Dissecting a cocoon of stardust: Scientists begin to tease out a hidden star’s secrets

Peering deep inside obscuring cocoons of stardust, astronomers are beginning to witness the birthing secrets of an unusual star.

Using a small, specially equipped telescope at the Kitt Peak National Observatory, as well as powerful radio telescopes such as the Very Large Array (VLA), UW–Madison astronomer Edward Churchwell has spent a career dissecting dense clouds of interstellar dust to reveal the mechanisms by which O stars, some of the largest and rarest in the cosmos, come to life.

By doing so, Churchwell and others who study the hidden giants are gaining important new clues to how the stars form, a phenomenon that, if unmasked, can help portray issues of cosmic importance such as how most of the mass in the universe gets lumped into stars and how objects from tiny neutron stars to massive galaxies spew out material in the form of bipolar jets.

“There are many puzzles in this whole area,” said Churchwell. And although they burn brightly in infrared wavelengths, “you can’t see them in visible light, and they are very rare.”

There are only about 20,000 O stars in the entire Milky Way, a galaxy with nearly 4 billion stars. And O stars, when they form, are hidden deep inside dense cocoons of dust, just a few light years across, called ultracompact H II regions, making them all but inaccessible to the bulk of astronomical telescopes. Moreover, they are distant from Earth. The closest is 1,500 light years away in the Orion Nebula.

But using sensitive infrared and radio telescopes to cut through the interstellar murk, Churchwell and a small cadre of other astronomers are accumulating enough evidence to construct a hazy picture of O star nurseries. Writing in the most recent issue of the Astrophysical Journal, Churchwell and his colleagues report observations of 21 ultracompact H II regions that are helping depict them as very large, but hollow dust cocoons with steep temperature gradients, and a burst of starmaking activity with dozens of nascent stars at their cores.

“Nobody knows how these stars are formed, but it’s clear they have to accrete a lot of mass over a short period of time,” said Churchwell of the jumbo baby stars, each being anywhere from 10 to 100 times more massive than the sun.

Smaller stars like the sun seem to form alone while massive stars like O stars come in bunches. Small infant stars also have what astronomers call accretion disks, a swirl of gas and stardust that surrounds a star and falls onto it over time. But how massive stars like O stars obtain the bulk of their mass, gaining the equivalent of a sun every thousand years, is a mystery, said Churchwell.

Solving these mysteries, he said, would go a long way toward explaining how material cycles in and out of stars, how galaxies evolve and a slew of unanswered questions in astrophysics.

One example, said Churchwell, is the mystery of bipolar jets, massive outflows of gas and dust from the polar regions of galaxies and stars, including O stars. How those jets are produced and driven is a central mystery of astrophysics. “There is more mass in these jets than the stars themselves, and if only 20 percent of the mass (of material around a star) falls into it, how do you throw the remaining 80 percent out in these jets? That is a big problem because there is something universal about these jets of outflowing material.”

Another conundrum posed by the stellar nurseries is why they are hollow. What force is it that carves out space within the ultracompact H II regions, leaving only a bevy of nascent stars at their cores? Is it the 2,000-kilometer-per-second solar winds from the massive stars that blow the dust away? “We’re not sure,” said Churchwell. “There are a number of possibilities.”

For Churchwell, who’s been observing jets in his stellar nurseries, one idea is that much of the matter around the star never makes it into the star itself, but through unidentified processes – possibly radiation and thermal pressure – is funneled outward into the jets.

Churchwell acknowledged that while there is still no definitive view of these star nurseries, the slow accumulation of facts through observation is beginning to flesh out theory. We don’t have all the answers yet, he said, but some of the pieces are beginning to fall into place.