20Aug2012

Hubble Views Head-On Cluster Collision

Image Credit: NASA, ESA, and E. Sabbi (ESA/STScI) Acknowledgment: R. O'Connell (University of Virginia) and the Wide Field Camera 3 Science Oversight Committee

Located in the Large Magellanic Cloud some 170,000 light years away, two star clusters are about to collide. The 30 Doradus Nebula has piqued astronomical interest since its discovery.

Its interesting for a good reason – it’s a prime example of how star-forming regions may have looked when our Universe was young. These new observations taken by the Hubble Space Telescope are important because they will help us further understand how star clusters once formed. Southern observers are very familiar with 30 Doradus – they call it the Tarantula Nebula. Once believed to be a single object, new data points to it being a composite of two clusters which differ in age by about a million years.

However, this May/December romance isn’t new… the entire complex has been actively forming stars together for more than 25 million years. How long with it last? No one is really sure, but this new information could show how two smaller systems merging into a larger one could create some of the biggest star clusters known.

Noticing Differences

Lead scientist Elena Sabbi of the Space Telescope Science Institute in Baltimore, Maryland, and her team began looking at 30 Doradus when studying runaway stars – kids that were kicked out of the nursery after they first formed. “Stars are supposed to form in clusters, but there are many young stars outside 30 Doradus that could not have formed where they are; they may have been ejected at very high velocity from 30 Doradus itself,” Sabbi said.

While looking at the distribution of low-mass stars as imaged by Hubble, Sabbi noticed something a little different about the cluster. Instead of having a regular shape, it appeared almost like two galaxies do when they merge. The shape was stretched, similar to what occurs when tidal pull is involved. Hubble had captured it. Now there was evidence of not only elongated structure, but a difference in age between the two clusters. In some theories, giant gas clouds which trigger star formation can sometimes break apart. Once split, the smaller clouds begin creating new stars – stars which then interact and cause the region to meld together again to form a larger system. This is the explanation which Sabbi and her team favour.

Cosmic Movement

However, that’s not all that’s going on in 30 Doradus. Astronomers also hypothesize that runaways stars were ejected from the core of the cluster as a result of dynamical interactions – a common occurrence during core collapse. If so, then the higher mass stars migrated toward the center of the cluster when interacting with their low-mass members. When the core becomes crowed, it becomes unstable and members are cast out. However, larger cluster R136 (located in the center of 30 Dor) is too young to have gone through core collapse. It’s a process left to the younger, smaller clusters and the high number of runaway stars in the region are better explained by the merger theory.

“Follow-up studies will look at the area in more detail and on a larger scale to see if any more clusters might be interacting with the ones observed.” says the team. “In particular the infrared sensitivity of NASA’s planned James Webb Space Telescope (JWST) will allow astronomers to look deep into the regions of the Tarantula Nebula that are obscured in visible-light photographs. In these areas cooler and dimmer stars are hidden from view inside cocoons of dust. Webb will better reveal the underlying population of stars in the nebula.”

Original Story Source: HubbleSite News Release.

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