Astronomy & Celestial Mechanics
the sky, computed
Concept

Galactic plane — where it appears

The plane of the Milky Way's disc, where its gas, dust and young stars are concentrated. It is the reference plane for galactic coordinates, and its dust makes it the part of the sky most surveys avoid.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Also named here as kinematic distance, tangent point — the same set of essays touches all of them, so they are one junction rather than several.

the Milky Way measured from inside it — the tangent-point construction. Left: the disc in plan, with the Sun at 8.2 kpc from the centre and four lines of sight at galactic longitudes 20°, 40°, 60°, 80°. Each one grazes a circle of radius R₀ sin l, and at that tangent point the whole circular velocity lies along the line of sight, so the largest velocity in the spectrum belongs to a radius the geometry fixes and no distance has to be measured. Right: the four radii and speeds that yields, on the model curve they are read against. The construction reaches only radii inside the Sun's, which is why the outer curve — the half of it that carries the argument — needs distances after all.

A galaxy measured from inside it

The Milky Way is the one galaxy nobody can photograph, and the only one whose rotation can be measured without knowing a single distance. A line of sight at a chosen longitude grazes a circle of known radius, and the fastest gas along it is orbiting there.

galaxies · Galactic structure One velocity, two distances — 4.7 and 9.5 kiloparsecs. The radial velocity of gas along a line of sight at galactic longitude 30 degrees, against distance from the Sun, for a flat rotation curve. The velocity rises to a maximum at the tangent point — where the line of sight is tangent to a circle of radius R₀ sin l — and falls again beyond it, so every velocity below the maximum corresponds to two distances. A cloud observed at 84 kilometres a second is at either 4.7 or 9.5 kiloparsecs, and nothing about its velocity says which. The two possibilities differ by a factor in distance and by its square in luminosity and mass, so the ambiguity is not a refinement — it decides whether a star-forming region is an ordinary one nearby or a monster on the far side of the Galaxy.

One velocity and two distances

Inside the Sun's orbit a line of sight crosses each galactocentric radius twice, and the two crossings have identical radial velocities. So a cloud's velocity gives two candidate distances, near and far, differing by a factor — and nothing about the velocity says which, though the choice decides whether the object is ordinary or extraordinary.

galaxies · Galactic structure

Named alongside it

The objects these essays reach for when they reach for this one.

Kinematic distanceTangent pointThe 21-centimetre lineDegeneracyDifferential rotationDistance ambiguityGalactic longitudeLocal standard of restMolecular cloudOort constantsRotation curveSelf-absorption

All concepts