The Pleiades is the most famous open star cluster in the sky — a tight gathering of hot blue stars in the constellation Taurus. To the naked eye most people count six or seven; binoculars reveal dozens, and a camera reveals the blue haze of dust that surrounds them.
Distance
~444 light-years
Age
~100 million years
Members
~1,000 stars
Span
~8 light-years
Greek myth names the brightest Pleiads after the seven daughters of Atlas and Pleione. In reality the cluster holds about a thousand stars, all born from the same collapsing cloud around 100 million years ago. The brightest are hot, massive blue-white stars only a few tens of millions of years old — astronomically speaking, infants that will burn out long before the Sun does.
The cluster is passing through an unrelated cloud of interstellar dust. The stars are far too cool to make the dust glow, so instead they scatter its light — the same way air makes the sky blue. This is a reflection nebula, and it is what gives the Pleiades their soft blue halo in photographs.
The Pleiades is not a constellation. Its stars really are close together in space, bound by their mutual gravity and drifting through the galaxy as one family. A constellation is a pattern seen from Earth: its stars can be hundreds or thousands of light-years apart with no physical connection at all.
From a dark site the cluster looks like a tiny, sparkling dipper about the width of your thumb at arm's length. Most observers resolve six stars; excellent eyesight and a dark sky can push that to eight or nine. A pair of binoculars shows several dozen, and M45 is one of the easiest objects to photograph with a simple camera.
Gaia's precise measurements reveal that the Pleiades is being pulled apart by the Milky Way's gravity. A thin tidal tail of stars trails behind the cluster, and heavier stars have already sunk towards its centre — a process called mass segregation. In a few hundred million years the cluster will disperse entirely.
For decades the Pleiades distance was disputed by about 10%. Gaia's parallax measurements — the tiny apparent shift of stars as Earth orbits the Sun — settled it at roughly 444 light-years, resolving a disagreement that had troubled astronomers for twenty years.
These words are often used interchangeably, but they describe very different things at very different scales.
| Object | What it is | Distance | Across | Made of | Looks like |
|---|---|---|---|---|---|
| The Pleiades | Open star cluster | ~444 ly | ~8 ly across | ~1,000 hot young stars born together, plus dust | Tight sparkle of blue stars, often six to nine naked-eye points |
| Orion Nebula (M42) | Emission nebula + embedded cluster | ~1,344 ly | ~24 ly across | Hydrogen gas, dust, and newborn stars including the Trapezium | Fuzzy greenish-white cloud around a small group of stars |
| Andromeda Galaxy | Spiral galaxy | ~2.5 million ly | ~220,000 ly across | Billions of stars, gas, dust, dark matter — and its own clusters and nebulae | A faint elongated smudge; binoculars show a bright core and two companions |
The key distinction: a nebula and a star cluster live inside a galaxy. A galaxy is the galaxy — billions of stars, gas and dust held together by gravity, often containing many nebulae and clusters of its own. All distances are in light-years (ly).
Image: artist's impression created for ANDROMEDA, not a telescope photograph. Distances and ages are rounded values from NASA, ESA and IAU reference data.
Figures are rounded averages from NASA, ESA and IAU reference data. Star positions on charts are schematic and approximate.
Check your understanding
Take the quiz