"Massive stars dominate the lives of their host galaxies through their ionizing radiation and supernova explosions," said Mordecai-Mark Mac Low, a curator in the American Museum of Natural History's Department of Astrophysics and an author on the paper. "All the elements heavier than iron were formed in the supernova explosions occurring at the ends of their lives, so without them, life on Earth would be very different."
Young massive stars, which have more than 10 times the mass of the Sun, shine brightly in the ultraviolet, heating the gas around them, and it has long been a mystery why the hot gas doesn't explode outwards. Now, observations made by a team of researchers using the Jansky Very Large Array (VLA), a radio astronomy observatory in New Mexico, have confirmed predications that as the gas cloud collapses, it forms dense filamentary structures that absorb the star's ultraviolet radiation when it passes through them. As a result, the surrounding heated nebula flickers like a candle.
Stars form when huge clouds of gas collapse. Once the density and temperature are high enough, hydrogen fuses into helium, and the star starts shining. The most massive stars, though, begin to shine while the clouds are still collapsing. Their ultraviolet light ionizes the surrounding gas, forming a nebula with a temperature of 10,000 degrees Celsius. Simple models suggest that at this stage, the gas around massive stars will quickly expand. But observations from the VLA radio observatory show something different: a large number of regions of ionized hydrogen (so-called HII regions) that are very small.
"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 lead 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 modeling has shown that this is because the interstellar gas around massive stars does not fall evenly onto the star but instead forms filamentary concentrations because the amount of gas is so great that gravity causes it to collapse locally. The local areas of collapse form spiral filaments. When the massive star passes through the filaments, they absorb its ultraviolet radiation, shielding the surrounding gas. This shielding explains not only how the gas can continue falling in, but why the ionized nebulae observed with the VLA are so small: the nebulae shrink when they are no longer ionized, so that over thousands of years, 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 Dr. Mac Low, a curator in the Museum's Department of Astrophysics. "We predicted that measurable changes could occur over times as short as a few decades."
The new study tested this theory with a 23-year-long experiment. The researchers used VLA observations of the Sagittarius B2 region made in 1989 and again in 2012. This massive star-forming region located near the Galactic center contains many small regions of ionized gas around high-mass stars, providing a large number of candidates for flickering. During this time, four of the HII regions indeed significantly changed 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."
The findings, made by scientists working at Agnes Scott College, Universität Zürich, the American Museum of Natural History, Harvard-Smithsonian Center for Astrophysics, National Radio Astronomy Observatory, European Southern Observatory, and Universität Heidelberg, were published recently in The Astrophysical Journal Letters.
Two of our Galaxy's most massive stars shown at the top of the page have been scrutinised in an impressive view by the NASA/ESA Hubble Space Telescope. They have, until recently, been shrouded in mystery, but the new image shows them in greater detail than ever before.
The image shows a pair of colossal stars, WR 25 and Tr16-244, located within the open cluster Trumpler 16. This cluster is embedded within the Carina Nebula, an immense cauldron of gas and dust that lies approximately 7500 light-years from Earth. The Carina Nebula contains several ultra-hot stars, including these two star systems and the famous blue star Eta Carinae, which has the highest luminosity yet confirmed. As well as producing incredible amounts of heat, these stars are also very bright, emitting most of their radiation in the ultraviolet and appearing blue in colour. They are so powerful that they burn through their hydrogen fuel source faster than other types of stars, leading to a "live fast, die young" lifestyle.
WR 25 is the brightest, situated near the centre of the image. The neighbouring Tr16-244 is the third brightest, just to the upper left of WR 25. The second brightest, to the left of WR 25, is a low mass star located much closer to the Earth than the Carina Nebula. Stars like WR 25 and Tr16-244 are relatively rare compared to other, cooler types. They interest astronomers because they are associated with star-forming nebulae, and influence the structure and evolution of galaxies.
WR 25 is likely to be the most massive and interesting of the two. Its true nature was revealed two years ago when an international group of astronomers led by Roberto Gamen, then at the Universidad de La Serena in Chile, discovered that it is composed of at least two stars (see the separate image). The more massive is a Wolf-Rayet star and may weigh more than 50 times the mass of our Sun. It is losing mass rapidly through powerful stellar winds that have expelled the majority of its outermost hydrogen-rich layers, while its more mundane binary companion is probably about half as massive as the Wolf-Rayet star, and orbits around it once every 208 days.
Massive stars are usually formed in compact clusters. Often the individual stars are so physically close to each other that it is very difficult to resolve them in telescopes as separate objects. These Hubble observations have revealed that the Tr16-244 system is actually a triple star. Two of the stars are so close to each other that they look like a single object, but Hubble's Advanced Camera for Surveys shows them as two. The third star takes tens or hundreds of thousands of years to orbit the other two. The brightness and proximity of the components of such massive double and triple stars makes it particularly challenging to discover the properties of massive stars.
WR 25 and Tr16-244 are the likely sources of radiation that is causing a giant gas globule within the Carina Nebula to slowly evaporate away into space, while possibly inducing the formation of new stars within it . The radiation is also thought to be responsible for the globule's interesting shape, prominently featured in earlier Hubble images, which looks like a hand with a "defiant" finger pointing towards WR 25 and Tr16-244 (see separate image).
These new observations were obtained by a team including astronomers from US, Chilean, Spanish, and Argentine institutions and led by Jesús Maíz Apellániz from the Instituto de Astrofísica de Andalucía in Spain. They are using Hubble as well as ground-based observatories in Spain, Chile, and Argentina to build a comprehensive catalogue of observations of all the massive stars in the Galaxy that are detectable at visible wavelengths.
The Daily Galaxy via AMNH and http://www.spacetelescope.org/
The publication can be viewed at: http://arxiv.org/abs/1312.7768
The Daily Galaxy via Astrophysical Journal Letters and AMNH