vla_nebula_011112 Stars are the grandest recyclers of material. Hydrogen becomes helium, old stars explode, new stars and planets are born, people emerge, and then, presumably, it all turns to dust again one day.
Some key steps in this never-ending cycle are inferred based on evidence obtained after the fact -- many processes in the life cycles of stars have never actually been studied much, because they often take place over eons.
Now a team of astronomers has spotted what they think are the early stages in the formation of a planetary nebula, a bubble of gas and dust that develops after a star explodes. Planetary nebulae have nothing to do with planets -- they got the name way back when crude telescopes made them look something like planets in a sky that had yet to be explored very deeply.
Using the National Science Foundation's Very Large Array (VLA) radio telescope, researchers examined a hot remnant star inside a shining bubble of glowing gas. They figure they've spotted the process of nebula formation within 100 years of its onset.
"This is the first time that anyone has seen a star that is so clearly going through this transformation stage," said Yolanda Gomez, an astronomer at the Institute for Astronomy at the National Autonomous University in Mexico City. "We believe this star began to enter its planetary-nebula phase only after 1984."
The research is reported in the November 15 edition of the journal Nature.
At the end of their lives, stars like our Sun eject gas into space before starting to contract under their own gravity into white dwarf stars. The contraction heats up the star, making it pour out energetic ultraviolet light. This UV light tears apart molecules in gas that was ejected earlier by the star, and it rips electrons from the atoms in the gas. This makes the gas glow, producing often-beautiful shining shells and other shapes.
Once the remnant star has heated up sufficiently to produce large amounts of UV light, molecules in the gas ejected earlier are destroyed rapidly.
"We are seeing radio waves emitted by water molecules in this planetary nebula," said Gomez. "The water molecules, we believe, are all destroyed within only 100 years of the beginning of this stage, so we are seeing this star during an extremely brief transition period of its life."
The object observed is called K3-35 and is 16,000 light-years from Earth in the constellation Vulpecula (the small fox). It has a doughnut-shaped ring of gas around its center and lobes of outflowing material, similar to structures seen in other planetary nebulae.
The researchers were surprised to find regions near the star in which water molecules are amplifying, or strengthening radio-wave emission at a frequency which corresponds to the manner that a laser amplifies light waves. They found these regions, called masers, in the doughnut-shaped structure surrounding the central star, as well as at the end of much larger lobes of gas extending from the star.
The doughnut-shaped ring has a radius of more than twice the distance from the Sun to Pluto. The masers at the ends of the lobes are more than 100 times more distant from the star.
By analyzing their observations as well as earlier data collected by other astronomers, the research team concludes that K3-35 has only just begun its transformation into a planetary nebula.
"This is extremely exciting, because we now have a 'laboratory' for watching this process take place over the next few years," said Luis Miranda of the Institute of Andalucia in Spain. "We don't fully understand everything we see in this object, but know that we are going to learn much valuable information about this process by watching it develop."
Guillem Anglada of the Harvard- Smithsonian Institute for Astrophysics in Cambridge, MA, and Jose Torrelles of the Institute for Space Studies of Catalunya in Barcelona, Spain, also participated in the study.
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