Galaxy Builder

Gas crowding into an arm forms stars, and the most massive of them are hot, blue and short-lived: they burn out within about ten million years, before they have drifted far from the arm that made them. Dark lanes of dust mark the edge where the gas enters an arm and is compressed; the young blue stars and glowing pink clouds of hydrogen lie just past the lanes, and the stars drift on across the arm as they age.

Inside corotation, the radius where the stars keep pace with the pattern, the gas and stars overtake the arms, so they enter each arm at its inner edge, where the dust lanes lie. Beyond corotation the pattern overtakes them and the sides swap. The model keeps that order but spreads it out: it draws its dust just upstream of the crest where its orbits crowd, and lights its young stars at the crest and for ten million years downstream of it, so in the model the blue edge is the outer one inside corotation.

Stars near the centre go round faster than stars far out. If the arms were made of the same stars for good, the inner part would lap the outer part and wind the arms tight within a few turns. Switch the arms to Material and watch it happen. Real spirals are not wound that tight, which is why astronomers concluded that arms are patterns the stars pass through: density waves (C. C. Lin and Frank Shu, 1964).

If a galaxy were only its stars and gas, its outer stars would orbit more and more slowly with distance, as the outer planets do around the Sun. When Vera Rubin, Kent Ford and their colleagues measured the rotation of spiral galaxies, the curves stayed flat far out (Rubin, Ford and Thonnard, 1980). Something unseen holds those stars in, and the leading explanation is a halo of dark matter. Turn the halo down and watch the outskirts slow and shear. In the Triangulum preset the measured curve reaches far past the stars: its gas keeps circling out to 23 kpc, and dark matter dominates outside about 7 kpc (Corbelli et al., 2014).

Stars many times the Sun’s mass burn through their fuel in about ten million years or less (Ekström et al., 2012), and they end as supernovae, so a supernova of that kind goes off soon after, and not far from, where its star formed. Aramyan and colleagues (2016) measured where 215 supernovae lie across the arms of their host galaxies: the core-collapse ones sit at a set place across the arm, in grand-design spirals, nearer the edge where the gas enters and stars form (the inner edge inside corotation, the outer edge beyond it), and near the arm’s peak in the rest, as short-lived stars formed in the arms would, while Type Ia supernovae, whose stars are older and less massive, are spread more widely around them.

The Milky Way has about 1.6 core-collapse supernovae a century (Rozwadowska, Vissani and Cappellaro, 2021). In the model a flash marks one: a young star ending its life about ten million years after it crossed an arm. So the model’s flashes come on the far side of its crest, further from where the gas enters than the measured ones. The model also flashes Type Ia supernovae, scattered over the old stars rather than tied to the arms: roughly one a second at ten million years a second on a desktop screen, which at the Milky Way preset is about a quarter as many as the core-collapse flashes, the ratio Li et al. (2011) give for a galaxy like ours. The screen shows far fewer than the real galaxy would: at the Milky Way preset on a desktop screen, each flash stands for about 44,000 real supernovae, and for about 160,000 on a phone or tablet (or any window 640 px wide or narrower), which draws a quarter of the stars. Other settings of the knobs, and the other galaxies, change the number.

The crowd of stars at the centre of the Milky Way is called the bulge, and it is bar-shaped, about 13,000 light years long. A supermassive black hole sits at the middle, about four million times the mass of the Sun.