The movement of the stars in the cluster NGC 346 offers clues to star formation in the early universe

The cluster NGC 346 seen by Hubble (Image NASA, ESA, A. James (STScI))
The cluster NGC 346 seen by Hubble (Image NASA, ESA, A. James (STScI))

Two articles – available here and here – published in “The Astrophysical Journal” report different aspects of a study on NGC 346, an open cluster of stars in the Small Magellanic Cloud, one of the satellite dwarf galaxies of the Milky Way. Two teams of researchers who share their respective leaders, Elena Sabbi of the Space Telescope Science Institute and Peter Zeidler of the AURA/STScI for ESA, used observations conducted with the Hubble Space Telescope and the VLT. They examined the spiraling motion of the cluster’s stars within it, a motion that appears to favor star formation.

Astronomers know many details of the mechanisms of star formation but there’s still a lot to learn especially about the mechanisms that favor or inhibit that formation. Understanding what happened to the very first generations of stars in the universe is even more difficult due to the difficulty of observing in detail primordial galaxies and stellar nurseries.

The Small Magellanic Cloud has a simpler chemical composition than the Milky Way and is more similar to that of primordial galaxies. For this reason, astronomers study star formation within this dwarf galaxy with particular interest. NGC 346 is an open cluster, which is a star cluster that formed inside the same giant molecular cloud of this dwarf galaxy. For this reason, it was the subject of various studies, including the ones now published in “The Astrophysical Journal”.

Elena Sabbi’s team measured the changes in the positions of the stars over more than 11 years. The stars in cluster NGC 346 are moving at an average speed of 3,200 kilometers per hour. This means that during the period examined, their average movement was about twice the distance between the Earth and the Sun.

Peter Zeidler’s team used the Multi Unit Spectroscopic Explorer (MUSE) instrument installed on the Very Large Telescope (VLT) to measure the radial velocity of stars. This allowed assessing whether they’re moving toward the Earth or away from it.

The combination of the results obtained with Hubble and VLT/MUSE made it possible to accurately map the movements of the stars in cluster NGC 346 in three dimensions. This allowed confirming the theory that the stars are spiraling inwards within the cluster. Peter Zeidler explained that this is the most efficient way in which stars and gas that fuel star formation can move towards the center of the cluster.

The Hubble archive proved invaluable with observations of the NGC 346 cluster conducted several years apart. Elena Sabbi explained that there are many star-forming regions that were observed by Hubble over the years and the observations can be repeated to conduct studies like this and advance our understanding of star formation.

The dynamics observed in the NGC 346 cluster are useful to improve models concerning stellar formation and evolution, in particular with conditions similar to those of the early universe. However, it’s important to study other clusters to understand whether other giant molecular clouds generated the same dynamics with spiral movements or there may be other mechanisms. More investigations could be conducted with the James Webb Space Telescope, which can resolve smaller stars in clusters such as NGC 346 and therefore offer an even more complete view of the region.

The cluster NGC 346 seen by Hubble with the indication of the spiral movement of stars and gas (Image NASA, ESA, A. James (STScI))
The cluster NGC 346 seen by Hubble with the indication of the spiral movement of stars and gas (Image NASA, ESA, A. James (STScI))

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