--- title: "Astronomy & Astrophysics" description: "19 topics in Science, in the order they build on each other." canonical: https://lightmysky.com/learn/science/areas/astronomy-and-astrophysics source: https://lightmysky.com/learn/science/areas/astronomy-and-astrophysics.md retrieved: 2026-09-02 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Astronomy & Astrophysics 19 topics in Science, in the order they build on each other. Page: https://lightmysky.com/learn/science/areas/astronomy-and-astrophysics - [Flux, Magnitudes and the Distance Ladder](https://lightmysky.com/learn/science/flux-magnitudes-and-the-distance-ladder-mt_q1vEUas3AY): What reaches a telescope is a flux, and turning it into a luminosity needs a distance, which is why every astronomical number rests on a chain of distance measurements. Parallax calibrates the first rung, and each later rung is calibrated against the one below it. - [Hydrostatic Equilibrium and the Structure of a Star](https://lightmysky.com/learn/science/hydrostatic-equilibrium-and-the-structure-of-a-star-mt_RKeMW8iX56): A star is a ball of gas where the pressure gradient balances gravity at every depth, which fixes how temperature and density rise towards the centre. The same balance explains why a heavier star burns hotter. - [Energy Transport and Nuclear Burning in Stars](https://lightmysky.com/learn/science/energy-transport-and-nuclear-burning-in-stars-mt_2rc4UAMyl-): Energy made by fusion in the core leaves by radiation or by convection depending on how steeply temperature falls, and the proton-proton chain and the CNO cycle dominate in different mass ranges. Photons take a very long time to escape. - [Stellar Spectra, Spectral Classes and Surface Composition](https://lightmysky.com/learn/science/stellar-spectra-spectral-classes-and-surface-composition-mt_4FBh5G7Gdc): A star's spectrum is a continuum crossed by absorption lines, and which lines appear depends more on the surface temperature than on what the star is made of. Sorting stars by those lines gives the spectral sequence, and the shape of each line carries rotation, pressure and motion along the line of sight. - [The Hertzsprung-Russell Diagram and Stellar Evolution](https://lightmysky.com/learn/science/the-hertzsprung-russell-diagram-and-stellar-evolution-mt_zMZDIB5TGc): Plotting luminosity against surface temperature sorts stars into a main sequence, giants and white dwarfs, and a star's track across the diagram is its life story. Position on the main sequence is set almost entirely by mass. - [General Relativity: Curved Spacetime and Its Tests](https://lightmysky.com/learn/science/general-relativity-curved-spacetime-and-its-tests-mt_-rZZxZwU2L): Gravity is treated as the curvature of spacetime rather than as a force, starting from the observation that free fall and weightlessness cannot be told apart. Light bending, orbit precession and clock rates are the tests. - [Stellar Death: White Dwarfs, Neutron Stars and Black Holes](https://lightmysky.com/learn/science/stellar-death-white-dwarfs-neutron-stars-and-black-holes-mt_k1oUF3mJlu): When fusion stops, what remains is held up by degeneracy pressure or by nothing at all, and which happens depends on the mass left behind. The Chandrasekhar limit is quantum statistics setting an astronomical boundary. - [Galaxies, Redshift and the Expanding Universe](https://lightmysky.com/learn/science/galaxies-redshift-and-the-expanding-universe-mt_-POhqrng-Q): Galaxy distances measured by standard candles and speeds measured by redshift give a linear relation whose slope sets the expansion rate. The expansion is of space itself, so there is no centre to point at. - [Galaxy Types, the Milky Way's Structure and Active Nuclei](https://lightmysky.com/learn/science/galaxy-types-the-milky-ways-structure-and-active-nuclei-mt_Eps5CXEB5C): Galaxies sort into spirals, ellipticals and irregulars, and our own is a barred spiral whose shape was worked out from inside it. A small fraction have centres far brighter than all their stars together, powered by accretion onto a massive black hole rather than by fusion. - [The Cosmic Microwave Background and the Light Elements](https://lightmysky.com/learn/science/the-cosmic-microwave-background-and-the-light-elements-mt_oz0MJXiuVt): A hot dense start predicts both a cooled thermal glow filling the sky and fixed proportions of hydrogen, helium and lithium, and both are measured. Together they are the strongest evidence for the Big Bang. - [Dark Matter, Dark Energy and What the Universe Is Made Of](https://lightmysky.com/learn/science/dark-matter-dark-energy-and-what-the-universe-is-made-of-mt_GKKPGcHqJH): Galaxy rotation curves and cluster masses need far more matter than shines, and the expansion is speeding up rather than slowing. Ordinary matter turns out to be a small part of the total. - [The Interstellar Medium and How Stars Form](https://lightmysky.com/learn/science/the-interstellar-medium-and-how-stars-form-mt_i2HyLf_Emy): The space between stars holds gas and dust in phases from cold molecular clouds to hot ionised bubbles, and dust reddens and dims everything seen through it. A cloud collapses when its own gravity beats its internal pressure, which sets a minimum mass for the collapse and starts the next generation of stars. - [The Scale Factor, the Friedmann Equation and the Age of the Universe](https://lightmysky.com/learn/science/the-scale-factor-the-friedmann-equation-and-the-age-of-the-universe-mt_v3VEzinvJe): Expansion is written as one function of time multiplying every distance, and the equation governing it comes from applying general relativity to a uniform universe. What the universe contains fixes how that function behaves, so the measured contents predict an age and a fate. - [The Sun as a Star: Photosphere, Corona and the Activity Cycle](https://lightmysky.com/learn/science/the-sun-as-a-star-photosphere-corona-and-the-activity-cycle-mt_AMIkiU7wRz): The Sun is the one star whose surface can be resolved, and its layers, granulation and sunspots are read as a magnetic field wound up by differential rotation. The corona is hotter than the surface below it, which is the open problem the observations frame. - [Tensors and the Metric on a Curved Manifold](https://lightmysky.com/learn/science/tensors-and-the-metric-on-a-curved-manifold-mt_9r9SG3NcZR): Coordinates are labels with no physical content, so physics is written with objects whose components transform in a fixed way under a change of labels. The metric holds the geometry: distances, angles, and which vectors count as parallel. - [Parallel Transport, Christoffel Symbols and the Geodesic Equation](https://lightmysky.com/learn/science/parallel-transport-christoffel-symbols-and-the-geodesic-equation-mt_bUEL2R91Vy): Comparing vectors at different points needs a rule for carrying one to the other, and the Christoffel symbols encode that rule for a given metric. A geodesic is a path that parallel-transports its own tangent, which is what free fall means. - [Curvature and the Einstein Field Equation](https://lightmysky.com/learn/science/curvature-and-the-einstein-field-equation-mt_1CxE6kRDt_): Curvature is measured by how a vector changes when it is carried around a closed loop, and the Riemann tensor records that. The field equation ties curvature to the energy and momentum present, so matter fixes the geometry that matter then moves through. - [The Schwarzschild Solution: Orbits, Redshift and the Horizon](https://lightmysky.com/learn/science/the-schwarzschild-solution-orbits-redshift-and-the-horizon-mt_dNZpRDeZUE): The geometry outside a spherical mass has one exact solution, and reading it gives orbital precession, light bending and the way clock rates depend on depth. At one radius the coordinates fail while the geometry does not, which is what a horizon is. - [Linearised Gravity and Gravitational Waves](https://lightmysky.com/learn/science/linearised-gravity-and-gravitational-waves-mt_z0tMZDGMHi): Small departures from flat spacetime obey a wave equation, so a changing mass distribution sends ripples of geometry that stretch one transverse direction while squeezing the other. Detectors measure that strain, and it is tiny because geometry couples weakly to matter.