7.1 Milky Way Components and Morphology

Galactic Structure

Central Components of the Milky Way

The Milky Way is not a uniform blob of stars. It has distinct structural components, each with different stellar populations, ages, and dynamics. Mapping these components is how we piece together the galaxy's formation history.

The galactic bulge forms the central region, roughly 2 kpc in radius. It's a dense, roughly triaxial (bar-shaped) concentration of predominantly older, metal-rich stars. The bulge's stellar population is a mix: mostly old stars with a wide metallicity spread, but it also contains younger stars near the plane. Its boxy/peanut-shaped morphology, revealed by infrared surveys like COBE/DIRBE and 2MASS, is strong evidence that the Milky Way is a barred spiral (type SBbc).

The galactic disk contains most of the baryonic matter and is where active star formation occurs. It extends roughly 15–20 kpc from the center and subdivides into two components:

• Thin disk: Scale height ~300 pc. Contains younger stars (ages up to ~8 Gyr), gas, dust, and the spiral arms. This is where Population I stars and ongoing star formation reside.
• Thick disk: Scale height ~1 kpc. Contains older, more metal-poor stars (ages ~8–12 Gyr) with higher velocity dispersions. Its origin is debated: possibilities include early turbulent disk formation, heating of the thin disk by minor mergers, or radial migration.

The galactic halo extends well beyond the disk, out to ~100 kpc or more. It contains:

• Sparse, old, metal-poor stars (Population II)
• ~150 known globular clusters
• A dominant component of dark matter, which accounts for the majority of the galaxy's total mass (~1–2 $\times 10^{12}$ $M_\odot$)
• Stellar streams and substructure from tidally disrupted satellite galaxies, direct evidence of hierarchical assembly

The halo's gravitational influence governs the overall rotation curve and dynamics of the entire galaxy.

Structural Features and Core

The Milky Way's spiral arms are density wave features in the disk where gas is compressed, triggering enhanced star formation. Because young, luminous O and B stars and H II regions trace these arms, the arms appear bright even though stars pass through them over time.

The major arms identified through radio (21 cm H I), CO surveys, and infrared observations include:

• Scutum-Centaurus Arm and Perseus Arm (the two major arms originating from the ends of the central bar)
• Sagittarius Arm and Outer Arm (secondary or inter-arm structures)
• The Sun sits in a minor structure called the Orion Spur (or Local Arm), between the Perseus and Sagittarius Arms, at a galactocentric radius of ~8.2 kpc.

The galactic center is the densest stellar environment in the Milky Way and hosts the supermassive black hole Sagittarius A*. It lies approximately 8.2 kpc (~26,700 light-years) from the Sun in the direction of the constellation Sagittarius. Heavy dust extinction along the line of sight ($A_V \sim 30$ mag) makes optical observation impossible, so the central region is studied primarily through:

1. Radio astronomy (synchrotron emission, molecular line emission)
2. Infrared astronomy (penetrates dust; used to track individual stellar orbits near Sgr A*)
3. X-ray astronomy (hot gas, accretion phenomena, X-ray binaries)

Stellar Clusters and Populations

Types of Stellar Clusters

Stellar clusters are gravitationally bound groups of stars that formed together, making them powerful tools for studying stellar evolution (all members share the same age and initial composition, but differ in mass).

Globular clusters are ancient, massive, tightly bound systems:

• Contain $10^5$–$10^6$ stars in a roughly spherical distribution, with half-light radii of a few parsecs
• Ages typically 10–13 Gyr, making them among the oldest objects in the galaxy
• Predominantly metal-poor ($[\text{Fe/H}] \sim -2.0$ to $-0.5$), though a metallicity bimodal distribution exists
• Orbit in the halo on eccentric, often highly inclined orbits
• Examples: M13 (NGC 6205) in Hercules, $\omega$ Centauri (NGC 5139, which may be the stripped nucleus of a dwarf galaxy given its unusual metallicity spread)

Open clusters are younger, less massive, and loosely bound:

• Contain $10^2$–$10^4$ stars, with typical radii of a few parsecs
• Found in the thin disk, often associated with recent star formation in spiral arms
• Ages range from a few Myr to several Gyr, though most dissolve within ~1 Gyr due to tidal interactions and encounters with giant molecular clouds
• Examples: the Pleiades (~100 Myr, ~400 members) and the Hyades (~625 Myr, the nearest open cluster at ~47 pc)

Stellar Population Characteristics

Baade's classification of stellar populations connects a star's chemistry, kinematics, and location to the galaxy's enrichment history:

• Population I stars are young to intermediate-age, metal-rich ($[\text{Fe/H}] \gtrsim -0.5$), and found in the thin disk on nearly circular orbits. The Sun ($[\text{Fe/H}] \approx 0.0$) is a typical Pop I star.
• Population II stars are old, metal-poor ($[\text{Fe/H}]$ down to $\sim -3$ or lower), and found in the halo and thick disk on eccentric, high-inclination orbits. They formed before the ISM was significantly enriched by supernovae.
• Population III stars are the hypothetical first generation, formed from pristine Big Bang nucleosynthesis material (essentially zero metals). They are predicted to have been very massive ($\sim 10$–$1000 \, M_\odot$) and short-lived. None have been directly observed, but their nucleosynthetic signatures may be imprinted on the most metal-poor Pop II stars.

This population framework is central to understanding chemical evolution: each generation of stars enriches the ISM with heavier elements through supernovae and AGB winds, so metallicity broadly increases with time and correlates with location in the galaxy (disk vs. halo).

Interstellar Medium and Central Black Hole

Composition and Dynamics of the Interstellar Medium

The interstellar medium (ISM) is the gas and dust filling the space between stars. By mass, it's roughly 70% hydrogen, 28% helium, and 2% heavier elements ("metals" and dust grains). Though diffuse, the ISM constitutes about 10–15% of the baryonic mass of the disk and is the reservoir from which new stars form.

The ISM exists in multiple thermal phases, maintained in rough pressure equilibrium:

Star formation occurs in the densest molecular cloud cores. The ISM is continuously stirred and recycled by stellar feedback: stellar winds, photoionization from massive stars, and supernova blast waves inject energy and metals, creating a cycle that regulates the star formation rate across the disk.

Supermassive Black Hole at the Galactic Center

Sagittarius A* (Sgr A*) is the compact radio source associated with the Milky Way's central supermassive black hole.

Its mass has been determined with high precision by tracking the orbits of individual stars (the "S-stars") in the near-infrared over decades. The star S2 (S0-2) completes an orbit in ~16 years with a pericenter distance of only ~120 AU. Fitting Keplerian (and post-Keplerian) orbits to these trajectories yields:

$M_{\text{Sgr A*}} \approx 4.0 \times 10^6 \, M_\odot$

This work earned Reinhard Genzel and Andrea Ghez the 2020 Nobel Prize in Physics.

Despite its enormous mass, Sgr A* is remarkably underluminous compared to active galactic nuclei, radiating at only $\sim 10^{-8}$ of its Eddington luminosity. It accretes at a very low rate, producing emission primarily in the radio and submillimeter (synchrotron from hot, magnetized plasma) with occasional X-ray flares.

The Event Horizon Telescope (EHT) released the first resolved image of Sgr A*'s shadow in 2022, confirming the predicted ring-like structure at a scale consistent with general relativistic predictions for a ~4 million solar mass black hole.

The central black hole's influence on the broader galaxy includes:

• Gravitational dominance within the central ~1–2 pc (the "sphere of influence")
• Possible past episodes of AGN-like activity, suggested by the Fermi Bubbles extending ~10 kpc above and below the galactic plane
• A laboratory for testing general relativity in the strong-field regime (gravitational redshift of S2 detected, Schwarzschild precession measured)