Astronomy and Astrophysics
DraftOther sections cover how to build and fly the instruments. This one covers what they are for: the objects, the physics and the methods by which anything is known about either.
Reading order
Stars come first because stellar physics underlies galaxies and compact objects alike. Telescopes and observational methods can be read at any point and may help before the rest.
The relations this section turns on
Mass determines a star’s entire history. A 30 solar mass star lives 2 million years; a 0.5 solar mass star outlives the current universe.
Three kilometres per solar mass. Sgr A* is 0.08 AU across, M87* is 128 AU, and both subtend roughly the same tiny angle from Earth.
Eighty-four parts per million for an Earth crossing a Sun. That number is what Kepler was built to measure.
Aperture governs both sensitivity and resolution, which is why the history of astronomy is a history of larger mirrors and, latterly, longer baselines.
Five things worth knowing
No low-mass star has ever died of old age. Stars below about half a solar mass have main sequence lifetimes longer than the age of the universe.
The Hubble tuning fork runs backwards. Ellipticals are generally the end state, produced by mergers that destroy ordered rotation, not the starting point.
Accretion beats fusion by more than an order of magnitude. Falling onto a black hole converts 6 to 42% of rest mass into radiation, against 0.7% for hydrogen fusion. That is why a region the size of the solar system outshines a galaxy.
The commonest exoplanets have no solar system analogue. Super-Earths and mini-Neptunes dominate the observed population, and there is a real deficit at 1.8 Earth radii separating the two.
GW170817 tested four fields at once. One event established the origin of short gamma-ray bursts, confirmed r-process nucleosynthesis, bounded the speed of gravity to a part in 10¹⁵, and gave an independent measurement of the Hubble constant.
Pages
Formation and fragmentation, the mass-luminosity relation that fixes everything downstream, the H-R diagram and cluster ages, post-main-sequence evolution, and the endpoints that enrich the universe.
02GalaxiesMorphological classification and why the sequence runs backwards, rotation curves as the first dark matter evidence, the structure of the Milky Way, mergers and quenching, and active nuclei.
03Black holesSchwarzschild and Kerr, what the horizon means and does not, formation and the supermassive origin problem, accretion and jets, event horizon imaging and gravitational wave detections.
04ExoplanetsDetection methods and the bias each carries, the population and its surprises, transmission spectroscopy, habitable zones honestly assessed, and why every biosignature has an abiotic explanation.
05CosmologyThe Friedmann equations and the three eras, the CMB as the strongest evidence, nucleosynthesis, large-scale structure, and the Hubble tension that current data cannot resolve.
06Dark matter and dark energyThe independent evidence and why the Bullet Cluster resists alternatives, candidates and the null results approaching the neutrino fog, where MOND works and where it fails, and the vacuum energy problem.
07Telescopes and instrumentsOptical design and segmentation, how adaptive optics shifted the ground-space balance, detectors across the spectrum, spectrographs and coronagraphs, and interferometry.
08Multi-messenger astronomyFour independent carriers of information, the detectors for each, what GW170817 settled in a single event, and the alert and follow-up infrastructure it required.
09Observational methodsMagnitudes and the distance modulus, the distance ladder and where its rungs are calibrated, spectroscopy, astrometry, time-domain surveys, and why systematics dominate modern results.