Globular Clusters

Globular clusters are spherical collections of stars bound tightly together by gravity. They’re among the oldest objects in the universe, containing hundreds of thousands to millions of stars packed into a region just 100 to 200 light-years across. The density at their cores is staggering - stars are separated by distances measured in light-weeks rather than light-years.

The most spectacular globular cluster visible from the northern hemisphere is M13 in Hercules. Through a small telescope, it appears as a fuzzy ball of light. Larger telescopes begin to resolve individual stars around the edges. M13 contains roughly 300,000 stars packed into a sphere about 145 light-years in diameter, located 25,000 light-years from Earth.

Globular clusters orbit in the halos of galaxies, following elongated paths that take them far above and below the galactic disk. The Milky Way has about 150 known globular clusters, though there may be more hidden behind dust clouds or on the far side of the galaxy. Large elliptical galaxies can have thousands of globular clusters.

The stars in globular clusters are old. Really old. Most formed 10 to 13 billion years ago, within a billion years of the Big Bang. This makes them older than the disk of the Milky Way itself. The age is evident in their stellar populations - they contain mostly low-mass red and yellow stars. The massive blue stars that would have formed alongside them burned out billions of years ago.

Here’s what makes their chemistry interesting: globular cluster stars are metal-poor. In astronomical terms, “metals” means any element heavier than helium. These clusters formed from primordial gas that hadn’t been enriched by many previous generations of stars. Their metallicity is typically 1/10th to 1/100th that of the sun. This low metal content affects their color - globular clusters appear distinctly yellow or orange compared to younger star clusters.

BTW the core of a globular cluster is an extreme environment. Stars are so densely packed that close encounters and even collisions occur. These interactions can produce exotic objects - blue stragglers, which appear younger than they should be because they’ve merged with or stripped mass from companion stars. X-ray binaries form when a normal star transfers mass onto a neutron star or black hole companion.

Globular clusters are remarkably stable structures. They’ve survived for over 10 billion years despite gravitational perturbations from passing molecular clouds and the tidal forces of the galaxy. Computer simulations show that clusters slowly lose stars over time through a process called evaporation, but most will survive for many more billions of years.

The distribution of globular clusters provides clues about galaxy formation. They orbit in two distinct populations - one in a flattened distribution aligned with the galactic disk, and another in a spherical halo. The halo population is older and more metal-poor. This suggests they formed very early in the galaxy’s history, possibly before the disk had fully formed.

Photographing globular clusters presents unique challenges. The bright, dense core can easily overexpose while the faint outer regions remain underexposed. Wide dynamic range is essential. The best images use techniques like HDR compositing - combining short exposures for the core with long exposures for the outer regions.

Omega Centauri is the largest globular cluster in the Milky Way, visible from the southern hemisphere. It contains about 10 million stars and may actually be the stripped core of a dwarf galaxy that merged with the Milky Way billions of years ago. Its unusual properties - multiple stellar populations with different ages and metallicities - support this theory.

The color-magnitude diagrams of globular clusters are textbook examples of stellar evolution. Plotting stars by brightness and color reveals distinct features - the main sequence, the red giant branch, the horizontal branch, and sometimes a blue horizontal branch. These diagrams allow astronomers to determine the cluster’s age and study stellar evolution in a population of stars that all formed at the same time.