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29 reading cards on astronomy and astrophysics. One self-contained explanation at a time, in chronological order.

  1. The Cepheid variable V1 in the Andromeda Galaxy, imaged by the Hubble Space Telescope — the same star Edwin Hubble identified in 1923 to prove that spiral nebulae lie beyond the Milky Way.
    2026-05-26

    The Cepheids

    How a pulsating star became the ruler of the universe.

    Some stars do not shine steadily. They swell and contract on a clockwork rhythm — and the rhythm itself, it turns out, tells us how far away they are.

  2. A high-resolution Lunar Reconnaissance Orbiter Camera image of a lobate thrust-fault scarp on the Moon, a low stair-step cliff cutting across the cratered surface, with boulder fields visible on the upthrown side
    2026-05-26

    A Moon Still Moving

    Its interior is still cooling — and its crust is still breaking.

    The Moon looks geologically dead. Its crust says otherwise.

  3. Artist's concept showing a millisecond pulsar at centre with its twisted blue magnetic field lines, and three rocky planets in orbit around it, with auroral light illustrated on the foreground planet's night side.
    2026-05-15

    Binary Pulsars

    Clocks in motion, watched by clocks — and sometimes, mid-experiment, the clock turns away.

    A pulsar locked in orbit with another compact body becomes a moving timepiece whose ticks can be tracked to nanoseconds. That precision is what makes binary pulsars the most demanding laboratory we have for gravity — and what occasionally reveals that one of them has stopped pointing at us.

  4. Full-disk view of Neptune taken by Voyager 2 in August 1989, showing the Great Dark Spot and its companion bright cloud, the fast-moving feature nicknamed Scooter, and bands of high-altitude cirrus
    2026-05-14

    Neptune

    The planet that was found with the point of a pen.

    In the summer of 1846, a French mathematician who had never looked through a telescope wrote a letter to Berlin, telling a German astronomer exactly where to point one. That night, a new planet appeared — within one degree of the predicted position. It was the first world in history discovered not by sight, but by pure mathematics.

  5. Composite image of the Crab Nebula — powered by a pulsar spinning 30 times per second at its core — combining X-ray, optical, and infrared data from Chandra, Hubble, and Spitzer
    2026-05-12

    The First Pulsar

    A signal that should not have existed.

    In the summer of 1967, a twenty-four-year-old graduate student in Cambridge found a rhythm buried in the radio noise — sharp, cold, perfectly repeating every 1.337 seconds. For weeks, no one could explain it. They half-jokingly labelled it LGM-1: Little Green Men.

  6. An interstellar generation ship rendered as a rotating O'Neill cylinder: centrifugal gravity, full radiation shielding, an axial artificial sun, a closed water cycle. Fourteen generations of inhabitants have shaped the inner landscape — strip agriculture, weathered wooden houses, a stream that returns to the lake it came from. This is the geometry physics permits, once you accept that the voyage outlasts a lifetime.
    2026-05-09

    The Slow Crossing

    What an interstellar voyage would actually require, with the technology we have today.

    It is not a hypothetical that has aged badly. The nearest other star is 4.24 light-years away, and the question of how long the trip would take, with what we actually have, returns an answer that almost no fiction has been honest about.

  7. Webb's First Deep Field — galaxy cluster SMACS 0723 imaged by the James Webb Space Telescope, with thousands of galaxies including faint, gravitationally lensed objects from the early universe
    2026-05-08

    Fossil Light

    Astronomy is the only science that cannot observe the present.

    Every photon that reaches a telescope is news from a moment that has already ended. The further the source, the older the news — and the more the universe in front of us is no longer there.

  8. Hubble image of globular cluster M80
    2026-05-07

    Stellar Collisions

    When stars meet — violently.

    A direct hit between two stars is exceedingly rare in the disc of the Galaxy. In the dense cores of globular clusters, it is routine.

  9. 18th-century Persian astrolabe
    2026-05-07

    The Astrolabe

    The Renaissance computer that fitted in the palm of your hand.

    Before there were watches, before there were sextants, there was an instrument that told you the time, your latitude, the position of the stars, and — if you were Muslim — the direction of Mecca. It fit in the palm of your hand.

  10. Hubble image of the Eagle Nebula Pillars of Creation
    2026-05-07

    Giant Molecular Clouds

    Stellar nurseries written in cold hydrogen and dust.

    Most of interstellar space is too hot and too sparse for atoms to ever meet. But cold, dense pockets exist — and that is where stars come from.

  11. Flammarion engraving of a man peering beyond the celestial sphere
    2026-05-07

    A Thought Experiment from the Middle Ages

    When medieval astronomers reached for the edge of everything.

    Aristotle insisted the universe ended somewhere. His medieval inheritors tried to imagine reaching past that end — and the question reshaped physics.

  12. An artist's concept of Europa Clipper flying above Europa's icy surface.
    2026-05-07

    The Oceans of Jupiter's Moons

    Beneath the ice, an entire hydrosphere waits in silence.

    The four points of light beside Jupiter, easily seen with binoculars, are not stars. Beneath the ice of at least one of them, an ocean is waiting.

  13. Hubble image of spiral galaxy NGC 3344
    2026-05-07

    Cosmic Pinwheels

    A field guide to the shapes of galaxies.

    The first time someone tried to put galaxies in alphabetical order was 1926. The order has changed remarkably little since.

  14. Noctilucent clouds observed from the International Space Station
    2026-05-07

    Noctilucent Clouds

    Ice crystals at the edge of space, glowing after sunset.

    There is a kind of cloud you can only see when the Sun has already set on you but is still rising on it. Eighty kilometres up, in air drier than any desert, ice has somehow learned to form.

  15. Galileo demonstrates his telescope to the Doge, as imagined by Giuseppe Bertini
    2026-05-07

    The Great Revolution

    How Kepler and Galileo dismantled fourteen centuries of cosmic certainty.

    Two men, working in different countries with different tools, finished what Copernicus had only begun. Within a single year — 1609 — the heavens stopped being a closed sphere and became something measurable.

  16. An artistic representation of weighing a galaxy
    2026-05-07

    How Much Does a Galaxy Weigh?

    The question is poorly posed — and the answer is stranger than the question.

    There is no scale large enough to weigh a galaxy. The methods we have invented instead reveal that most of the answer is something we cannot see.

  17. James Webb Space Telescope image of the Carina Nebula star-forming region NIRCam
    2026-05-07

    The Story of Stars

    From the dust of molecular clouds to the light that reaches our telescopes.

    Stars are not visible at the moment of their birth. To find one being made, we have to point a radio telescope at a place where, on the surface, nothing seems to be happening.

  18. The Sun imaged  by the NASA's Solar Dynamics Observatory
    2026-05-07

    Our Sun

    An extraordinarily complex star hiding behind an apparently simple glow.

    The Sun appears to the eye as a featureless ball of light. A century of observation has shown it to be the most layered, most paradoxical star we will ever study at close range.

  19. Artist's concept of Voyager 2 crossing the heliopause into interstellar space
    2026-05-07

    The Solar Neighbourhood

    Interstellar matter flows through our planetary system — and has for billions of years.

    The Solar System is not sailing through empty space. It is moving through a cloud — and through that cloud's history.

  20. Hubble image of the spiral galaxy NGC 5584 used for Cepheid distance calibration
    2026-05-07

    The Cosmic Distance Ladder

    Measuring the universe — one rung at a time.

    Astronomy began as the art of estimating how far things are. The trick is that no single method works at every scale — so we built a ladder.

  21. Natural-colour mosaic of Saturn and its ring system from the Cassini spacecraft
    2026-05-07

    How Saturn Got Its Rings

    A lost moon, a broken resonance, and a planet that devoured its own child.

    The rings of Saturn look ancient and are not. Two unrelated mysteries about the planet — their youth and its tilt — turn out to share a single, violent answer.

  22. Image of Io showing active volcanic plumes acquired during Galileo's ninth orbit (C9) around Jupiter
    2026-05-07

    Extraterrestrial Volcanoes

    Named after the God of Fire — and found on worlds far beyond our own.

    The conviction that Earth was the only living world geologically lasted as long as we had not looked closely. Voyager 1 ended it in three days.

  23. The terminator line seen from the Apollo11
    2026-05-07

    The Terminator

    The line that separates day from night — sweeping the Earth at 1,600 km/h.

    Twice every day, a moving line crosses every place on Earth at the speed of a jet airliner. We see it as a sunrise. The Moon shows the same line in cleaner form.

  24. Panoramic view of the Milky Way from ESO's Paranal Observatory
    2026-05-07

    The Mystery of the Milky Way

    Mapping a city from inside a foggy window.

    We have always lived inside the Galaxy, never outside it. Drawing its map has been like sketching a metropolis from a single window — and the picture is still unfinished.

  25. This image, acquired by the Left Mastcam-Z camera aboard NASA's Perseverance rover, reveals a rock displaying distinctive black-and-white banding on the Martian surface.
    2026-05-07

    Stones from Mars

    How we proved that pieces of the Red Planet have been falling to Earth all along.

    There are stones in our museums whose journey began on another world. The proof took twenty years to assemble — and one decisive measurement of trapped gas.

  26. Frank Drake
    2026-05-07

    Frank Drake ... Thank You

    The man who dared to listen — and taught us how to ask the right question.

    In 1960 a young astronomer pointed a radio telescope at two nearby stars and listened. He heard nothing. The act of listening, however, changed everything.

  27. Detail of Thorvaldsen's Copernicus Monument in front the Polish Academy of Sciences on Warsaw's Krakowskie Przedmieście.
    2026-05-07

    The Legacy of Copernicus

    Five centuries of demotion — from the centre of creation to the periphery of a spiral arm.

    For fourteen centuries the Earth sat at the centre of everything. The book that displaced it was printed the year its author died — and what it began still has not finished.

  28. Hubble high-definition panoramic mosaic of the Andromeda Galaxy showing over 100 million resolved stars
    2026-05-06

    The Andromeda Galaxy

    The nearest thing to a mirror the Milky Way has.

    The brightest object beyond the Milky Way visible to the unaided eye looks like a smudge of light. It is also the only galaxy heading our way — and we have an appointment.

  29. Earthset captured through the Orion spacecraft window as the crescent Earth sinks behind the Moon's far side during the Artemis II flyby
    2026-05-06

    Artemis II

    Fifty-four years of silence, then an Earthset.

    Half a century after Apollo 17, four astronauts boarded a spacecraft pointed at the Moon — and discovered the silence behind it had been waiting for them.