Massive Star Formation Mystery Solved
January 28, 2014

Astronomers Solve Massive Star Formation Mystery

Brett Smith for - Your Universe Online

Enormous young stars, which have over 10 times the mass of the Sun, shine brilliantly in ultraviolet wavelengths, energizing the gas around them, and it has long been a mystery why this superheated gas doesn't explode outwards.

According to a new study in The Astrophysical Journal Letters, a group of scientists using the Jansky Very Large Array (VLA), a radio observatory in New Mexico, has validated theories that say as the gas cloud deflates around a forming massive star, it forms heavy filamentary structures that take in the star's ultraviolet rays – resulting in the heated nebula flickering like a candle.

Stars are created when tremendous clouds of gas collapse. Once the heat and density of these collapsing clouds are high enough, hydrogen fusion creates helium, and the star begins to shine. Massive stars, unlike smaller stars, begin to shine while the clouds are still collapsing. Their ultraviolet radiation ionizes the nearby gas, forming a nebula with a temperature of around 18,000 degrees F. Models indicate the gas around immense stars will quickly enlarge at this stage of development. However, findings from the VLA radio observatory show something different – large highly compact regions of ionized hydrogen, called HII regions.

"In the old theoretical model, a high-mass star forms and the HII region lights up and begins to expand. Everything was neat and tidy," said study author Chris De Pree, a professor of astronomy and director of the Bradley Observatory at Agnes Scott College. "But the group of theorists I am working with were running numerical models that showed accretion was continuing during star formation, and that material was continuing to fall in toward the star after the HII region had formed."

Recent models had indicated this is due to the interstellar gas around massive stars forming localized spiral filaments. When the enormous star passes through the filaments, they absorb its ultraviolet energy, and protect the surrounding gas. This protection clarifies not only how the gas can continue plummeting down, but why the ionized nebulae seen with the VLA are so modest: the nebulae reduce in size when they cease to be ionized, so that over millennia, they appear to flicker like a candle.

"These transitions from rarefied to dense gas and back again occur quickly compared to most astronomical events," said Mac Low, a curator at the American Museum of Natural History’s Department of Astrophysics. "We predicted that measurable changes could occur over times as short as a few decades."

To reach their conclusion, the researchers used VLA observations of the star-forming Sagittarius B2 region made in 1989 and 2012. This region, found near the center of the Milky Way, holds many small regions of ionized gas around enormous stars, providing a number of candidates for observation. The team noted that four of the HII regions were found to have significant changes in brightness.

"The long term trend is still the same, that HII regions expand with time," De Pree said. "But in detail, they get brighter or get fainter and then recover. Careful measurements over time can observe this more detailed process."