NGC 346, an open star cluster in the Small Magellanic Cloud

NGC 346, an open star cluster in the Small Magellanic Cloud

Image Credit: Antonella Nota (ESA/STScI, STScI/AURA), NASA and ESA

NGC 346 is an open star cluster within an emission nebula of about 200 light-years across in the Small Magellanic Cloud (SMC) – a small satellite of our Milky Way – which lies some 200,000 light-years away from Earth in the southern constellation of Tucana (the Toucan).

It is the brightest star-forming region in the Small Magellanic Cloud, and contains HD 5980, the brightest star in the SMC. The nebula contains over 70,000 stars, of which the oldest are 4.5 billion years old and the youngest about 5 million years old.

The light, wind and heat given off by massive stars have dispersed the gas within and around the star cluster, forming the surrounding dramatic structure of arched, ragged filaments with a distinct ridge.

The hot, bright blue stars in the center are destroying the cloud that was their stellar nursery. Their energetic ultraviolet light heats the gas, making it glow. The pink color in the image is really the energized gas of the nursery being eaten away by the intense radiation.

The boundary between the cold gas and the heated gas appears as an intersecting dark ridge of dust, with a bright border that runs across the upper center and down the left side of the image, which contains several small globules that point back towards the central cluster. Astronomers have identified a population of embryonic stars strung along the intersecting dark dust ridge. Still collapsing within their natal clouds, the stellar infants’ light is reddened by the intervening dust.

Most of NGC 346′s smaller stars were created at the same time as the massive stars located at the centre of the region, all out of one collapsed cloud of matter. The intense radiation from the more massive stars ate away at the surrounding dusty cloud, causing gas to expand and compressing cold dust into new stars. This process is known as ‘triggered star formation’.

But the formation of a set of younger, small stars in the region could not be explained by this mechanism. By combining multi-wavelength data of NGC 346, astronomers were able to trace back the trigger of the formation of these young stars to a very massive star that blasted apart in a supernova explosion about 50,000 years ago.

Before it died this massive star spurred the stars into existence, but it triggered a different type of star formation compared to that which occurred around the center of the region. Fierce winds from the massive star, and not radiation, compressed the remaining dust in the parent cloud, forming new stars. This demonstrates that both wind- and radiation-induced star formation are at play in the same cloud, and that star formation is a very complicated process.

As time passes, the entire cloud will be heated by the young stars’ powerful radiation. Without any cold gas, star formation will cease. The result will be a cluster of stars somewhat like the one that appears at bottom, center. This cluster is about the same age as our Sun and is dominated by yellow stars. The hotter stars, such as the blue stars from its youth, have already exhausted their energy and died out.

NGC 346, therefore, is a picture book of star formation, showing the dark clouds where stars are born to their emergence as young stars to their passage into maturity.

This image was taken with the Advanced Camera for Surveys on the Hubble Space Telescope, using an infrared and two optical color filters.

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