The Milky Way in COSMOS: a reconstruction, labelled as one

Measured facts

Representation

Weighted density tracers, not a photograph

Tracer count

230,000

Deterministic seed

1129272141

Generated

2026-09-05T00:13:06.042Z

Binary SHA-256

bb34a7cd940fb240c352ff3a8fd5e9cfcbfa9fc737b8a42e5aa97df93aba8148

Model boundary

20 kpc Galactocentric radius

The five structural components

Each component is sampled from published parameter values rather than drawn by hand, and every component carries the citation for the constraint it implements.

Samples published analytic density laws with deterministic seed 0x434f534d.

Uses exponential thin/thick disc parameters and oblate power-law halo parameters from Jurić et al. (2008).

Uses the McMillan (2017) axisymmetric bulge density law and a 30° long-bar tracer constrained by Wegg et al. (2015).

Places the Sun at R0=8.275 kpc and z=20.8 pc, then rotates the model into the ICRS orientation used by the measured Gaia layer.

Tracer allocation and per-point display weights are rendering budgets, not estimates of component star counts, mass, or luminosity.

No spiral arms, dust extinction, warp, flare, streams, clusters, gas, or dark matter are encoded in version 1.

Thin stellar disc — 125,000 tracers

radialScaleLengthKpc: 2.6 · verticalScaleHeightKpc: 0.3 · maximumSampledRadiusKpc: 20https://doi.org/10.1086/523619

Thick stellar disc — 38,000 tracers

radialScaleLengthKpc: 3.6 · verticalScaleHeightKpc: 0.9 · localDensityRelativeToThinDisc: 0.12 · maximumSampledRadiusKpc: 20https://doi.org/10.1086/523619

Inner stellar halo — 20,000 tracers

densityPowerLaw: -2.8 · axisRatio: 0.64 · localDensityRelativeToThinDisc: 0.005 · sampledRadiusKpc: 0.5–20https://doi.org/10.1086/523619

Central bulge — 28,000 tracers

innerScaleKpc: 0.075 · cutoffRadiusKpc: 2.1 · densityExponent: -1.8 · axisRatio: 0.5 · maximumSampledRadiusKpc: 4https://arxiv.org/abs/1608.00971

Long bar — 19,000 tracers

halfLengthKpc: 5 · majorAxisAngleDegrees: 30 · thinScaleHeightKpc: 0.18 · superthinScaleHeightKpc: 0.045https://doi.org/10.1093/mnras/stv745

How it meets the measured catalogue

The reconstruction fades in between 40 pc and 1 kpc of observer distance from the Sun while the measured Gaia volume fades out. Both render during the overlap, so the transition is continuous rather than a scene replacement, and the two layers keep separate display units and separate provenance.

The solar placement uses a Galactocentric radius of 8.275 kpc and a solar height of 20.8 pc above the midplane, taken from published measurements, and the display asset subtracts the Sun at that point before applying the standard Galactic-to-ICRS rotation.

What version 1 deliberately omits

Empty or smooth regions in this reconstruction are not non-detections. They are structures COSMOS has not modelled yet, and the list is disclosed in the renderer itself.

• Spiral arms and arm-based structure
• Dust extinction and gas
• Disc warp and flare
• Stellar streams, open and globular clusters
• Satellite galaxies and the Magellanic system
• Dark matter
• Any structure beyond the declared 20 kpc Galactocentric boundary, which is not extrapolated

Limitations COSMOS discloses

• These points are weighted tracers of a density law. They are not stars, mass, luminosity or a count of objects.
• Spiral arms, dust extinction, gas, warp, flare, streams, clusters, satellite galaxies and dark matter are absent from version 1.
• The declared 20 kpc model boundary is not extrapolated into an outer halo that was never sampled.
• This is not an external photograph of the Galaxy. No spacecraft has taken one, and COSMOS never presents the reconstruction as an observation.