Solved: `Why nebulae around massive stars don`t disappear`

The birth of the most massive stars and the nebulae around them have posed an astrophysical riddle for decades.

Last Updated: Apr 21, 2010, 14:03 PM IST

Washington: The birth of the most massive stars and the nebulae around them have posed an astrophysical riddle for decades. Now, planetary scientists claim to have solved the mystery why their brilliant radiation doesn`t heat the infalling gas and blow it away.

New simulations by an international team have shown that as the gas cloud collapses, it forms dense filamentary structures that absorb the star`s radiation when it passes through them -- and as a result the surrounding heated nebula flickers like a candle flame.

"To form a massive star, you need massive amounts of gas. Gravity draws that gas into filaments that feed the hungry baby stars," team member Mordecai-Mark Mac Low of the American Museum of Natural History, said.

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. The ultraviolet light ionises 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, say the scientists.

So, in their research, the scientists ran gas dynamical simulations on supercomputers at the Texas Advanced Computing Center.

The results revealed that 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 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 ionised nebulae observed with radio telescopes are so small -- the nebulae shrink again as they
are no longer ionised, so that over thousands of years, the
nebula appears to flicker, almost like a candle.

"So far, these ionised 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," said member Thomas Peters of Heidelberg University.

"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," he added.

The findings have been published in the latest edition of `The Astrophysical Journal`.