Astrophysics Intermediate

Stars and galaxies emit across the entire electromagnetic spectrum, from radio waves to gamma rays. Each band reveals different physical processes.

The bands

Atmospheric windows

Earth's atmosphere is transparent only at optical and radio wavelengths. UV, X-ray, and gamma-ray astronomy require space telescopes.

The Hertzsprung-Russell (HR) diagram is the most important plot in stellar astrophysics: it shows stellar luminosity vs. surface temperature (or spectral type/colour).

Main sequence

About 90% of stars lie on a diagonal band from hot, luminous blue stars (upper left) to cool, dim red stars (lower right). This is the main sequence — stars burning hydrogen in their cores. The Sun is a G-type star in the middle.

Spectral types

OBAFGKM (plus L, T, Y for cool dwarfs): O blue ~30,000 K, G yellow ~5,800 K, M red ~3,000 K. A useful mnemonic: "Oh Be A Fine Guy/Gal, Kiss Me."

Giants and white dwarfs

Red giants and supergiants occupy the upper-right (cool but luminous — huge). White dwarfs are in the lower-left (hot but dim — small). The HR diagram is essentially a map of stellar physics.

Stars evolve as they consume fuel. The timeline depends strongly on mass.

Main sequence lifetime

t_MS ≈ (M/M_☉) / (L/L_☉) × 10 Gyr. Since L ∝ M⁴ approximately, t_MS ∝ M⁻³. A 10 M_☉ star lives ~30 Myr; the Sun ~10 Gyr.

Leaving the main sequence

When hydrogen in the core is exhausted, the core contracts (heats), and the envelope expands: the star becomes a red giant. The Sun will swell to ~200 times its current size, engulfing Mercury and Venus.

Helium burning and beyond

The core ignites helium fusion (triple-alpha process: 3 ⁴He → ¹²C). In stars > 8 M_☉, carbon, neon, oxygen, silicon burning follow in successive shells — an onion-like structure — until an iron core builds up. Iron cannot release energy by fusion. Collapse is inevitable.

Stars leave behind compact remnants whose nature depends on the original mass.

White dwarfs

Left by stars < 8 M_☉. Supported by electron degeneracy pressure. Maximum mass: Chandrasekhar limit M_Ch = 1.44 M_☉. No nuclear burning; cools over billions of years.

Type Ia supernovae

A white dwarf in a binary accreting matter from a companion can reach M_Ch and explode as a Type Ia supernova — a standard candle used to discover dark energy.

Neutron stars

Left by stars 8–20 M_☉ after core-collapse supernovae. Supported by neutron degeneracy pressure. Typical mass ~1.4 M_☉, radius ~10 km, density comparable to an atomic nucleus. Pulsars are rapidly rotating neutron stars with beamed radio/X-ray emission.

Black holes

Stellar remnants above ~3 M_☉ (Tolman-Oppenheimer-Volkoff limit): gravity overwhelms all degeneracy pressure, producing a black hole.

Galaxies are the building blocks of the large-scale universe, each containing billions to trillions of stars plus gas, dust, and dark matter.

Hubble classification

• Elliptical (E0–E7): smooth, oval, old stellar populations, little gas, range from ~10⁷ to 10¹³ M_☉.
• Lenticular (S0): disc with bulge, no spiral arms.
• Spiral (Sa–Sd / SBa–SBd): disc + arms + bulge; active star formation; the Milky Way is SBbc.
• Irregular: no clear structure; often disturbed by interactions.

Groups and clusters

Galaxies live in groups (the Local Group has ~80 members including MW and M31) and clusters (Virgo cluster: ~1,300 galaxies, 16 Mpc away). Clusters are the largest gravitationally bound objects.

About 10% of galaxies have exceptionally luminous nuclei powered by accretion onto a supermassive black hole (SMBH). These are active galactic nuclei (AGN).

The unified model

A central SMBH (10⁶–10¹° M_☉) is surrounded by an accretion disc (very hot, UV/X-ray bright) and a dusty torus. Broad- and narrow-line regions of ionised gas surround this. The direction we view from (through or outside the torus) determines the observed "type."

Classes of AGN

• Seyfert galaxies: moderate activity, spiral hosts.
• Quasars: most luminous, at cosmological distances; can outshine the entire host galaxy.
• Blazars: relativistic jet pointed toward us — extremely variable, highest energy gamma-rays.
• Radio galaxies: powerful jets extending to megaparsec scales.

Multiple independent lines of evidence point to the existence of large quantities of non-luminous, non-baryonic matter in the universe.

Evidence

• Galactic rotation curves (Rubin et al., 1970s): stellar rotation speed remains roughly constant far from the galactic centre instead of falling off as expected. A halo of dark matter extends far beyond the visible disc.
• Gravitational lensing: the Bullet Cluster shows the mass (from lensing) offset from the gas (from X-rays) — the mass moved like collision-less matter.
• CMB power spectrum: acoustic peak heights require non-baryonic matter.
• Large-scale structure: N-body simulations with cold dark matter (CDM) reproduce the observed cosmic web.

Candidates

• WIMPs (Weakly Interacting Massive Particles)
• Axions
• Sterile neutrinos
• Primordial black holes

No particle candidate has been directly detected yet.

The CMB is thermal radiation permeating all of space, a relic of when the universe first became transparent ~380,000 years after the Big Bang.

Recombination

Before recombination, the universe was a hot plasma — photons scattered constantly off free electrons (the universe was opaque). When it cooled to ~3,000 K, protons and electrons combined into neutral hydrogen, and photons streamed freely. We see this surface of last scattering as the CMB.

Temperature anisotropies

The CMB temperature is 2.725 K with fluctuations of only ~1 part in 10⁵. These anisotropies reflect primordial density fluctuations that grew into today's galaxies. The angular power spectrum has peaks from baryon acoustic oscillations.

Polarisation

The CMB is slightly polarised (E-modes from density fluctuations; B-modes from gravitational waves and lensing). Detecting primordial B-modes would directly probe inflation.