What Are Those Balls of a Million Stars? The Oldest Things in the Galaxy

These objects are gravitationally bound spheres containing anywhere from tens of thousands to several million stars, packed so densely towards the centre that individual stars cannot be separated in images taken from the ground. Our galaxy hosts around one hundred and fifty of them, distributed not in the disc where most stars live but in a roughly spherical halo surrounding the whole galaxy, on elongated orbits that carry them far out and back. They are old, with ages generally between eleven and thirteen billion years, and their stars are poor in the heavier elements that later generations inherited, which together indicate they formed very early in the history of the galaxy.

The contrast with the loose clusters found in the disc is systematic:

• •Hundreds of thousands of stars rather than hundreds
• •Tightly bound spheres rather than loose irregular groups
• •Located in the halo rather than in the disc
• •Ages of many billions of years rather than millions
• •Stars poor in heavy elements rather than of similar composition to the sun
• •Long-term survival rather than dispersal within a few hundred million years

All the stars in one of these objects formed at close to the same time from the same material, which makes them the single most useful natural laboratory in stellar astronomy. Because more massive stars burn out faster, the point at which stars are currently leaving the main phase of their lives depends only on the age of the population, so plotting brightness against colour and locating that turning point gives an age directly. Applying that to the oldest clusters produced ages that at one stage exceeded the estimated age of the universe, which was a genuine crisis resolved by improved distance measurements and by revised cosmology. The current figures sit comfortably below the universal age and provide an independent check on it.

The density near the centre of such an object is worth making concrete, because it is far outside ordinary experience. Stars near the core sit thousands of times closer together than stars near the sun, which means the nearest neighbour is a fraction of a light year away rather than several light years, and the night sky from a planet there would hold thousands of stars bright enough to cast shadows. Close encounters between stars actually happen, which is rare everywhere else, and those encounters produce objects found almost nowhere else, including unusually blue stars apparently formed by mergers and binary systems containing a neutron star and a companion. Stable planetary orbits are harder to maintain, since passing stars disturb them, which is one reason so few planets have been found in these systems.

Several substantial questions about these objects remain open. Their formation is not understood, since conditions in the present universe do not produce anything comparable and the necessary densities of gas are extreme. Many of them contain stars of more than one composition, which contradicts the assumption that all formed together and has prompted elaborate proposals involving multiple episodes of star formation within a single cluster. Whether they contain black holes of intermediate mass at their centres is disputed, with tentative evidence in a few cases. And a proportion of them appear to be the stripped cores of small galaxies swallowed by our own, which makes the population a mixture of objects with quite different histories.

Dense bound spheres of hundreds of thousands of ancient stars orbit in a halo around the galaxy rather than in its disc, with ages between eleven and thirteen billion years. Because all their stars formed together, the point where stars are leaving the main phase gives an age directly, which once produced ages exceeding the universe itself. Their formation, their mixed compositions and their central black holes all remain unsettled.