular clusters found within the halo.

Characteristics:
- The central bulge is largely composed of older stars and is significantly obscured by dust when viewed in visible light.
- Alternate observational methods are employed, such as:
- X-ray observations
- Infrared observations
- Radio frequency observations
- As materials move over time, they can be observed changing positions and forms.

Central Molecular Zone:
- Contains large amounts of gas and dust, which results in a high rate of star formation.
- Notable astronomical entity: Sagittarius A (Sgr A), a strong radio source related to the central black hole.
Mass Calculation:
- By tracking star orbits around the center, mass is calculated to be approximately $4,000,000 M_*$ (solar masses).
- This mass concentration suggests the presence of a black hole due to the immense density in a confined area.

Sagittarius A* (Sgr A*):
- Comparison with other black holes, such as M87, emphasizes Sgr A's characteristics.
Accretion Disks:
- Composed of dust and gas in orbit around the black hole, which does not get absorbed immediately.
- ISCO (Innermost Stable Circular Orbit) is a vital concept where the gravitational effects become significant.
- Friction among this material heats the dust and gas, leading to plasma formations which produce magnetic fields due to spinning charges.

Observational Discoveries:
- Identification of large gamma-ray sources located above and below the galactic disk.
- Hypothesis suggests that this material is ejected from Sgr A* through powerful magnetic field jets.
- Current evidence shows no ongoing activity from these jets.

Highlights and reiterations of the galaxy structure were made demonstrating:
- Bulge and Disk anatomy
- Halo compositions
- The positions of Sun and globular clusters relative to these structures.

Halo Composition:
- Contains old dim stars and is structurally different from the galactic disk.
- Globular clusters found in the halo, with significant distances and age—traditionally considered to be elder groups of stars compared to their disk counterparts.

Star Orbits:
- Halo star orbits characterized by a green color representation.
- Bulge and disk star orbits differentiated with distinct coloring, noting movements and interactions among various star populations complexly.

Formation Process:
- Stars originate from massive clouds of gas and dust. These clouds collapse to form clusters of stars.
- Star clusters typically have the following characteristics:
- Form similarly in age, arising from the same material.
- Generally, they exist in consistent locations within the galaxy.
- Mass Variations: Despite shared origins, individual stellar masses vary.
- Types of Clusters:
- Open Clusters:
  - Ranges from 10 to 1,000 stars, approximately 25 parsecs across.
  - Generally found in the galactic disk, primarily consists of younger stars.
- Globular Clusters:
  - Comprise 10,000 to 1 million stars, generally 10 to 30 parsecs across.
  - Traditionally located in the halo and consist of very old stars.

Stellar Evolution Observation:
- Averages and phases of stellar evolution observed through cluster Hertzsprung-Russell (H-R) diagrams denote:
- The evolution off the main sequence of massive stars over time.
- As clusters age, significant changes in the population structure noted through main-sequence turnoff points.
Age Determination:
- The age of a star cluster is invocatively determined from the main-sequence turnoff, where fewer massive stars appear at younger ages.

Composition Examination:
- Analysis of stars in globular clusters shows they are categorized as "metal-poor".
- Definition Clarification: (What are metals?)
- Astronomers refer to any element heavier than hydrogen and helium produced in stars.
- Source of Metals:
- Metals originate during stellar evolution, particularly during supernova events.
- Implications of Metal Deficiency:
- A lack of metals implies early star formation in the universe and hints at primordial stellar conditions.