29May2012

In the center of star-forming region 30 Doradus lies a huge cluster of the largest, hottest, most massive stars known. These stars, known as the star cluster R136, and part of the surrounding nebula are captured here in this gorgeous visible-light image from the Hubble Space Telescope. Credit: NASA, J. Trauger (JPL), J. Westphal (Caltech)

Ever wondered what caused some stars to be born giants, while others aren’t so grand? According to a team of astronomers in Germany, and a new computer simulation program, a star’s formation depends on its spawning grounds.

Modern theory tells us star formation occurs in interstellar space – arising from dark clouds of dust and gas. Stellar signature size and mass is hypothesized to arise from the type of conditions found in the birth environment. Much like the way temperature and type of clouds affect our terrestrial weather, it could be a slight sprinkle, a major downpour, or even a hail shower. Until recently, stars just seem to have formed randomly in the same fashion everywhere. “Sites of star formation are the bad weather regions in a galaxy and the forming stars are, in a very rough analogy, like the raindrops condensing out of this material”, comments team member Prof. Dr. Pavel Kroupa.

Thanks to these new modeling techniques, researchers now have the evidence they need to show the mass distribution of stars has a dependence on the conditions of the star forming regions. “Surprisingly, this evidence does not come to us from young regions of ongoing star formation, but from a very old class of objects, so called globular star clusters”, says Dr. Michael Marks, lead author of the new paper. “The number of observed stars less massive than our Sun in globular clusters is at odds with their structure.”

Now enter the neighborhood of globular clusters. We know these huge, gravitationally bound, spherical regions of stars surround not only the Milky Way, but other galaxies as well. One thing they all have in common is that star formation within them ceased billions of years ago. “Nevertheless, using our simulations we found that the connection between star formation and birth environment can be understood when invoking a process that occurs very early in the life of any cluster, called residual-gas expulsion”, continues Marks.

When a star is born, it begins to shine and its solar winds and radiation clear the region around it from gas and dust. This action punches holes in the environment and stars of varying mass are left behind. “This process leads to expansion of the whole aggregate of stars with the accompanying stripping of some of the stars from the cluster by the gravitational attraction of the young Milky Way. The faster the gas is blown out the stronger is the expansion and the more stars are removed”, Kroupa explains. He adds, “The imprint of this process is still visible in the present-day mass distribution”.

By studying current stellar population in globular clusters, researchers are able to surmise their original stellar content. Their findings point to a larger population of massive stars which are cataloged in other star forming region studies. “Otherwise the star birth region a globular cluster formed from is not destroyed quickly enough and the subsequent expansion is too weak to remove enough stars from the cluster”, says Marks. “If this had happened the distribution of masses of stars we see today would be quite different”. This discrepancy of massive stars forming in globular clusters dependent on environment matches with theoretical expectation… differences in content happen when star birth regions are extreme. “We do not observe these extreme environments in the present day, but these may have well been frequent when globular clusters were born around 12 billion years ago”, Marks states.

According to the team’s research, stars are going to keep on shaping themselves the same way – along with their variety of mass – throughout the Milky Way. Their work predicts that star’s differences will continue to be influenced by their environment, distributed in different sites in our galaxy. Kroupa summarizes their results: “With this work, we might have uncovered the long expected systematic differences in the star formation process”. But, the studies won’t stop there. The Bonn astronomers will continue to use their simulations to further study globular clusters and their long term evolution.

Original Story Source: Royal Astronomical Society News Release.

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