HOW THIS UNIVERSE IS MADE

How this universe is made

The sky of Fly the Milky Way is computed from the numbers in research papers and observational data, again every time the ship moves. Drawn from real numbers, we thought, the sheer distance to the stars is something you can feel. This page explains what each thing on screen is computed from, and how, with its sources.

On screen, information carries one of these marks:

  1. Star positions — 1.94 million observed stars
  2. Star colour comes from temperature
  3. Brightness changes with distance
  4. Dimming and reddening — a 3D dust map
  5. Planets, the Moon and satellites, where they are today
  6. The Galactic Centre — Sgr A* and the S-stars
  7. Where observation ends and the model begins
  8. Where AI was used, and where it was not

1Star positions — 1.94 million observed stars

ObservedThe stars come from the third data release of ESA's Gaia mission (Gaia DR3): every star brighter than G magnitude 11.5 whose parallax is more than five times its error (a well-measured distance), placed where it really is. Distances are the estimates of Bailer-Jones et al. (2021), which combine the parallax with the star's brightness and colour. Very bright stars that Gaia measures poorly are filled in from the new reduction of Hipparcos (van Leeuwen 2007).

Seen from Earth, these stars in 3D make the real night sky and its constellations. Leave Earth and Orion or the Big Dipper fall apart: the stars of one constellation lie at very different distances.

Sources: Gaia Collaboration (Gaia DR3, 2023) / Bailer-Jones et al. 2021, AJ 161, 147 / van Leeuwen 2007, A&A 474, 653

2Star colour comes from temperature

ComputedA star's colour is computed from its surface (effective) temperature. The temperature is Gaia's estimate from its spectra (GSP-Phot) or one derived from the star's colour; for 31 well-known bright stars it is the value measured in the literature, for example Betelgeuse 3,779 K (Borisov et al. 2023) and Rigel 12,100 K (Przybilla et al. 2010).

The steps

  1. Planck's law gives the light a body (a blackbody) at that temperature emits at each wavelength.
  2. Weighting it with the CIE 1931 colour-matching functions (how the eye sees colour, in 5 nm steps) and summing gives the three colour values XYZ.
  3. XYZ is converted to sRGB, the colour standard of screens.

B(λ, T) = (2hc² / λ⁵) · 1 / (exp(hc / λkT) − 1)

Real star colours are much paler than most pictures suggest. An "enhanced" colour option exists in the settings, but the default is the true colour.

Try it: how does a star's colour change with temperature?

This runs the same code that colours the stars in the app (brightness evened out to show the hue only).

Sources: CIE 1931 2° colour-matching functions (CVRL) / sRGB (IEC 61966-2-1) / Borisov et al. 2023, ApJS 266, 11 / Przybilla et al. 2010, A&A 517, A38 / solar temperature: IAU 2015 Resolution B3

3Brightness changes with distance

ComputedThe catalogue holds each star's own brightness (absolute magnitude), worked back from how bright it looks from Earth and how far it is. With the ship's distance to the star, every frame computes how bright it looks now:

m = M + 5 log₁₀(d / 10 pc)

Light falls off with the square of the distance: twice as far, a quarter as bright. Fly closer and a star brightens; fly away and it fades.

The size of each star's dot is adjusted for legibility, but the total light the dot puts on screen is exactly what its magnitude says. That is why the light of many faint stars and the faint glow of the Milky Way add up correctly, in the same units.

Try it: move the same star further away

4Dimming and reddening — a 3D dust map

ComputedInterstellar dust between a star and the ship dims its light, blue more than red, so stars behind dust look fainter and redder. It looks like a sunset, by a similar mechanism.

Around the Sun the dust is the 3D map that Edenhofer et al. (2024) built from Gaia and other observations; beyond the map a smooth dust disc model takes over (thickness from Drimmel & Spergel 2001). The dust is summed along the line of sight from the star to the ship, and each colour is dimmed by its own ratio (Cardelli et al. 1989).

Seen from Earth, each star keeps exactly the extinction Gaia estimated for it. ModelThe change as the ship moves and looks through different dust is computed from the map and the model.

Sources: Edenhofer et al. 2024, A&A 685, A82 / Cardelli, Clayton & Mathis 1989, ApJ 345, 245 / Drimmel & Spergel 2001, ApJ 556, 181

5Planets, the Moon and satellites, where they are today

ComputedThe planets and the Moon are placed for today's date with the ephemeris library astronomy-engine, and the results are checked against NASA JPL's Horizons ephemeris service. Voyager 1 is where it is now, too.

ObservedThe planets' surfaces are maps made from images taken by spacecraft such as Cassini and Voyager.

ComputedThe satellites around Earth are propagated with the standard SGP4 method from the real orbital data published by CelesTrak (orbital elements from the US Space Force's tracking). The date of the data is shown on screen.

Sources: astronomy-engine (Don Cross) / JPL Horizons / NASA/JPL/Space Science Institute and others / CelesTrak (T. S. Kelso), SGP4 (Vallado et al. 2006)

6The Galactic Centre — Sgr A* and the S-stars

ObservedAt the centre of the Milky Way is Sgr A*, a black hole of about 4.3 million solar masses, about 8.3 kiloparsecs (about 27,000 light-years) from the Sun. Both are the 2024 measurements of the GRAVITY Collaboration, and that distance sets the scale of the whole galaxy in the app.

Around it, S2 (one orbit in about 16 years) and the other S-stars move on their observed orbits: S2 from GRAVITY (2024), the others from the orbital elements of Gillessen et al. (2017). Observing these stars led to the 2020 Nobel Prize in Physics. ComputedClose to the black hole, the bending of light by gravity (gravitational lensing) is computed too.

Sources: GRAVITY Collaboration 2024 / Gillessen et al. 2017, ApJ 837, 30

7Where observation ends and the model begins

Gaia mostly measures stars fairly close to the Sun. Most stars on the far side of the galaxy, and stars hidden by dust, are not observed. Those parts are filled in with physical models.

On screenHow it is made
ObservedStar positions, brightness, temperaturesThe Gaia DR3 and Hipparcos catalogue (1.94 million stars)
ModelThe faint glow of the Milky WayThe light of countless stars too faint to see one by one, computed from a galaxy model: discs, the bar (Wegg et al. 2015), the spiral arms (Reid et al. 2019) and more
ModelStars in distant regionsStars generated from the same galaxy model as the glow, drawn as the stars you would see there without changing the total light. Never named, and they can be switched off
ModelThe shape of the whole galaxyThe spiral seen from outside is a model based on observations. The display settings mark every layer drawn from a model with "Model"

When the ship enters a region where model stars dominate, a notice says so. Adding stars or pushing the colours would look more dramatic, but it would no longer be the real galaxy, so we don't.

Sources: Wegg, Gerhard & Portail 2015, MNRAS 450, 4050 / Reid et al. 2019, ApJ 885, 131 / Bahcall & Soneira 1980 (luminosity function) and others

8Where AI was used, and where it was not

All sources, and reporting errors

Every dataset with its source and licence is listed in the app under "About & credits" and in the detailed sources and licences (ASSET_CREDITS.md). If you find an error in a computation or in this page, please tell us at info@3pt.co.jp or with the feedback form in the app.

Fly the Milky Way