A ‘Treasure Trove’ Of Ancient Wandering Stars In The Fifth Brightest Galaxy In The Sky

A huge catalogue of 40,000 previously unknown globular clusters—mysterious clumps of ancient stars —has been revealed around the closest elliptical galaxy to us.

Presented to the Astronomical Society of America last week by Allison Hughes, a doctoral student at the University of Arizona, the paper is a fascinating glimpse into one of the best-looking galaxies in the night sky and what these clumps of ancient stars tell us.

What is a globular cluster?

Globular clusters are the oldest visible objects in the Universe and some of the most fascinating celestial objects of all. Dense volumes of hundreds of thousands (sometimes millions) of stars of similar age, globular clusters are thought to have formed at the same time as the first galaxies, shortly after the birth of the Universe about 13.8 billion years ago.

There are about 150 globular clusters in the Milky Way, orbiting far away from the galactic plane. They no longer produce new stars, but there are planets. 

What is Centaurus A?

Also known as NGC 5128, it’s the fifth brightest galaxy in the sky about 12 million light years from Earth in the constellation of Centaurus.

The closest elliptical galaxy to us, it’s known for three things:

• The dust lane across its middle.
• A giant jet blasting away from the supermassive black hole at its center.
• A new class of super-massive “dark” globular star clusters left behind by past collisions and mergers with smaller galaxies.

It all looks a bit of a mess, but that’s largely because we have a great view of it. “Centaurus A may look like an odd outlier, but that’s only because we can get close enough to see its nitty gritty details,” said Hughes. “More likely than not, both elliptical and spiral galaxies like the Milky Way are messier than we realize if we look a little deeper.”

Elliptical galaxies like Centaurus A are much more common in the cosmos than orderly spiral galaxies like the Milky Way and the Andromeda galaxy.

What’s been discovered?

Globular cluster-hunters have long targeted Centaurus A due to its richness and closeness to Earth, but the outer reaches of the galaxy are largely unexplored—until now. Approximately 40,000 globular clusters were found out to nearly half a million light-years from the galaxy’s center.

Why are globular clusters important?

Almost every galaxy has globular clusters. They offer clues about the history of the galaxy hosting them, specifically about previous mergers and collisions with other galaxies. “Globular clusters are interesting because they can be used as tracers of structures and processes in other galaxies where we can’t resolve individual stars,” said Hughes. “They hold on to chemical signatures, such as the elemental composition of their individual stars, so they tell us something about the environment in which they formed.”

Stars from the same globular cluster move in space together, so must have come from the same dwarf galaxy that once collided with the present giant galaxy being studied.

How to see a globular cluster from your home

Some of the Milky Way’s 150 globular clusters are visible at night. The one to go for is M13, the “Great Globular Cluster,” which is best seen in summer. It’s around 25,000 light-years distant in the constellation of Hercules.

To find it, locate bright star Vega, the brightest star in the asterism of the “Summer Triangle”, and then locate the curve of seven stars that comprise the constellation of Corona Borealis.

The constellation of Hercules is between them; find its square-shaped “keystone” of four stars. To find M13, range binoculars down the side of the keystone, going a third of the way between the star Eta Herculis and Zeta Herculis; you’ll see a misty patch that’s distinctively bright in the center—it’s the combined brightness of as many as a million stars. 

Wishing you clear skies and wide eyes.