Credit: ESA/Hubble & NASA, A. Sarajedini, G. Piotto

The subject of today’s ESA/Hubble Picture of the Month is an ancient inhabitant of our galaxy. This sparkling scene is of a globular cluster: a collection of tens of thousands to millions of stars, all tightly bound together under the influence of gravity. Astronomers know of more than 150 globular clusters in our galaxy, though there may be others yet to be discovered, hidden from view by dust or densely packed fields of stars. This particular globular cluster is NGC 6723, sometimes called the Chandelier Cluster. Much like its namesake, this cluster sparkles with countless lights — but each ‘lightbulb’ in this chandelier is an individual star 27 000 light-years away in the constellation Sagittarius (the Archer). Globular clusters like NGC 6723 contain some of the oldest stars in our galaxy. The ages of these clusters often exceed 10 billion years old, and some are nearly as old as the Universe itself. Globular clusters are thought to be some of the first structures to have formed in our galaxy, coalescing potentially billions of years before the thin disk of stars in which our Sun orbits. The details of how globular clusters formed, however, are not yet certain. Astronomers initially thought that all stars in a globular cluster formed at the same time in a single flourish of star formation. This would mean that all stars in a globular cluster would be the same age and be made of the same mixture of chemical elements. Now, thanks to observations from telescopes like Hubble, researchers know that these seemingly simple stellar populations have more complex histories than originally thought. Hubble first observed NGC 6723 as part of an ambitious survey dedicated to demystifying the properties of globular clusters in our Milky Way galaxy. In this observing programme (#10775, PI: Sarajedini), researchers used Hubble to study 65 globular clusters in our galaxy in visible and near-infrared light. These data allowed researchers to study everything from the ages of globular clusters to the process through which massive stars sink to the centre of a star cluster and lower-mass stars drift toward the cluster outskirts. This survey has been immensely scientifically valuable, and these observations have inspired several hundred published research papers. In a later observing programme (#13297, PI: Piotto), researchers set their sights again on many of these same clusters, including NGC 6723. This time, they used Hubble’s unique sensitivity to ultraviolet light to detect the subtle variations in chemical composition between the stars of globular clusters and determine the age spread among the clusters’ stars. For NGC 6723, researchers found evidence of two closely-spaced periods of star formation, the second occurring within 634 million years of the first. (‘Closely-spaced’ is relative; 634 million years is a blink of an eye for a star cluster that is more than 10 billion years old!) Thanks to these findings, astronomers are on the path to understanding how and when globular clusters formed — and Hubble observations of celestial chandeliers like NGC 6723 are lighting the way. [Image Description: A globular cluster. It is made up of many thousands of bright stars, tightly-packed in the centre and more spread out at the corners, but filling the entire view. The stars are coloured either orange or bright blue, with the blue stars mainly concentrated in the centre. Orange stars are located mainly around the edge, and also vary in size from small dots to glowing stars with four points, based on their position in the foreground or background of the cluster.] Links Pan video: NGC 6723

Provider: Hubble Space Telescope | ESA

Image Source: https://esahubble.org/images/potm2606a/

Curator: ESA/Hubble, Baltimore, MD, United States

Image Use Policy: Creative Commons Attribution 4.0 International License