Doesn’t take a village to make a star

U. MICHIGAN (US) — The most massive stars in the universe are able to form essentially anywhere, even in isolation, without the benefit of a large stellar cluster nursery.

Using the Hubble Space Telescope, scientists zoomed in on eight of these giants, which range from 20 to 150 times as massive as the sun.

The stars appeared to be in the Small Magellanic Cloud, a dwarf galaxy that is one of the Milky Way’s nearest neighbors.

Five of the eight had no neighbors large enough for Hubble to see and the remaining three were in tiny clusters of ten or fewer stars.

The research is reported in Astrophysical Journal.

The most massive stars direct the evolution of their galaxies as their winds and radiation shape interstellar gas and promote the birth of new stars.

Their violent supernovae explosions create all the heavy elements that are essential for life and the Earth, which is why astronomers want to understand how and where these giant stars form, says Sally Oey, associate professor of astronomy at University of Michigan.


Star 302 as viewed through the Hubble Space Telescope, which can zoom in roughly 40 times. From the ground, everything within the circle appears to be one star.
(Credit: Joel Lamb)

Their origins are a matter of debate, she says.

One theory is that the mass of a star depends on the size of the cluster in which it is born, and only a large star cluster could provide a dense enough source of gas and dust to bring about one of these massive stars.

The opposing theory, and the one that this research supports, is that these monstrous stars can and do form more randomly across the universe—including in isolation and in very small clusters.

“Our findings don’t support the scenario that the maximum mass of a star in a cluster has to correlate with the size of the cluster,” Oey says.

The researchers acknowledge the possibility that all of the stars they studied might not still be located in the neighborhood they were born in. Two of the stars they examined are known to be runaways that have been kicked out of their birth clusters.

But in several cases, the astronomers found wisps of leftover gas nearby, strengthening the possibility that the stars are still in the isolated places where they formed.

The research was funded by NASA and the National Science Foundation.

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