Large stars are solid out of clusters

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The largest stars in the universe tend to be loners, and new research has shown why. Although massive stars are born in groups of many smaller brothers, they are quickly kicked out and forced to live their lives alone.

Stars tend to come in clumps. That's because a massive cloud of gas and dust that collapses into stars has more than enough to form an entire cluster that contains both large and small ones.

Once formed, the brightest stars in the cluster remove any remaining debris by the intensity of their radiation, leaving behind what is known as an open cluster. Open clusters are bundles of newly hatched stars that are no longer bound by gravity. Over time, they slowly drift away from each other and spread across the galaxy.

But first, the gravitational interactions of the smaller stars conspire to drive out the massive stars and send them on high-speed tracks far away from the cluster in record time.

This is the story, presented in a recent paper in the Astrophysical Journal, which explains why up to 75% of stars heavier than 8 solar masses are not associated with other stars, unlike their smaller cousins .

To test whether massive stars really do form in isolation (“in the field” in astronomy jargon), the team behind the paper also looked for massive stars that were surrounded by a retinue of smaller stars. They found that less than 5% of the massive stars had such a wake.

Since a random gas cloud has enough material to form a giant star, it almost certainly has enough material to form a few smaller ones as well. This result suggests that less than 5% of the massive stars actually form on the field and were really once members of a much larger association.

The results have important implications for our understanding of how all stars (both large and small) are formed and, more importantly, how they live their lives.

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