Galaxies & the Milky Way: Island Cities of Stars, Weighed by Their Spin

On a dark night away from city lights, a pale band crosses the sky. Peoples everywhere have read it. In Chinese tradition it is the Silver River. The Inca read animal shapes in its dark dust clouds, and Aboriginal Australians saw an emu stretched along its dark lanes. All were looking at the same thing: the Milky Way's disk, seen edge-on from a point inside it.

It took until the 1920s to learn that the band is only one galaxy among a great many. In 1923 and 1924 Edwin Hubble used Cepheid stars to show that the Andromeda "nebula" was far outside the Milky Way, and the universe suddenly held islands of stars. Today the study of galaxies asks how these systems were built, how fast they turn, what holds them together and how they change.

This piece starts from inside the Milky Way, where we stand, and moves outward to its neighbours and to its centre. It goes beyond the dark-matter rotation curve of Astrophysics & Cosmology (34/88) rather than repeating it: here the question is the structure of galaxies, why they have their shapes, and what happens when they meet.

The Milky Way from inside: a galactic year

The Milky Way is a flat disk with a bulge and a bar at the middle, wrapped in a thin, nearly invisible halo of old stars. The stellar disk is about 100,000 light-years across, and the Sun sits at roughly 8.2 kiloparsecs, about 27,000 light-years, from the centre (a kiloparsec is 3,262 light-years). The distance was pinned down by watching the star S2 orbit the black hole at the centre (GRAVITY collaboration, about 8.28 kpc in 2021). The Sun moves around the centre at about 230 km/s, with estimates from the Gaia satellite in the range of roughly 229 to 236.

Worked: 2π × 8.2 kpc is 1.59 × 10¹⁸ km. Divide by 230 km/s: 6.9 × 10¹⁵ seconds, or about 219 million years. A "galactic year" is often quoted as 225 to 250 million years, depending on the speed and distance chosen. Since the Solar System formed 4.57 billion years ago, it has gone round about 21 times. The last time the Sun was where it is now, dinosaurs were new.

Without JavaScript: a star at 8.2 kpc moving at 230 km/s takes 219 million years per lap and has made about 21 laps since the Solar System formed.

Notice what the slider shows. Hold the speed fixed and a star twice as far out takes twice as long, because it has twice as far to go. Real disks are close to this: speeds stay about the same over a wide range of radii. That flatness is the whole reason galaxies need extra mass, taken up in the next section.

Mapping the disk from inside is hard, because dust hides the centre in visible light. Infrared and radio see through it. The European Space Agency's Gaia satellite measured the positions and motions of about 1.8 billion stars in its third data release (13 June 2022), the most detailed map of our galaxy so far. Gaia stopped observing on 15 January 2025. Its data show that the disk is warped and rippling, and that the halo is full of streams of stars torn from smaller galaxies. One of them, the Gaia–Enceladus (or "Sausage") merger, brought in a galaxy about 8 to 11 billion years ago; the Sagittarius dwarf galaxy, found in 1994, is being pulled apart now.

Weighing a galaxy by how fast it turns

Orbits give mass. The faster a star circles at a given radius, the more gravity must be pulling it inward from the mass inside. Worked: with the Sun's numbers, the speed of 230 km/s at 8.2 kpc gives a mass inside the Sun's orbit of 1.0 × 10¹¹ Suns, from just the speed and radius.

Apply it far out, where stars and gas are thin on the ground, and the mass still grows. That is the dark-matter evidence of 34/88. Here is a second and stronger test. Across thousands of spiral galaxies, the speed at which the outer parts rotate predicts the total mass of the stars and gas, with little scatter. The baryonic Tully–Fisher relation says the mass of stars plus gas grows as the fourth power of the flat rotation speed: double the speed and the visible mass is 16 times bigger. Brent Tully and Richard Fisher found the original relation for luminosity in 1977; Stacy McGaugh and colleagues later showed that it is tightest when gas is counted too. The fit gives about 47 Suns for each (km/s)⁴.

Without JavaScript: at 230 km/s, the mass inside 30 kpc is about 3.7 × 10¹¹ Suns and the Tully–Fisher relation gives about 1.3 × 10¹¹ Suns of stars and gas; at 40 km/s inside 6 kpc, the mass is about 2.2 × 10⁹ Suns and the relation gives 1.2 × 10⁸.

The slider tells a clear story: the smaller the galaxy, the larger the share of invisible mass inside its measured radius. The relation is tight and has a slope near four. In the standard dark-matter picture, that tightness has to arise from how galaxies form in their halos, and its precision is an open challenge for simulations. A competing idea, Mordehai Milgrom's MOND (1983), predicts a slope of exactly four as a law. Each side points to the data it explains best, and clusters and the cosmic microwave background, which MOND struggles with, are the main evidence against it. The Milky Way's total mass, dark halo included, is roughly 1 to 2 trillion Suns, uncertain by a factor of about two.

The galaxy zoo: shapes that record how stars were made

In 1926 Edwin Hubble sorted galaxy photographs into a scheme that later became the tuning fork. On one prong: ellipticals, from round (E0) to flat (E7). On the other: spirals, from tight arms and a big bulge (Sa) to loose arms and a small bulge (Sc), with a parallel prong of barred spirals. Between them: lenticulars (S0), disks without arms. Hubble did not mean it as an evolutionary sequence, and it is not one, but the shapes do carry history.

The shapes go with colour. Ellipticals are red because their stars are old; they have little cold gas, and make few new stars. Spirals are bluer because their arms are lit by young, hot stars from gas that is still collapsing. Plot colour against brightness for many galaxies and they fall into two groups, a red sequence and a blue cloud, with a sparse "green valley" between, where galaxies are shutting off star formation. What switches it off (a black hole's heating, the loss of gas in a cluster, mergers) is under study. The Milky Way is a barred spiral near the green valley, with a bar that Spitzer Space Telescope maps confirmed in 2005.

Without JavaScript: E0 round and old; E5 flattened; S0 disk without arms; Sa tight arms and a big bulge; Sc loose arms and many young stars; SBb has a central bar.

Spiral arms: a traffic jam, not a river

Spiral arms pose a puzzle. Stars near the centre go round faster than stars far out. If the arms were fixed chains of stars, they would wind up like a coil and vanish into tight rings within a few hundred million years. The Milky Way has gone round some 50 times in its life, and the arms are still open. This is the winding problem.

The classic answer, by C. C. Lin and Frank Shu in 1964, is that arms are not made of fixed stars but are a wave of extra density that moves through the disk at its own speed, like a traffic jam on a motorway. Cars enter, slow, leave, and the jam stays in place. Stars and gas pile up as they pass through, and gas compressed in the wave forms new stars, which is why arms are lit blue. Estimates of the Milky Way's pattern speed run roughly from 20 to 30 km/s per kiloparsec, and not everyone agrees. Computer simulations also show arms that form, break and re-form over a few hundred million years, and grand-design spirals such as M51 are tied to the tidal pull of a nearby companion. Several mechanisms probably operate in different galaxies.

Without JavaScript: in 1 billion years a star at 4 kpc goes round about 9 times and one at 12 kpc about 3 times, so an arm made of fixed stars would be wound round about six extra times.

Galaxies collide: the Milky Way and Andromeda

The Andromeda galaxy (M31) is about 2.5 million light-years away and the nearest large galaxy. Its light is shifted toward blue, so it is approaching us at about 110 km/s. Gravity speeds up the approach. Whether the galaxies actually collide depends on the sideways motion, which is hard to measure because Andromeda's apparent drift across the sky is almost impossibly small. Hubble Space Telescope measurements published in 2012 by Sangmo Tony Sohn, Jay Anderson and Roeland van der Marel found sideways motion small enough to be consistent with zero, and predicted a merger in about 5 billion years. A later analysis in 2025 by Till Sawala and colleagues, which includes the pull of the galaxy M33 and the Large Magellanic Cloud and allows for measurement uncertainty, concluded that there is only about a 50% chance of a merger within the next 10 billion years.

Without JavaScript: at the default mass, with no sideways motion, the galaxies fall straight together in about 3.3 billion years; with 100 km/s sideways, they miss by about 180 kpc on the first pass.

If they do merge, nothing like a crash of stars occurs: the gaps between stars are so wide that stars almost never collide. The two disks are scrambled, gas is compressed and a burst of star formation follows, and in the end the pair settle into one big, round galaxy, an elliptical. This is how many ellipticals are thought to have formed. Mergers are a main theme of modern galaxy formation, and the Milky Way's halo already holds the remains of galaxies it absorbed.

The monster in the middle: weighing Sagittarius A* with one star

At the centre of the Milky Way a dense cluster of stars orbits something dark. For nearly three decades two teams, led by Reinhard Genzel and Andrea Ghez, tracked individual stars through the dust with infrared telescopes in Chile (the European Southern Observatory) and Hawaii (Keck). The star S2 goes round in 16.05 years on an elongated orbit, coming within about 120 AU of the object, which is roughly four times Neptune's distance from the Sun, at about 2.5% of the speed of light.

Kepler's third law, from 36/40, does the rest: mass equals the cube of the orbit's size in AU over the square of the period in years, in Suns.

Worked: S2's orbit has a of about 1,035 AU and P = 16.05 years. The cube is 1.11 × 10⁹, the square of the period is 257.6, so M is 4.3 million Suns, in agreement with the published 4.3 million. That mass packed inside an orbit smaller than our Solar System is too compact to be anything but a black hole. Genzel and Ghez shared the 2020 Nobel Prize in Physics for this, with Roger Penrose. In 2022 the Event Horizon Telescope published the first image of its glowing ring of gas.

Without JavaScript: a = 1,035 AU and P = 16.05 years give about 4.3 million Suns. The Earth around the Sun gives 1, which checks the formula.

Almost every large galaxy seems to have such a black hole, and the more massive its bulge's stars, the heavier it is: the black hole's mass tracks the stars' random speeds in the bulge, a relation found around 2000. That is odd, because the hole's pull reaches only a tiny part of the bulge. It hints that galaxies and their central holes grew together, perhaps through the energy the hole releases when it feeds. Astronomers are still working out how.

What a graduate just sees

• A galaxy's spin is a scale: orbital speed gives the mass inside, and a lap for the Sun takes about 220 million years.
• The visible stars are a minority of the mass inside a galaxy's disk, and smaller galaxies are more dark-matter dominated.
• Galaxy shape records history: red and round means old stars and few new ones, blue and spiral means gas and star formation.
• Spiral arms are waves of density, not rivers of stars, but how they start is still debated.
• Big galaxies are built by merging smaller ones, and collisions mix gas and stars rather than smashing stars.
• Kepler's law at the very centre of a galaxy weighs its black hole.
• Most of what we call "the Milky Way" is a view from inside, so mapping it needs infrared, radio and careful statistics.

Where it shows up next

• Astrophysics & Cosmology (34/88): galaxies are the tracers by which the expansion, dark matter and the cosmic web are measured.
• Compact Objects & High-Energy Astrophysics (36/43): central black holes switch on as quasars when they feed, and galaxy clusters shine in X-rays.
• Astronomical Data & Computation (36/44): Gaia's billions of stars and the Vera C. Rubin Observatory's survey in Chile are galaxy science run as data science.
• In the world: radio arrays such as MeerKAT in South Africa, which imaged the Galactic Centre in 2022 and is a precursor to the Square Kilometre Array, and the stars in your sky.

If you want to go deeper

• 📘 Textbook. Linda Sparke and John Gallagher, Galaxies in the Universe: An Introduction (Cambridge University Press, 2nd edition 2007), a standard undergraduate text. James Binney and Michael Merrifield, Galactic Astronomy (Princeton University Press, 1998), for the Milky Way in detail. James Binney and Scott Tremaine, Galactic Dynamics (Princeton, 2nd edition 2008), for graduate study.
• 📘 Gentler. Ken Croswell, The Alchemy of the Heavens (Anchor Books, 1995), a readable account of how we mapped the Milky Way.
• 🌐 Free tools. Zooniverse, where Galaxy Zoo volunteers classify real galaxy images, and the free planetarium program Stellarium for finding the Milky Way's band.
• 🌐 Free course. MIT OpenCourseWare 8.902 Astrophysics II (Fall 2004), covering galaxy dynamics, active galaxies and cosmology.