Globular clusters are roughly spherical collections of extremely old stars, and around 150 of them are scattered around our galaxy. Hubble is one of the best telescopes for studying these, as its extremely high resolution lets astronomers see individual stars, even in the crowded core. The clusters all look very similar, and in Hubble’s images it can be quite hard to tell them apart – and they all look much like NGC 411, pictured here. And yet appearances can be deceptive: NGC 411 is in fact not a globular cluster, and its stars are not old. It isn’t even in the Milky Way. NGC 411 is classified as an open cluster. Less tightly bound than a globular cluster, the stars in open clusters tend to drift apart over time as they age, whereas globulars have survived for well over 10 billion years of galactic history. NGC 411 is a relative youngster — not much more than a tenth of this age. Far from being a relic of the early years of the Universe, the stars in NGC 411 are in fact a fraction of the age of the Sun. The stars in NGC 411 are all roughly the same age, having formed in one go from one cloud of gas. But they are not all the same size. Hubble’s image shows a wide range of colours and brightnesses in the cluster’s stars. These tell astronomers many facts about the stars, including their mass, temperature and evolutionary phase. Blue stars, for instance, have higher surface temperatures than red ones. The image is a composite produced from ultraviolet, visible and infrared observations made by Hubble’s Wide Field Camera 3. This filter set lets the telescope “see” colours slightly further beyond red and the violet ends of the spectrum.

The current mystery in astronomy is dark matter. Billions of dollars in research funds are spent, and many astronomers study this all-consuming mystery. 

Since 1884, astronomers have realized the motion of galaxies suggests there is unseen matter in the universe. Over time, astronomers began to suspect this wasn’t simply material that we couldn’t see, but something entirely different. Astronomers madly search for dark matter without knowing its exact nature. 

It’s the hot mystery in astronomy and physics today. The dark matter mystery overshadows a second mystery, a mystery as old as dark matter. 

Stars collect in clusters. Gas and dust collect in great clouds, some glowing and some unseen. All this collects into massive assemblies called galaxies, and galaxies collect in clusters of galaxies. These collections are held together by gravity.

Another collection, and one of my favorites to observe, are globular clusters. Globular clusters are at the heart of the second mystery. These spherically shaped clusters contain a hundred thousand to a million stars. Their name derives from the Latin globulus, “a small sphere.” 

Nearly all galaxies have globulars. They are as old as the host galaxy itself. They are low in heavy elements because they formed early in the universe’s history. 

Over 150 globulars reside in the Milky Way, and they are scattered in its outer fringes. A few are observable in binoculars, but their great distance makes them appear like “a small sphere.” The Milky Way’s sister, the Andromeda galaxy, contains over 500 globulars. Some giant galaxies have over 10,000 globulars.

How globulars form is not fully understood. Galaxies typically flatten into disks or at least a highly oval shape because of the spin in the initial clouds of gas from which their stars formed. Globulars don’t flatten into a disk because they have little spin. The mystery is why not? Everything in the universe spins; planets, our solar system, stars, and galaxies. Why not globular clusters? That is the mystery. Maybe, just maybe, understanding this mystery would be a clue to understanding dark matter.

This image of a globular cluster in the Milky Way was taken using the Hubble Space Telescope and released in 2013 by NASA.