Spectral classes hot→cool: OBAFGKM — 'Oh Be A Fine Girl/Guy, Kiss Me.' Sun = G2V.
OBAFGKM Spectral Classes
The stellar spectral sequence — temperature, color, and what absorption lines reveal
O: >30,000 K — blue, ionized helium lines (Rigel). B: 10,000–30,000 K — blue-white, helium lines (Spica). A: 7,500–10,000 K — white, strong hydrogen (Vega, Sirius). F: 6,000–7,500 K — yellow-white, calcium (Procyon). G: 5,200–6,000 K — yellow, calcium + metals (Sun = G2V). K: 3,700–5,200 K — orange, molecular bands (Arcturus). M: 2,400–3,700 K — red, TiO bands (Betelgeuse). 'V' = main sequence (luminosity class). Extended: L, T, Y for brown dwarfs. Each class has 0–9 subdivisions.
O
Blue, >30,000 K — ionized He
B
Blue-white, 10–30k K — He lines
A
White, 7.5–10k K — H lines
F
Yellow-white, 6–7.5k K
G
Yellow, 5.2–6k K — Sun
K
Orange, 3.7–5.2k K
M
Red, <3,700 K — TiO molecules
Hertzsprung-Russell Diagram
HR diagram: luminosity (y-axis) vs temperature (x-axis, hot LEFT). Main sequence diagonal. Giants upper right. White dwarfs lower left.
Hertzsprung-Russell Diagram
The most important diagram in stellar astronomy — reveals stellar life stages
X-axis: surface temperature (decreasing left to right — hot on left). Y-axis: luminosity (increasing upward). Main sequence: 90% of stars — hydrogen-burning diagonal from hot-luminous (O) to cool-dim (M). Giants/Supergiants: upper right — large, cool, evolved. White dwarfs: lower left — small, hot, evolved. The HR diagram is not a timeline — stars don't move along it continuously. It shows where stars spend most of their time. Mass determines position on main sequence: more massive = hotter, brighter, shorter-lived.
Main sequence
H-burning diagonal — 90% of stars
Giants
Upper right — evolved, large, cool
Supergiants
Top of diagram — most luminous
White dwarfs
Lower left — hot, tiny, dead
Our Sun
G2V — middle of main sequence
Low-Mass Stellar Evolution
Low-mass stars (< ~8 M☉): main sequence → red giant → planetary nebula → white dwarf → black dwarf.
Low-Mass Stellar Evolution
The life cycle of stars like our Sun — from birth to slow cooling death
Main sequence: H → He fusion in core (~10 billion years for Sun). H exhausted: core contracts, heats → shell burning → star expands → red giant. Helium flash: sudden He ignition in degenerate core. He burning: carbon and oxygen formed. Asymptotic giant branch: double-shell burning, thermal pulses, heavy mass loss. Planetary nebula: outer envelope expelled. White dwarf: remaining carbon-oxygen core, Earth-sized, ~100,000 K → cools over billions of years. Black dwarf: theoretical final state (cooled white dwarf) — none exist yet (universe too young).
High-Mass Stellar Evolution
High-mass stars (> ~8 M☉): supernova → neutron star or black hole (> ~25 M☉). Nuclear burning through iron.
High-Mass Stellar Evolution
How massive stars live fast, die violently, and seed the universe with heavy elements
Massive stars: much shorter lives (O stars ~3 million years). Burn through H, He, C, Ne, O, Si in concentric shells (onion structure). Iron: no energy from fusion → iron core grows. Core collapse: when Fe core > 1.4 M☉ (Chandrasekhar limit) → collapse in 0.1 seconds. Type II supernova: bounce creates shock wave → outer layers expelled. Nucleosynthesis: all elements heavier than iron made in supernova. Remnant: neutron star (< ~25 M☉) or black hole. Supernovae enriched the galaxy with heavy elements — we are made of stardust.
Chandrasekhar Limit
Chandrasekhar limit: 1.4 M☉ — above this, white dwarfs collapse. Basis for Type Ia supernovae as standard candles.
Chandrasekhar Limit
The mass limit for white dwarfs — and why it makes Type Ia supernovae standard candles
Subrahmanyan Chandrasekhar (1930, Nobel 1983): electron degeneracy pressure supports white dwarfs up to 1.4 solar masses. Above this: collapse inevitable. Type Ia supernova: white dwarf in binary accretes mass beyond limit → thermonuclear explosion — same peak luminosity everywhere (standard candle). Used to discover dark energy (1998). Neutron stars: supported by neutron degeneracy pressure up to ~2–3 M☉ (Tolman-Oppenheimer-Volkoff limit). Above TOV limit: black hole inevitable.
Nuclear Fusion in Stars
Stellar fusion: pp-chain (Sun), CNO cycle (massive stars). Energy = mass deficit × c². 4H → He-4 + energy.
Stellar Nucleosynthesis
How stars convert hydrogen to helium — and eventually forge all elements up to iron
Proton-proton (pp) chain: dominates in stars < 1.5 M☉. 4 protons → He-4 + 2 positrons + 2 neutrinos + energy. Mass deficit: He-4 is 0.7% lighter than 4 protons → E = mc². Sun converts 4 million tons/sec to energy. CNO cycle: dominates in massive stars — carbon/nitrogen/oxygen act as catalysts. Helium burning: 3 He-4 → C-12 (triple-alpha). Carbon burning: C → Ne, Mg. Each stage produces heavier elements up to iron. Iron: most stable nucleus — fusion would require energy input. End of the line.
Neutron Stars
Neutron stars: ~1.4 M☉ in ~20 km diameter. Density: 1 teaspoon = billion tons. Spin up to 700 Hz (millisecond pulsars).
Neutron Stars
The densest visible objects in the universe — a city-sized remnant of a supernova
Formed in core-collapse supernovae. Mass: ~1.4 M☉. Radius: ~10 km. Density: neutrons packed at nuclear density — 1 tsp ≈ 10⁹ kg. Strong magnetic fields: up to 10¹⁵ Gauss (magnetars). Pulsars: rotating neutron stars with radio beams sweeping like a lighthouse — extremely regular → used as cosmic clocks. Millisecond pulsars: spun up by accretion in binary → hundreds of rotations/second. GW170817: neutron star merger detected in gravitational waves + light → kilonova → heavy elements (gold, platinum) created.
Black Holes
Black holes: gravity so strong even light can't escape. Event horizon: point of no return. Schwarzschild radius = 2GM/c².
Black Holes
The most extreme objects in physics — predicted by Einstein, confirmed by observation
Event horizon: boundary from which nothing escapes, r_s = 2GM/c². Stellar black holes: from > ~25 M☉ stellar collapse. Intermediate: 100–10⁵ M☉ (evidence accumulating). Supermassive: 10⁶–10¹⁰ M☉ in galactic centers. First image: M87* (2019, Event Horizon Telescope), Sgr A* (2022). Hawking radiation: quantum effect → black holes slowly evaporate (not yet observed). Tidal forces (spaghettification) at stellar black holes. No 'singularity' in quantum gravity theories. Information paradox: still unresolved.
Variable Stars
Variable stars: change brightness. Cepheids (pulsation) → standard candles. Novae: binary mass transfer explosions.
Variable Stars
Stars whose brightness changes — and how they revolutionized distance measurement
Intrinsic variables: Cepheids (pulsation period 1–100 days, period ∝ luminosity — Henrietta Leavitt 1908), RR Lyrae, Mira (red giant pulsation). Eruptive: T Tauri (young), flare stars. Extrinsic: eclipsing binaries (Algol), rotating spotted stars. Leavitt's period-luminosity law: revolutionized distance measurement — Cepheids are standard candles to ~100 Mpc. Hubble used Cepheids in Andromeda to prove it was a separate galaxy (1924). Type Ia supernovae: even brighter standard candles → dark energy discovery.
Star Formation
Stars form in molecular clouds. Jeans instability: cloud collapses when gravity > pressure. Protostar → T Tauri → main sequence.
Star Formation
How molecular clouds collapse into newborn stars — from gas to nuclear fusion
Molecular clouds: cold (10–30 K), dense, mostly H₂ and CO. Jeans instability: if cloud mass > Jeans mass, gravity wins over thermal pressure → collapse. Collapse: conservation of angular momentum → rotation → protoplanetary disk. Protostar: heating by gravitational contraction (not yet fusion). T Tauri stage: nuclear reactions begin, strong stellar winds clear surrounding nebula. Main sequence: hydrogen fusion begins → hydrostatic equilibrium. Time to main sequence: ~50 million years for Sun-like star. HII regions: ionized gas glowing around hot young stars (Orion Nebula).
Binary Stars
~50% of Sun-like stars in binary systems. Mass transfer can create novae, X-ray binaries, and Type Ia supernovae.
Binary Star Systems
More than half of all stars have companions — with dramatic consequences
Visual binaries: both stars resolved (Albireo). Spectroscopic binaries: Doppler shifts reveal orbital motion. Eclipsing binaries: brightness dips as stars transit each other — gives radii and masses. Mass transfer: evolved giant fills Roche lobe → mass flows to companion. Cataclysmic variables: white dwarf + main sequence → accretion, nova explosions. X-ray binaries: neutron star or black hole + companion → accretion disk → X-ray emission. Type Ia supernova: white dwarf accretes → exceeds Chandrasekhar limit → thermonuclear explosion. Gravitational wave source: compact binary mergers.
Stellar Distances
Distances: parallax (nearby), Cepheids (intermediate), Type Ia SN (distant). 1 parsec = 3.26 light-years.
Measuring Stellar Distances
The cosmic distance ladder — the foundation of all extragalactic astronomy
Parsec: distance at which 1 AU subtends 1 arcsecond. 1 pc = 3.26 ly. Nearest star: Proxima Centauri, 1.3 pc = 4.24 ly. Parallax: trigonometric, accurate to ~10 kpc with Gaia. Proper motion: star's real movement across sky. Spectroscopic parallax: spectral type → luminosity → distance (less accurate). Standard candles: Cepheids, RR Lyrae, Type Ia SN. Light-year vs parsec: astronomers use parsecs; popular science uses light-years. Megaparsec (Mpc) = 3.26 million light-years used for extragalactic distances.