This NASA/ESA Hubble Space Telescope image features
the globular cluster NGC 6723, sometimes called the Chandelier Cluster.
ESA/Hubble & NASA, A.
Sarajedini, G. Piotto
The subject of today’s NASA/ESA Hubble Space Telescope image
is an ancient inhabitant of our galaxy. This sparkling scene features a globular cluster: a collection of tens of thousands to millions of stars, all tightly
bound together under the influence of gravity. There are more than 150 globular
clusters in our galaxy, though there may be others still undiscovered, hidden
from view by dust or densely packed fields of stars.
This globular cluster, NGC 6723,
sometimes called the Chandelier Cluster, is much like its namesake because it
sparkles with countless lights. However, 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. These clusters have ages that often
exceed 10 billion years old, and some are nearly as old as the universe itself.
Astronomers think globular clusters are some of the first structures that
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 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 program (#10775,
PI: Sarajedini), researchers used Hubble to study 65 globular clusters in our
galaxy in visible and near-infrared light. That data allowed researchers to study everything from the ages of
globular clusters to the process through which massive stars sink to the center
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 program (#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.
Text Credit: ESA/Hubble
