Newly fashioned stars don’t blast away the fabric as beforehand assumed. Why do they cease rising?

We thought we understood how stars are made. It turns out we don’t. Not entirely, anyway. A new study recently conducted using data from the Hubble Space Telescope sends astronomers back to the drawing board to rewrite the accepted model of star formation.

What we know about star formation is that they are born from huge clouds of hydrogen gas. The gas is clumped and compressed by gravity, increasing pressure and temperature until the mass becomes large enough to trigger nuclear fusion. But stars don’t seem to absorb all of the gas around them. Something is preventing them from reaching enormous sizes.

So far, the accepted model has assumed that the excess gas is blown away from the star in extremely strong solar winds that are shaped and directed by magnetic fields that shoot out of the star’s poles. “There are remarkable U- or V-shaped structures that extend north and south of a protostar,” explains Nolan Habel, one of the researchers at the University of Toledo. “They are actually hollowed out cavities carved into the surrounding gas by hurricane-like winds or jets of material ejected from the poles of the protostar.”

Protostars are born in the Orion Nebula as seen by the Hubble Space Telescope. Photo credit: NASA, ESA, STScI, N. Habel, and ST Megeath (University of Toledo).

It was believed that these powerful jets would eventually remove all of the excess gas and the stars would only absorb about 30% of the material around them. But the new study turns that theory on its head. The voids created by the jets do not seem to grow steadily over time, which means that on their own they cannot possibly explain why stars stop growing. There must be some other mechanism that will help remove the remaining gas surrounding a protostar.

The team made this discovery by examining a sample of 304 protostars at various stages of formation in the Orion Nebula (the region of star formation closest to Earth). The astronomers sorted the stars by age and then used Hubble images to measure the shape and volume of the cavities created by jets. They expected the voids to grow over time, as the model suggests. But they didn’t.

“We find that at the end of the protostellar phase, when most of the gas from the surrounding cloud fell onto the star, some young stars still have fairly narrow cavities,” said Tom Megeath, another researcher on the team. This contradicts all popular theories of star formation and requires further investigation to find out what is happening. Why do stars stop growing if not because of their polar rays?

A wide view of the Orion Nebula as captured by the Hubble Space Telescope. Photo credit: NASA, ESA, M. Robberto (Space Telescope Science Institute / ESA) and the Hubble Space Telescope Orion Treasury project team.

The team has worked out some possible alternatives. The gas clouds in which stars form are not evenly dense. They have higher density “filaments” that protostars form, and vibrations in these filaments can throw the protostars away. We also know that stars don’t always form alone: ​​About half of all sun-like stars have a binary partner. It is conceivable that two or more protostars that form close to each other disturb each other through gravity and push them away from their starting material.

At the moment these are just theories. Larger and better tools are likely to be needed to find a concrete answer. The James Webb Space Telescope, due to be launched later this year, could give astronomers the clues they need to solve the puzzle. In the meantime, there are a number of astrophysics textbooks that need revised editions.

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