Why a nearby star and a distant galaxy share a constellation

A constellation is a direction on the sky, not a place in space

The 88 constellations are areas of the sky, fixed by the International Astronomical Union in 1930 so that every point belongs to exactly one. Anything that happens to lie in that direction belongs to the constellation, whatever its distance: a star in the next street of the galaxy, a nebula across the spiral arm, a galaxy far beyond our own.

Distances here are in light-years (ly), the distance light travels in a year, about 5.9 trillion miles. Light from a star 100 light-years away left it 100 years ago; light from a galaxy 50 million light-years away left before the first apes.

A flat star chart hides the depth because the eye cannot judge distance at all past a few hundred yards. Two points of light side by side may be a pair in orbit or millions of light-years apart.

Ursa Major: the Big Dipper's stars are about 80 light-years away, its galaxies up to 82 million

Five of the seven Big Dipper stars (Merak, Phecda, Alioth, Mizar and Megrez) belong to the Ursa Major Moving Group, stars born together about 80 light-years from us and still traveling through space as a loose family. Dubhe and Alkaid, at the two ends, are not members.

The deep-sky objects of Ursa Major are a different story. The Owl Nebula, M97, a planetary nebula inside our galaxy, is about 2,600 light-years away. Everything else on the Messier list here is a galaxy: Bode's Galaxy, M81, and the Cigar Galaxy, M82, at about 11.5 to 12 million light-years; the Pinwheel Galaxy, M101, at 22 million; and M109 at 82 million. Seen in the same binocular field, the stars of the Dipper and M81 are a million times different in distance.

See Ursa Major.

Sagittarius looks toward the center of the Milky Way and past it to another galaxy

The Teapot stars of Sagittarius are near neighbors, 78 to 510 light-years away. Behind them the line of sight runs along the Milky Way toward its center, about 26,000 light-years away, so the constellation is packed with objects at every depth:

• Open clusters and nebulae in the next spiral arm in, the Sagittarius Arm: M25 at 2,100 light-years, the Lagoon Nebula, M8, and the Trifid Nebula, M20, at about 4,100 to 4,300.
• The Small Sagittarius Star Cloud, M24, a window through the dust to stars about 10,000 light-years away.
• Globular clusters orbiting the galactic center: M22 at 10,800 light-years, M69 and M70 at 29,000 to 30,500, beyond the center itself.
• M54 at 85,700 light-years, which belongs to a different galaxy: the Sagittarius Dwarf Spheroidal Galaxy, being pulled apart by the Milky Way on the far side of the center.
• Barnard's Galaxy, C57, a dwarf galaxy of the Local Group 1.5 million light-years away.

See Sagittarius and the Milky Way's structure.

Orion is the exception: most of its bright stars and its nebula belong to one star-forming region

Orion is one of the few constellations where the pattern is partly real. Rigel, the three Belt stars (Alnitak, Alnilam and Mintaka), Saiph and the Orion Nebula, M42, lie between about 650 and 2,000 light-years away, most of them members or neighbors of the Orion OB1 association, a group of young, hot stars born from the same giant molecular cloud. Betelgeuse at 500 light-years and Bellatrix at 250 are foreground stars, and Tabit, the brightest star of Orion's shield, is only 26 light-years away. Hatysa, the star at the tip of Orion's sword, is listed at 2,330 light-years from an uncertain parallax and is probably closer, near the Orion Nebula.

Nothing in Orion's Messier list lies outside the Milky Way, so the whole constellation spans a factor of less than 100 in distance, against a million or more for Ursa Major. See Orion.

In 14 constellations the farthest object is more than a million times farther than the nearest star

For each constellation, the nearest and farthest of its bright stars and its Messier and Caldwell objects with a known distance. Distances are in light-years.

Stars are the brightest of each constellation in the site's star dataset. Constellations with no Messier or Caldwell object are left out.

Constellations along the Milky Way hold nebulae and clusters; those away from it hold galaxies

In 34 of the 62 constellations the farthest object is in the disk of the Milky Way, within 100,000 light-years. These lie along the band of the Milky Way (Scutum, Puppis, Carina, Norma, Vulpecula), where the galaxy's own dust hides whatever lies beyond and the nebulae and clusters of the disk fill the view. In the other 28, the farthest object is another galaxy or a globular cluster far out in the galaxy's halo. The great galaxy fields of Virgo, Coma Berenices, Leo, Ursa Major and Canes Venatici all lie well away from the Milky Way, where the view out of the galaxy is clear.

Parallax measures the stars; standard candles and redshift measure the galaxies

• Parallax: the small shift of a nearby star against the background as Earth orbits the Sun. The European Space Agency's Gaia spacecraft measured parallaxes for about 1.5 billion stars, giving distances to most of the stars and open clusters here.
• Standard candles: stars of known true brightness, such as Cepheid variables and the brightest red giants, whose apparent faintness gives the distance to nearby galaxies such as M81 and M101.
• Redshift: the stretching of a galaxy's light as the universe expands. It gives the distance to the far galaxies, such as NGC 4889 in the Coma Cluster.

Each step is calibrated on the one before, and uncertainty grows with distance: a few percent for a nearby star, 10 percent or more for a galaxy.