Life and death across a spiral arm: gas is squeezed into stars as it enters an arm, and those stars age and die downstream, which paints every spiral in bands.
At a glance
| supernovae in the Galaxy | 1.63 per century, all types |
|---|---|
| of which Type Ia | 25%model |
| a remnant stays visible for | about 100,000 yearsmodel |
| remnants at any moment | about 1,630model |
| massive stars explode | 3 to 40 million years after they formmodel |
| stars and gas orbit at | 233 km/smodel |
| the spiral pattern turns at | 25 km/s per kpcmodel |
| the two match (corotation) | 9.3 kpc from the centremodel |
Read off the piece’s own catalogue, the same values it draws with. Model marks a property of the piece’s galaxy model rather than a measurement.
About Life and death across a spiral arm
A spiral arm is not a fixed crowd of stars. It is a wave of crowding that stars and gas pass through, the way traffic passes through a slow stretch of road. Inside the Sun’s orbit the gas overtakes the pattern; far outside it, the pattern overtakes the gas. Where the gas runs in, it is compressed and shocked, and that shock is the dark lane of dust along one edge of an arm.
The compressed clouds collapse into clusters. The heaviest stars light the gas around them into a pink nebula that lasts only as long as they do. Then the cluster shows bare and blue, drifting on out of the arm; as its hottest stars die it turns white, then cream, and slowly loosens into the general field of stars. The order across an arm, dark lane, pink knots, blue clusters, gold disc, is that sequence of ages laid out in space.
The heaviest stars end as supernovae while their cluster is still young, so the explosions stay close to the arms; white dwarfs in the old disc add a quieter share of their own. Each leaves an expanding shell of hot gas that slows, cools and fades into its surroundings. Over a human lifetime the Galaxy makes very few, and dust hides most of them. On the time scale of its turning they never stop.
What you see in the piece
Speed up the clock at galaxy scale and the complexes stream through the arms: pink just behind the dust lanes, blue further on, cream far downstream, then gone. In the radio and X-ray bands the arms fizz with supernova remnants, a fresh set every frame, because a single frame spans more than a remnant’s whole life. The panel counts the supernovae between today and the date on the clock; the flashes themselves are not drawn, since each would last far less than a frame.
Measured or modelled
The arms are modelled as a density wave turning at a single pattern speed, with the gas on a flat rotation curve, and the age of the gas is the time since it last crossed a dust lane. Both speeds, and so the radius where they match, are uncertain; near that radius the model’s gas crosses arms so rarely that its star formation fades, a prediction of density-wave theory that the real Galaxy only partly follows. The supernova rate is a measured estimate; where each one goes off is the model’s.
Questions
How often does a supernova happen in the Milky Way?
About 1.63 per century across the whole Galaxy, by the estimate the piece uses. Most are hidden from us by the dust in the disc.
How many supernova remnants are there in the Milky Way?
A remnant stays visible for about 100,000 years, so at any moment the piece’s model holds about 1,630 of them.