Stars Spiral Inward to the Cores of Stellar Nurseries

Astronomers studying a stellar cluster within the Small Magellanic Cloud (SMC) have found young stars spiraling in towards the center of the cluster. The cluster, NGC 346, is an open cluster embedded within a glowing cloud of gas, which is typical of stellar nurseries – places where new stars are formed. The outer spiral arm of this star forming region appears to be funneling gas, dust and new stars into the center, which researchers describe as an efficient way to fuel the birth of new stars.

The SMC is a small satellite galaxy of the Milky Way, visible with the naked eye to Southern hemisphere observers under dark skies. It is about 200 000 light years away, and contains a number of nebulae and clusters. One of these, NGC 346, combines a population of bright, new stars, and their still-collapsing stellar nursery of gas and dust, which continues to produce new stars.

The region is only 150 light years across, and has a mass of about 50 000 Suns. Its unusually high rate of star-formation, and its intriguing shape, have been an interesting puzzle to astronomers for some time. This latest image offers some fresh clues to help us understand what’s going on. It combines observations from the Hubble Space Telescope (HST) and the European Southern Observatory’s (ESO) Very Large Telescope (VLT).

NGC 346: Stellar formation region in the SMC
Image Credit & License: NASA, ESA, Hubble; Processing: Judy Schmidt
NGC 346: Star Forming Cluster in the SMC. Image Credit & LicenseNASAESAHubbleProcessing: Judy Schmidt

Elena Sabbi of the Space Telescope Science Institute in Baltimore, and leader of the study, had this to say:

“Stars are the machines that sculpt the Universe. We would not have life without stars, and yet we don’t fully understand how they form. We have several models that make predictions, and some of these predictions are contradictory. We want to determine what is regulating the process of star formation, because these are the laws that we need to also understand what we see in the early Universe.”

Sabbi and her team used a series of HST images taken over the course of 11 years to calculate the movements of the stars within the NGC 346 cluster. By comparing this sequence of images, they were able to measure the movements of stars within the cluster. Over the 11 years, the stars moved an average of 320 billion kilometers, slightly more than twice the distance from the Earth to the Sun. This works out to a surprisingly slow speed of only 3200 kilometers per hour. The HST produces a particularly high image resolution, and is extraordinarily accurate, making these measurements possible.

Meanwhile, a second team used the Multi Unit Spectroscopic Explorer (MUSE) instrument on the VLT, to take measurements of the radial velocities of the same stars. The team, led by Peter Zeidler of AURA/STScI for the European Space Agency, use the spectrographic instrument to measure the speeds at which those stars are moving towards or away from Earth.

“What was really amazing is that we used two completely different methods with different facilities and basically we came to the same conclusion independently,” said Zeidler. “With Hubble, you can see the stars, but with MUSE we can also see the gas motion in the third dimension, and it confirms the theory that everything is spiraling inwards.”

This spiral motion seems to arise naturally, as it is the easiest and most efficient way for material to make its way to the high-density central regions of the nebula.

“A spiral is really the good, natural way to feed star formation from the outside towards the center of the cluster,” explained Zeidler. “It’s the most efficient way that stars and gas fueling more star formation can move towards the center.”

The SMC is useful to physicists trying to understand star formation because it has a much simpler chemical composition than our own galaxy, and so better matches the conditions of the early universe. The simpler chemistry of stars formed at that time caused them to burn hotter and faster, and having an environment in our galactic backyard that matches those conditions is very handy for scientists. By observing stars of different ages in the various stellar nurseries in the SMC, we can better understand the burst of star formation that is believed to have occurred throughout the Universe when it was just a few billion years old.

These latest observations tell us that star formation in this early period happened in a way that is very similar to how modern stars form right now in our own Milky Way.

“The Hubble archive is really a gold mine,” said Sabbi. “There are so many interesting star-forming regions that Hubble has observed over the years. Given that Hubble is performing so well, we can actually repeat these observations. This can really advance our understanding of star formation.”

The next step is to repeat these observations with the NASA/ESA/CSA James Webb Space Telescope (JWST). JWST will be able to resolve smaller, cooler stars than what were included in the current measurements. Over time, researchers will be able to repeat the observations, including the lower mass stars. This will serve to confirm and verify the current work, and expand upon it to include more of the stellar population. This will allow a comparison of the behavior of high and low mass stars, and bring a more complete understanding of the dynamics of these stellar nurseries.