The Hubble tuning fork diagram — classifying galaxies by shape and structure
Elliptical (E0–E7): smooth, round to elongated, mostly old red stars, little gas/dust, little star formation. Lenticular (S0): disk but no spiral arms. Spiral (Sa–Sc): arms tighter→looser, bulge larger→smaller, more gas/younger stars. Barred spiral (SBa–SBc): central bar instead of round bulge — Milky Way is SBbc. Irregular: no clear structure (Magellanic Clouds). Note: Hubble's tuning fork was NOT an evolutionary sequence — galaxies don't evolve along it.
E0–E7
Elliptical — round to elongated
S0
Lenticular — disk, no arms
Sa–Sc
Spiral — tight to loose arms
SBa–SBc
Barred spiral — central bar
Irr
Irregular — Magellanic Clouds
The Milky Way
Milky Way: barred spiral (SBbc), ~100,000 ly wide, ~300 billion stars, central SMBH Sgr A* (4 million M☉).
The Milky Way
Our home galaxy — its structure, size, and the supermassive black hole at its center
Diameter: ~100,000 ly. Thickness: ~1,000 ly (disk), ~100,000 ly (dark matter halo). ~300 billion stars. Barred spiral — central bar ~27,000 ly long. Our location: Orion Arm, ~26,000 ly from center. Galactic center: Sgr A* — supermassive black hole, 4 million solar masses (Nobel 2020, Ghez & Genzel). Galactic rotation: ~220 km/s at Sun's location. Dark matter halo extends to ~600,000 ly. Satellite galaxies: Large and Small Magellanic Clouds.
Dark Matter in Galaxies
Galaxy rotation curves: stars at edge orbit as fast as inner stars — requires dark matter halo.
Galaxy Rotation Curves
Vera Rubin's discovery that changed our understanding of galaxy mass distribution
Keplerian prediction: orbital speed should drop at galaxy edges (like planets — outer planets orbit slower). Observation (Vera Rubin, 1970s): rotation curves flat — outer stars orbit just as fast as inner stars. Explanation: dark matter halo extending well beyond visible galaxy. Dark matter: 5–10× more mass than visible matter in typical galaxy. Bullet Cluster: dark matter (lensing) separated from hot gas (X-ray) during collision — direct evidence dark matter ≠ hot gas.
Local Group
Local Group: ~80 galaxies including Milky Way + Andromeda + Triangulum. Andromeda approaching — merge in ~4.5 bya.
The Local Group
Our galactic neighborhood — and the coming collision with Andromeda
~80 galaxies in ~10 Mly diameter volume. Dominant members: Milky Way (Milky Way Group) and Andromeda/M31 (Andromeda Group). M31: ~2.5 Mly away, ~1 trillion stars (more than MW), approaching at ~110 km/s. Andromeda–Milky Way collision: ~4.5 billion years → Milkomeda. Individual stars unlikely to collide (vast empty space between them). Triangulum (M33): third largest. Satellites: Magellanic Clouds, Sagittarius Dwarf, Canis Major Dwarf. Local Group is part of Virgo Supercluster → Laniakea Supercluster.
Active Galactic Nuclei
AGN and quasars: supermassive black holes actively accreting — most luminous objects in the universe.
Active Galactic Nuclei
When supermassive black holes feed — outshining entire galaxies from a region smaller than the solar system
AGN types: Seyfert galaxies (moderate activity), quasars (extreme — can outshine host galaxy 100×), blazars (jet pointing at us). Unified model: all AGN are same phenomenon viewed from different angles. Accretion disk: infalling matter heats to millions of degrees → X-ray, UV, visible emission. Relativistic jets: plasma ejected at near light speed. Quasars: most at high redshift (early universe) → SMBHs were more active then. Evidence: radio galaxies, X-ray observations, VLBI imaging.
Galaxy Mergers
Galaxy mergers: common — Milky Way already absorbed Gaia-Enceladus dwarf galaxy ~10 bya.
Galaxy Mergers and Evolution
Galaxies are not isolated — they grow by eating smaller ones
Most massive galaxies grew through mergers. Milky Way: Sagittarius Dwarf currently being torn apart; Gaia-Enceladus merger ~10 bya left chemical signature in halo stars. Merger signatures: tidal tails, shells, disturbed morphology. Elliptical galaxies: thought to form from spiral mergers (quench star formation). Starburst galaxies: mergers trigger intense star formation. N-body simulations: reproduce merger morphology. Antennae Galaxies (NGC 4038/39): current collision — visible in small telescopes. Hierarchical structure formation: big = assembled from smaller.
Galaxy Clusters
Galaxy clusters: largest gravitationally bound structures. Hot gas (X-ray), dark matter (70–80%), galaxies (~5%).
Galaxy Clusters
The largest gravitationally bound structures in the universe
Cluster contents: galaxies (~5%), hot intracluster gas (15–20%, X-ray emitting, T~10⁷–10⁸ K), dark matter (70–80%). Mass: 10¹⁴–10¹⁵ solar masses. Examples: Virgo Cluster (nearest, ~65 Mly), Coma Cluster, Perseus Cluster. Brightest Cluster Galaxies (BCG): giant ellipticals at cluster center, formed by repeated mergers. Sunyaev-Zel'dovich effect: CMB photons gain energy from hot gas → cluster detection. Galaxy clusters form at intersections of cosmic filaments (cosmic web).
Supermassive Black Holes
Every massive galaxy has a SMBH at its center. Mass correlates with bulge mass — co-evolution.
Supermassive Black Holes
The monsters at the heart of every large galaxy — and their surprising link to galaxy evolution
Evidence: stellar orbits near Sgr A* (Nobel 2020), M87* imaged by Event Horizon Telescope (2019). Masses: millions to billions M☉. M-σ relation: SMBH mass correlates with galaxy bulge velocity dispersion — tight relationship implies co-evolution. Quenching: AGN feedback (jets/winds) can heat/expel gas → stops star formation → explains why massive galaxies stopped growing. Event horizon: point of no return. Hawking radiation: quantum effect causing black holes to slowly evaporate (not yet observed).
Cosmic Web
Cosmic web: filaments, sheets, voids. Galaxies form in filaments at intersections (nodes = clusters).
The Cosmic Web
The large-scale structure of the universe — a vast web of filaments and voids
Matter in the universe is not uniformly distributed. Dark matter filaments connect galaxy clusters. Voids: vast empty regions (100s of Mly across). Sheets/walls: 2D structures (e.g., Great Wall). Nodes: galaxy clusters at filament intersections. Redshift surveys (SDSS, 2dF): mapped cosmic web in 3D. Millennium Simulation: dark matter-only simulation reproduced web structure. Baryonic matter follows dark matter scaffolding. Largest structures: ~500–1000 Mly — consistent with homogeneity at larger scales.
Voids
Vast empty regions — 100s of Mly
Filaments
Dark matter threads connecting clusters
Sheets/walls
2D structures — Great Wall
Nodes
Galaxy clusters at intersections
Stellar Populations
Pop I: young, metal-rich (disk). Pop II: old, metal-poor (halo, globular clusters). Pop III: first stars — no metals.
Stellar Populations
Three stellar generations that trace the chemical enrichment history of galaxies
Population I: metal-rich (like Sun), young, found in galactic disk and spiral arms, still forming. Population II: metal-poor (only H, He, trace Li from Big Bang), old (~10–13 bya), found in halo and globular clusters. Population III (theoretical): first stars — no metals at all, likely massive and short-lived, enriched the universe with first heavy elements. Metallicity: astronomers call all elements heavier than He 'metals.' Each stellar generation enriches gas for the next. Milky Way thick disk and bulge mostly Pop II.
Galaxy Formation
Galaxy formation: dark matter halos collapse first, gas cools inside → stars form. Feedback halts growth.
Galaxy Formation and Evolution
How galaxies assembled from primordial gas over billions of years
Dark matter halos: collapsed first from density fluctuations. Gas falls into potential wells → cools → forms stars. Bottom-up (hierarchical) assembly: small structures merge into large. Early universe: more mergers, higher star formation rate. Cosmic star formation rate peaked at z~2 (~3 billion years after Big Bang). AGN feedback: SMBH energy input quenches star formation in massive galaxies. Stellar feedback: supernovae drive winds, expel gas. Green valley galaxies: transitioning from blue star-forming to red quiescent.
Gravitational Lensing
Gravitational lensing: mass bends light. Strong (arcs), weak (shear), micro (brightness). Einstein rings.
Gravitational Lensing
How massive objects bend light — and how we use this to map dark matter
Einstein (1915): mass warps spacetime → light bends. Confirmed 1919 solar eclipse. Strong lensing: multiple images, arcs, Einstein rings — requires precise alignment. Weak lensing: subtle shape distortions of background galaxies — maps dark matter statistically. Microlensing: temporary brightening as foreground mass passes — detects dark matter candidates, exoplanets. Hubble Frontier Fields: used massive clusters as natural gravitational telescopes. Lensing independently confirms dark matter distribution matches X-ray hot gas offset in Bullet Cluster.