Step one
Two surveys, one star map
Gaia DR3 measured over a billion stars with sub-milliarcsecond precision — positions,
parallaxes, proper motions, colours. But Gaia's detectors saturate on the brightest
stars: Sirius, Vega, Betelgeuse. These are exactly the stars people recognise.
The Hipparcos mission (ESA, 1989–1993) was purpose-built for bright-star astrometry.
Its 118,000 stars anchor the naked-eye sky that Gaia cannot reliably reach.
About 100,000 stars appear in both catalogues. The pipeline produces exactly one
canonical row per physical star — choosing the better measurement for each, handling
binary systems, and manually curating the handful of cases that defeat automation
entirely (including the Sun, which does not appear in either catalogue).
How the merge works →
Step two
Cleaned, corrected, and positioned
Each star gets a distance — from the catalog parallax where reliable, from
Bailer-Jones probabilistic estimates where not, from photometric modelling where
those fail, and from a conservative prior as a last resort. Each tier is flagged.
Positions are propagated to a common epoch (J2016.0) using proper motions, and
converted to Sun-centred Cartesian coordinates in parsecs. Temperature comes from
spectroscopic measurements where available, falling back through colour cascades
to a default.
The result is a HEALPix-partitioned Parquet table: one row per star, a fixed schema,
quality flags packed into each record.
Step three
Indexed for the browser
A browser cannot download a billion-row table. The merged data is encoded into a
spatial octree — a tree structure that divides 3D space into nested cells across
fourteen levels. Each star is placed into a level based on its brightness: the
brightest stars sit in the shallowest, largest cells, while fainter stars go into
progressively deeper, smaller ones. The threshold at each level is tuned to how
the human eye perceives starlight.
When you navigate the viewer, it loads cells level by level. Bright-star cells
have large visibility radii and load from anywhere in the scene. Faint-star cells
only load when you fly close enough that those stars would actually be visible.
Everything else stays on the server until you need it.
