The whole bright star catalogue — 9096 objects down to sixth magnitude — and the official boundaries of all 88 constellations. Turn on your location and the map shows the sky that is over you now. Switch the colouring from apparent brightness to true luminosity and it becomes clear which of the bright stars are genuinely brilliant and which are merely close.
Sources
Positions, magnitudes, spectra and proper motions come from the Bright Star Catalogue, 5th ed. (Hoffleit & Warren, 1991). Distances use Hipparcos parallaxes (ESA, 1997): BSC5's own parallaxes predate the satellite and give negative distances for the far stars. Constellation boundaries are Davenhall & Leggett (1989), precessed here from B1875 to J2000. Star names are the official catalogue of the IAU Working Group on Star Names (CC BY). Accessed through the VizieR service, CDS, Strasbourg.
Frequently asked
Why exactly 9096 stars?
That is the complete Yale Bright Star Catalogue — everything brighter than about sixth magnitude, which is precisely what an unaided eye separates on a dark night. Fainter stars exist in other catalogues, but there are billions of them, and on a sky map they would turn into an even wash.
What is true luminosity and how does it differ from brightness?
Apparent magnitude only says how much light reaches us, which depends both on the star's output and on its distance. Absolute magnitude brings every star to a common distance and shows how powerful it really is. Switch between the modes: Sirius is among the brightest stars in the sky only because it is next door, while Rigel pours out forty thousand times the Sun's light from eight hundred light years away.
Why is the distance of some stars given as a range?
Distance is measured by parallax — the tiny shift of a star against more distant ones over half a year. The further the star, the smaller the shift, and for the most distant it becomes comparable to the measurement error. Deneb's parallax carries a 56 per cent uncertainty, so a range is the honest answer rather than a single number.
Why shift time by thousands of years?
Stars move. Each travels its own path, and the constellations are temporary shapes. Over a hundred thousand years the Plough becomes unrecognisable, because five of its stars travel together and the two at the ends go the other way.
A constellation is not a group of related stars but a region of sky. Since 1930 the International Astronomical Union has divided the celestial sphere into 88 areas bounded by lines of constant right ascension and declination, and every point of sky belongs to exactly one of them. Those are the boundaries drawn on this map.
A star's apparent brightness says almost nothing about the star. Sirius looks brightest only because it is eight light years away, while Deneb, far more powerful, barely makes the top twenty because it is hundreds of times further off. Switching the colouring from apparent to absolute magnitude separates the near neighbours from the true beacons at a stroke.
The atlas works offline once loaded: the catalogue, the constellation boundaries and the names all live in the application, with no external service and no key. The sky's orientation is computed from your latitude, longitude and the time, through sidereal time and the transformation from equatorial to horizontal coordinates.