Star clusters are among the most striking sights in the night sky. Look toward Taurus on a clear night and you may spot the Pleiades — a small gathering of brilliant stars known since ancient times as the Seven Sisters.
Now compare that delicate scattering with M13, the Great Hercules Cluster. Through a telescope, M13 appears as an enormous swarm, becoming so densely crowded toward its centre that countless points of light seem to merge into a glowing ball.
The two objects could hardly look more different. The Pleiades is a loose gathering dominated by young, hot blue stars and is only around 100 million years old. By contrast, M13 contains hundreds of thousands of stars and is more than 10 billion years old, with aging red giants among its brightest members.
Yet both are star clusters — examples of astronomy’s two main types. The Pleiades is an open cluster, while M13 is a globular cluster. More importantly, their stars really belong together. They are not chance patterns created by our viewpoint from Earth, but stars with a shared birthplace and history — genuine stellar families moving through the Milky Way.
Our own Sun almost certainly began life alongside other stars too. Its original siblings have long since scattered through the Galaxy, while the Pleiades remains together and M13 has survived for more than 10 billion years.
Why did their stories turn out so differently? It begins with the way stars are born.
How a Star Cluster Is Born
Stars are almost never born alone. Inside a vast molecular cloud — a cold, dark reservoir of gas and dust that can stretch for many light-years — gravity causes denser regions to collapse and fragment into dozens or even thousands of star-forming pockets. A single stellar nursery can therefore become the birthplace of an entire generation of stars.
At first, the youngest groups may still be hidden within the cloud that created them. Only as radiation and stellar winds from the newborn stars clear away the surrounding gas and dust do they gradually emerge into view.
The stars share a birthplace and form at roughly the same time from broadly the same material. What they don’t share is the same mass — or the same future. Some are small, cool stars destined to shine for immense periods; others are hot, massive stars that race through their lives far more quickly.
And the stellar family itself has a future. Will it remain together as a cluster, or will its stars eventually scatter across the Galaxy?
Being Born Together Doesn’t Guarantee Staying Together
A newly formed group of stars is not automatically a permanent cluster. Its stars are moving within the group, and their mutual gravity must be strong enough to keep them gravitationally bound. If it is, the group can remain together as a recognizable star cluster.
If not, a much looser stellar family may result. Stellar associations are groups of stars that share a common origin and move through space in related ways, but are weakly bound or unbound. Over time, their members spread out into the wider stellar population of the Galaxy.
But even becoming a gravitationally bound cluster does not guarantee survival forever. Stars can escape, while encounters with molecular clouds and the gravitational pull of the Milky Way gradually reshape and erode a cluster.
Some stellar families disperse relatively quickly. Others remain together for billions of years. Being born together gives stars a shared history. Gravity determines whether that history continues as a recognizable cluster.
The Two Main Types of Star Cluster
Astronomers divide star clusters into two main types: open clusters and globular clusters. Both are gravitationally bound stellar families, but they can differ dramatically in size, age, structure and location within the Milky Way. Open clusters are generally loose populations associated with the Galaxy’s disk, while globular clusters are much denser, typically ancient systems distributed far more widely around the Galaxy.
Open Clusters Are Young Families in the Milky Way’s Disk
Open clusters are relatively loose stellar families, typically containing tens to thousands of stars. They are found mainly in the Milky Way’s disk, close to the gas and dust from which new stars continue to form. This helps explain why open clusters are generally much younger than globular clusters.
The Pleiades is a beautiful example. At only around 100 million years old, it is dominated visually by hot, luminous blue stars. Its members are spread loosely across space rather than crowded into a dense sphere, giving the cluster the open appearance behind its name.
But “younger” does not mean that every open cluster is young. The Hyades, also in Taurus, shows us what happens as an open cluster grows older. At around 600–700 million years old, it is several times the age of the Pleiades, and among its most conspicuous members are several orange giant stars that have already evolved away from the main sequence.
The cluster itself has changed too. The Hyades has gradually been losing members, with stars that once belonged to it now spread far beyond its familiar central group. This is part of the eventual fate of many open clusters: their relatively loose structure allows stars to escape until the cluster gradually disperses into the Milky Way’s disk.
Put the Pleiades and Hyades side by side and you are therefore seeing more than two different patterns of stars. You are seeing two open clusters at different stages of their lives. Their stars change as they age, but the clusters themselves evolve too.
Globular Clusters Are Survivors from the Milky Way’s Distant Past
Globular clusters take us into a very different stellar environment. Instead of the loose arrangement of an open cluster, hundreds of thousands of stars can be packed into a roughly spherical swarm, becoming increasingly crowded toward its centre.
They are also extraordinarily old. Many of the Milky Way’s globular clusters formed more than 10 billion years ago, long before the Sun and Solar System existed. The hot, massive stars they were born with have long since exhausted their fuel, leaving behind an ancient stellar population in which evolved stars such as red giants are prominent.
Their location tells another part of the story. Open clusters are concentrated in the Milky Way’s disk, where stars are still being born today. Globular clusters have a much broader distribution, including the Galaxy’s halo far above and below the disk. Their great ages and locations make them survivors from a much earlier period in the Milky Way’s history.
Their enormous mass and tightly packed structure also help explain their longevity. Gravity binds a globular cluster far more strongly than a typical open cluster, allowing many to remain recognizable systems for billions of years — although even globular clusters slowly evolve and lose stars.
M13, which we met at the beginning, gives us a glimpse of that difference. Through a small telescope it can appear as a compact glow; with greater aperture, more and more individual stars emerge around its edges, while its densely populated centre remains a brilliant concentration of ancient stars.
Globular clusters once helped reveal something even more surprising: the Sun is nowhere near the centre of the Milky Way.
In the early 20th century, astronomer Harlow Shapley mapped globular clusters and discovered that they were not centred around us. Instead, their great swarm was centred far away in the direction of Sagittarius.
The idea was beautifully simple. Globular clusters surround the Milky Way, so Shapley reasoned that the centre of their swarm should also mark the centre of the Galaxy. The result placed the Sun far out in the Milky Way rather than anywhere near its centre.
How Do Astronomers Know Which Stars Belong to a Cluster?
A cluster may look like an obvious gathering of stars, but appearances can be deceptive. From Earth, an unrelated foreground or background star can appear right in the middle of a cluster simply because it lies along the same line of sight. Aldebaran provides a perfect example. The bright orange star appears alongside the Hyades and seems to form part of its familiar V-shaped pattern in Taurus — but at about 65 light-years away, Aldebaran lies well in front of the Hyades, whose centre is roughly 150 light-years from Earth. It does not belong to the cluster at all.
Astronomers therefore look for stronger evidence. Do the stars lie at similar distances and move through space together?
Measurements of proper motion — a star’s gradual movement across our sky — reveal whether apparently neighbouring stars are actually travelling in related ways.
ESA’s Gaia spacecraft has transformed this work by measuring the positions, distances and motions of vast numbers of stars with extraordinary precision. Astronomers can also compare their motion toward or away from us and their chemical composition. Combined, these clues should reveal a broadly consistent picture of a shared origin.
How Astronomers Measure the Age of a Star Cluster
Because a cluster’s stars formed at roughly the same time but with different masses, astronomers can use the way those stars have aged to estimate the age of the entire cluster.
Massive stars burn through their fuel faster and leave the main sequence — the long, stable stage of a star’s life — much sooner than lower-mass stars. The most massive stars still remaining on the main sequence therefore act like a marker on the cluster’s clock: the more massive those surviving stars are, the younger the cluster must be.
Astronomers measure this more precisely by plotting the cluster’s stars on a Hertzsprung–Russell (H–R) diagram, which compares their luminosity and temperature. The point where stars begin leaving the main sequence is called the main-sequence turnoff. Its position allows astronomers to estimate how long ago the cluster formed.
This is what helps distinguish the Pleiades from the Hyades. The young Pleiades still contains hot, massive main-sequence stars, while in the older Hyades the more massive stars have already evolved into later stages — a visible record of the hundreds of millions of years separating the two clusters.
Star Clusters Are Among the Easiest Deep-Sky Objects to Observe
Star clusters are some of the most accessible deep-sky objects for beginners. Several can be seen with the naked eye, while even modest observing equipment brings many more within reach.
The best-known naked-eye example is the Pleiades (M45), perhaps the most recognizable star cluster in the night sky. Under reasonably dark skies, it appears as a compact gathering of bright stars, while binoculars reveal many more members and show just how much larger the cluster is than the familiar Seven Sisters pattern.
The Hyades provides a very different experience. It is so close to us that its stars are spread across a large area of sky, making binoculars — or even the naked eye — more useful than a telescope at high magnification. Although bright orange Aldebaran looks like one of the Hyades’ most prominent stars, as we saw earlier, it does not actually belong to the cluster.
The Beehive Cluster (M44) in Cancer offers another easy target. Under dark skies it can be glimpsed with the naked eye as a faint, misty patch, but binoculars reveal its true nature: a loose swarm of individual stars where only a haze was visible before.
Globular clusters offer a very different view. While the stars of an open cluster appear scattered across the eyepiece, a globular cluster crowds toward a bright, densely packed centre.
M13 in Hercules is a classic example. In binoculars or a small telescope under dark skies, it appears as a small fuzzy patch. Increasing telescope aperture begins to resolve individual stars around its edges, while the densely packed centre retains its concentrated glow.
A Star Cluster Is Not a Constellation—or a Galaxy
Star clusters, constellations and galaxies can all appear to be groupings of stars, but they describe very different things.
A constellation is primarily a pattern or region of the sky as seen from Earth. The stars forming its familiar shape can lie at vastly different distances and need not have any physical relationship with one another. A star cluster, by contrast, is a real physical grouping. Its stars share a common origin and, in a bound cluster, are held together by their mutual gravity.
A galaxy operates on an enormously greater scale. The Milky Way contains hundreds of billions of stars, together with gas, dust, dark matter and many star clusters of its own. A cluster is therefore not a small galaxy: it is a stellar family living within a galaxy.