Astronomy & Celestial Mechanics
the sky, computed
Concept

Blue cloud — where it appears

The part of the colour–magnitude diagram of galaxies holding those still forming stars, whose young massive stars make them blue. A sparsely populated valley separates it from the red sequence. Galaxies leave it when star formation stops and seldom move back.

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

Two populations, and the valley between them. 420 model galaxies, 42 per cent of them drawn from a tight red sequence tilted by -0.08 magnitudes of colour per magnitude of brightness, the rest from a broad blue cloud. Only 3.3 per cent land in the band of width 0.20 magnitudes midway between the two sequences, against 17 per cent in the same band laid on the blue cloud and 27 per cent on the red sequence. The bimodality is the strongest statement the galaxy census makes: a galaxy is usually forming stars or has stopped, rarely in between, and since colour tracks the age of the newest stars, the emptiness of the middle is a statement about a timescale — whatever ends star formation does it in a few hundred million years, not a few billion.

Two colours, and almost nothing between

Plot a few hundred thousand galaxies by colour and brightness and they do not fill the plane. They pile into a tight red sequence and a broad blue cloud with a near-empty gap between, and an empty gap is a statement about how fast something happens.

galaxies · Galaxy populations Two routes through the plane of colour and shape. Colour against the bulge's share of the light. The solid track is a disc that stops forming stars, starting with a bulge share of 0.15, with dots at 0, 1, 2, 4 and 8 billion years: it moves steeply up and then slowly right, passing through the part of the plane where red spirals are found. The other track is a spheroid with a recent burst, which starts blue with its shape fixed and moves straight up. The upper left and lower right are the two minorities a colour cut and a shape cut disagree about.

A galaxy that goes red before it changes shape

Sort galaxies by colour and by shape and the two lists agree about four times in five. The fifth galaxy is the interesting one: red spirals that have stopped forming stars and kept their discs, and blue spheroids still fading from a burst. The disagreement is a clock. A disc that stops forming stars goes red within a billion years, and the same fading raises its bulge's share of the light without a single star moving, which means part of what has been called a change of shape is only a change of lighting.

galaxies · Galaxy populations

Named alongside it

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

Morphology density relationQuenchingRed sequenceBimodalityBulge to total ratioColour magnitude diagramGreen valleyMetallicityStar formation historyStellar population

All concepts