making them favourite observational targets for astronomers

Hubble has observed its fair share of these cosmic baubles,

imaging and studying them to explore their secrets.

This new image from Hubble shows a globular cluster known as Messier 15

As well as containing over 100 000 old stars, this cluster hosts something dark and mysterious at its heart.

Hubblecast 69: What has Hubble learned from star clusters?

Star clusters are some of the most beautiful objects in our skies

Hubble has viewed many of these over the years — but it is not all style over substance

These clusters are incredibly useful to the astronomers studying them

What can they tell us about our Universe?

There are two main types of star cluster:

Open,

and Globular.

Most stars, including the Sun, are thought to have formed within open star clusters

groups of thousands of stars loosely bound together by gravity

destined to be spread throughout their host galaxy once the cluster matures and most of its gas disperses.

Globular clusters are different; they are huge balls of old stars that orbit the centres of galaxies

Many of them were once little galaxies, cannibalised by their larger companions over the history of the Universe.

Hubble has explored many globular clusters

The Milky Way alone has over 150 of these starry satellites, and Hubble’s sharp vision can resolve the individual stars.

This has allowed Hubble to explore these star clusters in unprecedented detail.

Hubble has produced the deepest, most detailed images ever of a galaxy outside our own: the Andromeda galaxy,

spotting individual star clusters and even resolving their separate stars.

It has detected a population of white dwarfs within NGC 6397

These ancient, dying stars are the dimmest stars ever seen in any globular cluster.

They led to the first accurate measurements of cluster ages.

Hubble has also spotted the largest collection of globular clusters ever found,

a staggering 160 000, swarming around at the heart of the galaxy cluster Abell 1689.

It has probed the Milky Way’s clusters to understand how different stars age at different rates,

and investigated why some clusters seem to host multiple generations of stars while others contain stars of mostly the same age.

Now, Hubble has looked towards the globular cluster Messier 15,

one of the oldest globular clusters known in our galaxy, at around 12 billion years old.

This image of Messier 15 is reminiscent of a dazzling firework display, with golden and bright blue stars swarming together across the frame.

Messier 15 houses a planetary nebula known as Pease 1 — making it the first globular ever known to contain one of these objects

Even now, only three other similar clusters have been found to host such a planetary nebula.

However, all it not as it seems —

this sparkling bauble has hidden secrets

Astronomers studying Messier 15 with Hubble in 2002 found there to be something dark and mysterious lurking at its heart.

There are two possible explanations for this intriguing finding

It could either be a collection of dark, incredibly dense neutron stars, or a rare and exotic intermediate-mass black hole

Studying these unusual black holes could tell us about how such objects grow and evolve within both star clusters and galaxies.

Hubblecast is produced by ESA/Hubble at the European Southern Observatory in Germany.

The Hubble mission is a project of international cooperation between NASA and the European Space Agency.

www.spacetelescope.org

Transcribed by ESA/Hubble. Translation --