What Galaxies Are Made Of

From shapes to parts

In the last guide we lined the galaxies up along Hubble's tuning fork and learned to name them by their looks — smooth ellipticals on the handle, graceful spirals fanning out along the prongs, with lenticulars and irregulars filling the gaps. But a name for a shape is only the cover of the book. A galaxy is not a smear of light; it is an assembled object with distinct working parts, each made of particular stuff, each with its own history. This guide opens the book and reads the parts.

We have the perfect specimen to dissect: our own Milky Way. Two guides ago, in the Milky Way rung, you walked its disk, bulge and halo from the inside. The lesson there generalises beautifully — those same parts, in different proportions, build nearly every large galaxy in the sky. So when we name a galaxy's anatomy here, you are not meeting strangers; you are recognising your own neighbourhood, scaled up and seen from outside.

Anatomy of a spiral

Start with a spiral, because it has the most parts on display. The dominant feature is the disk: a flat, rotating plate of stars and gas, typically tens of thousands of light-years across yet only a few hundred light-years thick — proportionally far thinner than a sheet of paper. Everything in the disk orbits the centre in the same direction, in roughly circular paths, like cars on a vast roundabout. The disk is where a spiral does its living: it holds the cold gas, and it is where new stars are born.

Swelling out of the disk's centre is the bulge — a rounder, denser pile of mostly older stars, like the yolk at the heart of a fried egg whose white is the disk. Threading through many bulges is a straight bar, a cigar-shaped river of stars that funnels gas inward toward the centre. And wrapped around the whole flat system is the stellar halo: a sparse, roughly spherical sprinkling of the galaxy's oldest stars, often bound up into tight ancient swarms called globular clusters. Disk, bulge, bar, halo — four parts, and we have not yet touched the most beautiful one.

The most beautiful part is the spiral arms themselves — and here lurks a lovely misconception. The arms are not rigid pinwheel blades made of a fixed set of stars; if they were, the disk's differing orbital speeds would wind them up into a tight coil within a few rotations, and they would have unwound any sign of pattern long ago. Instead the leading idea, density-wave theory, says an arm is a slow-moving traffic jam: a wave of slightly higher density that stars and gas drift into and out of, like cars bunching up at a slow patch on a motorway. The cars (and stars) keep moving on; the jam itself rolls along at its own gentler pace.

That picture explains why the spiral arms shine so brightly. Where gas piles up in the density wave, it is squeezed hard enough to collapse and ignite fresh stars. The most massive of those new stars are brilliant, blue and short-lived — they burn out before they can drift far from the arm that made them. So the arms are lit up not because more stars sit there, but because the youngest, hottest stars are born and die there, tracing the wave like foam on the crest of a slow ocean roller.

Blue and young, red and old

Now we can answer the question that the Hubble sequence only posed: why does a spiral look blue and an elliptical look red? The colour of a whole galaxy is the blended light of billions of stars, and its overall hue is set by the hottest, brightest stars in the mix — exactly the lesson of blackbody colours from the light rung, where hot means blue and cool means red. A galaxy that is still forming stars always contains a fresh supply of those massive blue stars, so it glows blue. A galaxy that stopped forming stars long ago has lost them all to death, leaving only the cool, red, long-lived stars behind — so it glows red.

So colour is really a clock. Massive blue stars live only millions of years; small red stars live for many billions. A galaxy's colour therefore tells you, at a glance, whether it has made stars recently. Astronomers make this quantitative with a colour index — the difference in brightness measured through a blue filter and a red one — turning a hue into a number that ranks galaxies from actively star-forming to long dead. This is why colour, not shape, is often the deeper way to classify a galaxy.

Underneath the colour sits the real engine: gas. Stars form only from cold, dense gas, so a galaxy keeps making stars only as long as it holds a reservoir of it. The thin disk of a spiral is exactly such a reservoir, brimming with the interstellar gas you met two rungs ago — much of it cold hydrogen that we can map directly through its 21-centimetre radio glow. Ellipticals, by contrast, are nearly dry: their gas has been used up, blown out, or heated until it can no longer collapse. No cold gas means no new stars; no new stars means no blue light. The whole blue-spiral / red-elliptical split comes down to who still has fuel.

There is one more layer worth naming: not just the age of a galaxy's stars but their chemical makeup. The oldest stars, born when the universe had barely begun cooking heavy elements, are metal-poor; later generations, enriched by the ashes of stars that died before them, are metal-rich. Astronomers fold age and chemistry together into the idea of stellar populations — old, metal-poor halo stars on one hand, younger, metal-rich disk stars on the other. Reading a galaxy's populations is like reading tree rings: each generation records the conditions of the era that made it.

The part you cannot see

We have catalogued everything that shines — disk, bulge, bar, arms, halo. Yet weigh a galaxy by its motion and almost all of it is missing from that list. In the Milky Way rung you met this puzzle at home; the same lesson holds for galaxies across the sky. Measure how fast stars and gas orbit at each distance from a galaxy's centre and you draw its rotation curve. By Newton's gravity, once you pass the bright visible matter the orbital speed should fall off with distance, as the planets slow with distance from the Sun. It does not. The curve stays stubbornly flat, far beyond the last visible stars.

Orbital speed set by the mass enclosed inside radius r:

     v(r) = sqrt( G * M(<r) / r )

  Visible matter only:  past the bright galaxy, M(<r) stops growing
     ->  v should DROP as 1 / sqrt(r)

  What rotation curves actually show:
     v(r) stays ~flat, ~150-300 km/s, far beyond the stars
     ->  for flat v, M(<r) must keep RISING (roughly as r)

  So unseen mass keeps accumulating where there is no light:
     a roughly spherical DARK-MATTER HALO around the whole galaxy

Work the shape of that flat curve backwards and it tells you what is missing: a vast, roughly spherical dark-matter halo that envelops the whole bright galaxy and reaches far past its edge — the visible disk and bulge sitting inside it like a coin dropped into a beach ball. This is not unique to spirals. Weigh ellipticals by the swarming speeds of their stars, weigh whole galaxies by how they bend the light of objects behind them, and the same answer returns: every large galaxy seems to live inside a halo of mass several times heavier than everything we can see. The luminous galaxy, the part this whole subject works to understand, is the minority partner.

Putting the galaxy back together

Step back and the dissection becomes a portrait. A spiral is a thin, gas-rich disk still busy making blue stars along its density-wave arms, plus an older bulge, often a bar, all of it wrapped in a sparse stellar halo and a far larger invisible one. An elliptical is what you get when the disk and the cold gas are gone: a fat, slowly churning ball of old red stars whose orbits point every which way, its star-forming days long over. Same basic ingredients — stars, gas, dust, dark matter — combined in different proportions and at different stages of life.

That raises the obvious next question, and it is the thread the rest of this rung will pull. If a blue spiral is a galaxy with fuel and a red elliptical is one that ran out, what shuts the fuel off? The turning-off of star formation has a name — quenching — and how it happens, through running out of gas, having it stripped away in a crowded cluster, or having it heated by the galaxy's own central black hole, is one of the liveliest questions in the field. We will follow that thread in the formation guide at the end of this rung.

First, though, a surprise that the next guide makes its subject. Knowing all these parts, you might expect each galaxy to be a one-off, its pieces mixed in any old ratio. Instead galaxies obey tight relations: a spiral's rotation speed predicts its total brightness; an elliptical's size, brightness and stellar motions lock together on a single 'fundamental plane'. These regularities are too clean to be accidents — they are fossil rules left by how galaxies form. That is where we go next.