New observations of striking clouds of gas and dust reveal the engine behind the beauty, some of the hottest and most massive stars ever detected.
The photographs, taken by the Very Large Telescope at the European Southern Observatory's Paranal site in Chile, show four separate nebulae in two small satellite galaxies of the Milky Way, known as the small and large Magellanic clouds.
Nebulae can be castoffs from stellar explosions or interstellar clouds building toward future star formation. What makes many of them so visible and beautiful is largely understood, but that's not the case with some that appear to be strongly heated and show an amazingly high degree of excitation, astronomers say. Gas in these poorly understood nebulae is energized by large amount of ions, atoms that have been stripped of one or more electrons.
The new observations, announced last week, show nebulae that are powered by stars 20 times more massive and far hotter than our Sun.
Three of the nebulae harbor so-called Wolf-Rayet stars. The other surrounds a massive O-type star. All the stars will live short lives, shining up to 10 million times more brightly than our Sun and using their energy up in millions of years, rather than billions. Some appear to have orbiting stellar companions.
The stars emit energetic particles in stellar winds that can be a billion times stronger than what emanates from our Sun. These winds pressurize the surrounding material and force it into bubble-like shapes. Some of these bubbles are evident in the new pictures only as arcs of color.
Astronomers said the new images show that the stars are responsible for the intense energy noted in the clouds. Three of the stars emit enough ultraviolet energy to completely ionize helium atoms. The ions then capture an electron, which gives rise to the characteristic radiation of single ionized helium -- seen only in the most energetic nebulae.
Blue regions in the images trace high rates of single ionized helium emission. Red and green correspond to more common nebular emissions from atomic hydrogen and doubly ionized oxygen, respectively.
The newly observed mechanism fails to explain the fourth nebula, called N44C.
"We were able to fully understand three nebulae, but we must now look more closely at N44C," said Yal Naz of the Institut d'Astrophysique et de Gophysique Lige in Belgium. Further observations are planned.
Details of the findings will be published in the European research journal Astronomy & Astrophysics. Also working on the study were Grgor Rauw, Jean Manfroid and Jean-Marie Vreux of the Lige Institute and You-Hua Chu from the University of Illinois.
advertisement
