Simulations solve a 20-year-old riddle about why nebulae around masssive stars don't disappear

Mar 16, 2010
This is a simulated observation of a massive star viewed along the plane of the disk. This visualization of dust emission traces the density and temperature of the gas cloud that surrounds the star. The regions that are currently ionized (in red) and have been ionized in the past (blue structures) show how the nebula flickers. Credit: Peters, et al. 2010

The birth of the most massive stars -- those ten to a hundred times the mass of the Sun --has posed an astrophysical riddle for decades. Massive stars are dense enough to fuse hydrogen while they're still gathering material from the gas cloud, so it was a mystery why their brilliant radiation does not heat the infalling gas and blow it away.

New simulations by researchers affiliated with the University of Heidelberg, American Museum of Natural History, the National Autonomous University of Mexico, and the Harvard-Smithsonian Center for Astrophysics show that as the gas cloud collapses, it forms dense filamentary structures that absorb the star's radiation when it passes through them. A result is that the surrounding heated flickers like a candle flame. The research is published in the current issue of The Astrophysical Journal.

"To form a massive star, you need massive amounts of gas," says Mordecai-Mark Mac Low, a co-author and curator in the Department of Astrophysics at the Museum. "Gravity draws that gas into filaments that feed the hungry baby stars."

Stars form when huge clouds of gas collapse. Once the central density and temperature are high enough, hydrogen begins to fuse into helium and the star begins to shine. 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. This suggests that the growth of a massive star should taper off or even cease because the surrounding gas should be blown away by the heating.

First author Thomas Peters, a researcher at the Center of Astronomy at the University of Heidelberg and a former Annette Kade Fellow at the Museum, and colleagues ran gas dynamical simulations on supercomputers at the Texas Advanced Computing Center funded by the National Science Foundation and at the Leibniz and Jülich Computing Centers in Germany. The team's results show that interstellar gas around 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 while falling to the star. The local areas of collapse form spiral filaments. When the massive star passes through them, they absorb its ultraviolet radiation, shielding the surrounding gas. This shielding explains not only how gas can continue falling in, but why the ionized nebulae observed with radio telescopes are so small: the nebulae shrink again as they are no longer ionized, so that over thousands of years, the nebula appears to flicker, almost like a candle.

"So far, these ionized nebulae were just thought to be expanding bubbles of hot gas, and the measured size of these bubbles was used by observers to infer the age of its central star," says Peters. "Our results are of particular importance because the simulations show that there is, in fact, no direct relation between the size of the nebula and the age of the massive star, so long as the star is still growing. This is the case over a significant fraction of the total lifetime of a massive star."

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More information: doi:10.1088/0004-637X/711/2/1017

Provided by American Museum of Natural History

4.4 /5 (14 votes)

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mysticshakra
2.5 / 5 (10) Mar 16, 2010
Sorry, but simulations do not solve or prove anything except the faith and/or incompetence of the "scientist" using them. Try using observations next time.
solrey
4 / 5 (8) Mar 16, 2010
Ionized gas is plasma. Since it's plasma, in a magnetic field, the filamentary structures are field aligned currents flowing between the dense highly ionized central core and the surrounding diffuse lightly ionized plasma. Experiments show that an electric field forms between charge sheath layers that develop at the boundaries of different plasma "species". Electromagnetic fields act on charged particles 39 orders of magnitude stronger than gravity. I think plasma physics offers a more realistic description than gas and gravity.
ShotmanMaslo
5 / 5 (8) Mar 16, 2010
Sorry, but simulations do not solve or prove anything except the faith and/or incompetence of the "scientist" using them. Try using observations next time.


When the simulations agree with observations, like in this case, they do solve many things. Science is not only about observing phenomena, but also about explaining them, and thats where simulations are immensely useful.
deatopmg
2.4 / 5 (5) Mar 16, 2010
The evidence, not the simulations, may suggest that a stars primary energy generation mechanism could be other than H - H fusion.
As solrey says; the electromagnet field's effect on the charged particles is 1,000,000,000,000,000,000,000,000,000,000,000,000,000 times stronger than gravity. This being the case, is gravity of any importance?
PhysicsLver21
5 / 5 (2) Mar 16, 2010
Of course... with out hydrostatic equilibrium the star would just collapse or not form altogether. With gravity their its kind of like a counter force acting against the increased pressure from nuclear fusion taking place in the core
ShotmanMaslo
5 / 5 (6) Mar 16, 2010
I am no astrophysicist, so take what I say with a big dose of salt, but the reason why EM effects on such a long scale are neglected could be that even plasma tends to be electricaly neutral - it is a mess of particles with half of them positive and half of them negative - overall charge is zero - no force.

It would have tremendous effects if we were talking about an object with positive charge and an object with negative charge - ions attract. But macroscopic objects are almost completely neutral, and any difference in charge tends to cancel out quickly.

On the other hand gravity never cancels out.
solrey
2.3 / 5 (3) Mar 16, 2010
The charge sheath layers and the electric field between double layers in a non-current carrying plasma greatly reduce the speed of charge neutralization. This has been well demonstrated in the lab with RF induced non-current carrying plasma. Even lacking an external voltage potential, an electric field can develop between two different plasmas. If the difference between plasma species is large enough, the electric field will accelerate particles beyond the critical ionization velocity of the plasma and will further reduce the rate of charge neutralization, or even maintain charge separation, on a time scale that could last for billions of years in large volumes of space.