Galaxies

Foundations

Bigger than a cluster

A galaxy looks, in a photograph, like a very large crowd of stars, and it is tempting to think that is all it is. It is not. A galaxy is a system: stars, yes, a few hundred billion of them in ours, but also gas and dust between the stars, a black hole of millions of solar masses at the centre, and, holding all of it together, an invisible halo of dark matter that outweighs everything else five to one. The stars are the part that shines. They are not the part that dominates.

When the universe got large

Until a hundred years ago it was reasonable to believe the Milky Way was the whole universe. The faint spiral smudges seen in telescopes might be nearby clouds or might be distant “island universes”; astronomers formally debated the question in 1920 and could not settle it. Edwin Hubble settled it. Between 1923 and 1929, using a kind of pulsating star whose brightness reveals its distance, he showed that the Andromeda spiral lies far beyond the Milky Way’s edge, a galaxy of its own, and that the sky is full of them. The universe became, in a few years, a hundred thousand times larger than anyone had known.

Our own

The Milky Way is a barred spiral: a flat disc about a hundred thousand light-years across, with a bar of stars through its centre and arms winding outward, wrapped in a faint sphere of old stars and globular clusters. The Sun sits in the disc, about twenty-seven thousand light-years from the centre, orbiting once every two hundred and thirty million years. It has made about twenty circuits since it formed. At the centre, hidden behind dust, is a black hole of four million solar masses, Sagittarius A*. Stars have been watched orbiting it for thirty years, and in 2022 its shadow was photographed by a telescope the size of the Earth.

The disc is where the action is: the gas, the dust, the star formation of the last chapter, at about one and a half solar masses a year. The spiral arms are not fixed groups of stars but patterns moving through the disc, like a traffic jam that persists while individual cars pass through it. Gas piles up in the arms, and that is where the bright young stars are born, which is why the arms shine.

The invisible part

Measure how fast the disc rotates at different distances from the centre and you find, as Vera Rubin did for other galaxies in the 1970s, that the outer parts move as fast as the inner parts. The visible matter cannot do that; there must be far more mass, spread far beyond the stars. The Milky Way’s dark halo extends to perhaps a million light-years and holds about a trillion solar masses. The galaxy sits inside it like a city inside a fog. Every galaxy does.

How galaxies grew

Galaxies were not born whole. In the Dark Ages, the ripples of the afterglow grew into small clumps of dark matter, gas fell into them, and the first stars formed. The clumps merged into larger clumps, and those into larger still, over billions of years: galaxies were built from the bottom up. The Milky Way’s own history is written in the motions of its stars. The Gaia satellite, which has measured the positions and speeds of over a billion of them, found in 2018 the scattered remains of a galaxy about a quarter of the Milky Way’s mass that it swallowed some ten billion years ago. Smaller galaxies are being absorbed today; streams of their stars still trail across our sky.

The next big event is a matter of debate. Andromeda, our nearest large neighbour, is approaching at a hundred and ten kilometres per second. For a decade the standard prediction was a collision in about four and a half billion years. A 2025 analysis that took the uncertainties seriously found only a coin-flip chance of a merger within ten billion years. Whether the Milky Way ends its life as part of a giant elliptical or drifts on alone is open.

Where we live

The Milky Way belongs to the Local Group, a loose family of about a hundred galaxies dominated by Andromeda and ourselves. The Local Group lies on the edge of the Virgo cluster, a swarm of a thousand galaxies fifty million light-years away. Virgo, in turn, is part of Laniakea, a supercluster of a hundred thousand galaxies half a billion light-years across, named in 2014 with a Hawaiian word for “immense heaven”. And Laniakea is one knot in a web: galaxies strung along filaments and sheets around enormous empty voids, a pattern that grew from the ripples in the afterglow and stretches across the whole observable universe. Counting the faintest, there are hundreds of billions of galaxies, perhaps two trillion, each a system like ours.

Deeper

The Milky Way by the numbers

Distance from the Sun to the Galactic centre: 8.18 kiloparsecs, measured directly in 2019 from the orbit of a star around the central black hole. Rotation speed at the Sun’s radius: about 230 kilometres per second, giving an orbital period near 230 million years. Stellar mass: about 6 × 10¹⁰ solar masses, with a total virial mass of roughly 10¹² solar masses within about 200 kiloparsecs. Star formation rate: 1.65 ± 0.19 solar masses per year. Central black hole: 4.15 million solar masses. The number of stars is quoted anywhere between a hundred and four hundred billion because it depends on how many faint red dwarfs are assumed; the mass is far better known than the count.

Hubble’s fork

Hubble’s 1926 scheme sorts galaxies into ellipticals, from round to cigar-shaped, spirals with and without bars, and irregulars. He drew it as a tuning fork and wondered whether galaxies evolved along it; they do not, but the classes are physically meaningful. Ellipticals are old, red, gas-poor and dominated by random motions, typically the products of major mergers. Spirals are rotating discs with ongoing star formation. About two thirds of large disc galaxies, including the Milky Way, have bars. Dwarf galaxies, the most numerous kind, mostly fall outside the scheme.

Rotation and the halo

For a disc of stars and gas alone, orbital speed should fall with radius beyond the bulk of the mass, as the planets’ speeds do in the solar system. Observed rotation curves stay flat to the last measurable point, implying enclosed mass rising in proportion to radius, and hence a density falling as one over radius squared: the signature of an extended halo. In the Milky Way the halo is traced by the motions of globular clusters, satellite galaxies and stellar streams out to hundreds of kiloparsecs.

Density waves and star formation

Lin and Shu proposed in 1964 that spiral arms are quasi-stationary density waves, regions of slightly enhanced gravity that orbit the centre at a fixed pattern speed while stars and gas pass through them. Gas entering an arm is compressed, cools, and forms stars; the brightest of those die before they leave, so the arms are outlined by young blue stars and glowing gas. Modern simulations show arms that are more transient and recurrent than the original theory, but the essential picture, that arms are patterns rather than material structures, holds.

Hierarchical assembly

In the cold dark matter model, small halos collapse first and merge into larger ones, and galaxies follow their halos. Mergers between galaxies of comparable mass destroy discs and build ellipticals; minor mergers thicken discs and add to the halo of old stars. Gaia’s measurement of stellar motions revealed a large population of stars on highly elongated orbits, chemically distinct from the disc, that share a common origin: a galaxy of about 10⁹ solar masses in stars, now called Gaia-Enceladus, accreted roughly ten billion years ago and responsible for much of the inner halo and the thick disc.

Structure on the largest scales

The Local Group’s galaxies orbit a common centre of mass; the Group falls toward Virgo; Virgo and its neighbours flow toward the Great Attractor region; and the whole assembly, Laniakea, is defined by the region within which galaxy motions converge. On still larger scales galaxy surveys map filaments hundreds of millions of light-years long, and the statistical pattern of that web, including the baryon acoustic scale from lesson 1.5, matches what gravity should have grown from the afterglow’s ripples in a universe with the measured amounts of dark matter and dark energy.

Frontier

The next merger

Whether Andromeda and the Milky Way collide depends on the sideways component of Andromeda’s motion, on the masses of both galaxies and their halos, and on the third large member of the Local Group, the Triangulum galaxy, whose gravity tugs on the pair. The 2025 analysis found that plausible ranges of these quantities give outcomes from a merger in five billion years to no merger at all. Gaia’s continuing measurements of Andromeda’s stars are narrowing the sideways velocity. Active research.

Too many satellites, or too few

The cold dark matter model predicts thousands of small halos around a galaxy like ours; only a few dozen satellite galaxies are seen. Either most small halos never formed stars, which the physics of reionisation and stellar feedback can explain, or the model needs adjusting on small scales. The count of faint satellites found by deep surveys has been rising, and the tension has eased but not vanished.

How galaxies stop

Large ellipticals stopped forming stars long ago, and their central black holes are suspected of doing the stopping, heating or expelling the gas that would otherwise cool and form stars. How a black hole a billionth of the galaxy’s size regulates the whole system is one of the central questions in galaxy evolution.

Transcript and shot list

Target runtime about 6 minutes. Built to take apart the crowd-of-stars picture, rebuild the galaxy as a system, then keep zooming out until the galaxy itself is one dot in a web. Shot types: ANIM is rendered from code with Manim, AI is a generated shot from Higgsfield with the shared style reference, REAL is agency imagery with credit, CAM is the narrator.

[00:00] S01 · REAL · The Andromeda galaxy, wide field (NASA, ESA, or an observatory image; confirm credit line).

This is Andromeda, two and a half million light-years away, the most distant thing you can see with your naked eye. A century ago nobody knew what it was. And most people, looking at it now, think they know exactly what it is: a very big crowd of stars. It isn’t. The stars are the part that shines. They are not the part that matters.

[00:30] S02 · ANIM · Cosmic calendar; markers at 8 January (first galaxies), 20 January (Milky Way’s oldest stars), March to April (the disc forms).

The first galaxies were assembling by the 8th of January on the cosmic calendar. The Milky Way’s oldest stars date from around the 20th. Its disc, the part we live in, took shape through March and April.

[00:47] S03 · ANIM · Typographic: 1920, the Great Debate; then a pulsating star in Andromeda with a distance readout jumping to millions of light-years; the universe expands outward a hundred thousand times.

In 1920 astronomers held a formal debate about whether the Milky Way was the whole universe, and couldn’t settle it. Edwin Hubble did. Between 1923 and 1929, using a kind of pulsating star whose rhythm reveals its distance, he showed that Andromeda lies far beyond the Milky Way’s edge. A galaxy of its own. And the sky was full of them. In a few years, the universe became a hundred thousand times bigger than anyone had known.

[01:23] S04 · ANIM · The Milky Way from above: bar, arms, disc; the Sun’s position 27,000 light-years out; its orbit traced; a counter of twenty circuits; zoom into the centre and the black hole’s shadow.

Our own galaxy is a barred spiral: a disc a hundred thousand light-years across, a bar of stars through the middle, arms winding out. The Sun sits here, twenty-seven thousand light-years from the centre, going round once every two hundred and thirty million years. About twenty laps since it formed. At the centre, hidden behind dust, a black hole of four million solar masses. In 2022 its shadow was photographed by a telescope the size of the Earth.

[01:59] S05 · ANIM · A spiral arm as a traffic jam: stars pass through it; gas piles up; bright blue stars ignite inside the jam.

The arms aren’t fixed groups of stars. They’re patterns moving through the disc, like a traffic jam that persists while every car passes through. Gas piles up in the jam, and that’s where the bright young stars are born. That’s why the arms shine.

[02:19] S06 · ANIM · A rotation curve: expected falling, measured flat; the galaxy shrinks inside a vast translucent halo.

Now the part you can’t see. Measure how fast the disc rotates at different distances and the outer parts move as fast as the inner ones. The visible matter can’t do that. There must be far more mass, spread far beyond the stars. The Milky Way’s dark halo reaches out perhaps a million light-years and holds about a trillion solar masses. The galaxy sits inside it like a city in a fog. Every galaxy does. The stars are the part that shines. They’re not the part that dominates.

[02:00] S07 · ANIM · Small clumps of dark matter merge into larger ones over a clock of billions of years; a galaxy assembles; a smaller galaxy is swallowed and its stars scatter into a halo.

Galaxies weren’t born whole. In the Dark Ages, small clumps of dark matter gathered gas and made the first stars, then merged into larger clumps, and those into larger still, over billions of years. From the bottom up. The Milky Way’s history is written in the motions of its stars. The Gaia satellite, which has measured over a billion of them, found in 2018 the scattered remains of a galaxy a quarter of the Milky Way’s mass that we swallowed ten billion years ago. Streams of stars from smaller victims still trail across our sky.

[03:43] S08 · ANIM · Andromeda approaches the Milky Way at 110 km/s; two futures branch: a merger into an elliptical, and a near miss; a coin flips.

The next big event is a live argument. Andromeda is approaching at a hundred and ten kilometres per second. For a decade the prediction was a collision in four and a half billion years. A 2025 analysis that took the uncertainties seriously found only a coin-flip chance of a merger within ten billion. Whether we end as part of a giant elliptical or drift on alone: open.

[04:14] S09 · ANIM · Zoom out: the Local Group; the Virgo cluster; Laniakea’s flow lines converging; then the cosmic web of filaments and voids across the observable universe.

Zoom out. The Milky Way belongs to the Local Group, a loose family of about a hundred galaxies dominated by Andromeda and us. The Local Group sits on the edge of the Virgo cluster, a thousand galaxies fifty million light-years away. Virgo is part of Laniakea, a hundred thousand galaxies half a billion light-years across, named in 2014 with a Hawaiian word for immense heaven. And Laniakea is one knot in a web: galaxies strung along filaments around enormous empty voids, a pattern grown from the ripples in the afterglow, across the whole observable universe. Hundreds of billions of galaxies. Perhaps two trillion. Each a system like ours.

[05:04] S10 · REAL · A galaxy-survey map of the cosmic web (SDSS, DESI or a simulation frame; confirm credit line).

This is a map of a slice of it, every dot a galaxy. The web is not a metaphor. It’s a structure, the largest there is, and the afterglow’s ripples are its seed.

[05:19] S11 · ANIM · Labels: ESTABLISHED for what a galaxy is and how it grew; ACTIVE RESEARCH for the Andromeda merger, the missing satellites and how galaxies stop forming stars.

The scorecard. What a galaxy is, that most of its mass is dark, that it grew by mergers from the bottom up, and how the web is arranged: established. Whether Andromeda hits us, why there are fewer small satellite galaxies than the model predicts, and how a black hole a billionth of a galaxy’s size can shut off its star formation: active research.

[05:48] S12 · AI into ANIM · Andromeda again, now with the Milky Way’s disc in the foreground; the calendar returns.

So look at Andromeda again. A few hundred billion stars, a black hole, gas, dust, and a trillion solar masses of something invisible, arriving at a hundred and ten kilometres per second, maybe. Everything we know about it we learned from its light. Next lesson: how to read starlight.

Shot list

One AI shot, nine code-rendered shots, two agency images. Estimated Higgsfield use: 2 to 4 generations including retries.